Detailed working notes on ICAO Electronic Terrain and Obstacle Data (eTOD). This folder expands the summary in topics/etod.md into per-aspect files so each can be read independently.

Files in this folder#

• overview.md — what eTOD is, where it sits in the ICAO/ATM framework, and why digital terrain data matters for modern aviation.
• components.md — the building blocks of eTOD: terrain dataset, obstacle dataset, coverage Areas, data quality attributes, data product specification, and metadata.
• blocks.md — the four coverage Areas (1, 2, 3, 4) treated as the maturity/coverage tiers of eTOD, with numeric requirements tables and a mermaid area-nesting diagram.
• threads.md — the six functional axes that together constitute eTOD: terrain, obstacle, data quality/integrity, collection and maintenance, exchange (AIXM), and downstream applications (PBN/TAWS/charting).
• modules.md — anatomy of one eTOD strand: the Area 2 obstacle data collection-to-publication chain with a worked aerodrome example.
• enablers.md — CNS, surveying technology, ISO standards, AIXM exchange, regulation, training, and institutional arrangements required for State eTOD compliance.
• performance_objectives.md — KPA-keyed performance table and KPIs for eTOD data quality and State provision.
• timeline.md — historical evolution: SARPS introduction, Amendment phases, PANS-AIM First Edition, and Area provision milestones.
• references.md — consolidated ICAO and authoritative external references for all content in this folder.

Reading order#

Start with overview.md to understand what eTOD is and why it matters, then blocks.md for the four-Area framework and numeric requirements, then threads.md for the functional axes. components.md provides the structural detail; modules.md gives a worked example. enablers.md is useful for State implementation planning. performance_objectives.md links eTOD quality to measurable outcomes. timeline.md gives date context for amendment history. references.md is the citation master list.

Source basis#

Content is grounded in:

• ICAO Annex 15 (Aeronautical Information Services), Sixteenth Edition (2018), Chapter 5, §5.3.3 and Foreword amendment table.
• ICAO Doc 10066 (PANS-AIM), First Edition (2018), Chapter 5, §5.3.3.2 and Appendices 1, 6, and 8.
• ICAO Doc 8126 (Aeronautical Information Services Manual), Chapter 3 and Chapter 10.
• ICAO Annex 6, Part I, §6.15 (GPWS/TAWS requirements).
• ICAO Annex 14, Volume I, Chapter 4 (obstacle limitation surfaces).
• ICAO Annex 4 (Aeronautical Charts) §3.8.2.
• ICAO Doc 8168 (PANS-OPS), Volume II — obstacle clearance criteria.

What eTOD is#

Electronic Terrain and Obstacle Data (eTOD) is the set of standardised digital data sets that represent, in machine-readable form, the Earth's terrain surface and the three-dimensional extent of fixed or mobile obstacles above defined data collection surfaces.

The data sets are organised into four coverage Areas (1 through 4), each calibrated to the operational application it serves. Area 1 covers an entire State territory with coarse resolution sufficient for en-route and airspace planning. Area 2 covers the vicinity of individual aerodromes with higher resolution and tighter accuracy for approach, departure, and terminal-area applications. Area 3 covers the aerodrome movement area for surface operations. Area 4 covers the precision approach corridor for CAT II/III runways with the highest accuracy requirement.

eTOD is defined as a mandatory digital data set type in Annex 15, Chapter 5. The Standards (what must be provided) and Recommended Practices (what should be provided) are stated in §5.3.3. The numerical accuracy, resolution, integrity, and post-spacing values for each Area are specified in PANS-AIM (Doc 10066), Appendices 1 and 8.

Where eTOD sits in the ICAO/ATM framework#

eTOD occupies the base layer of the Digital AIM (DAIM) data product family. It is one of five digital data set types mandated by Annex 15, Chapter 5 alongside:

• AIP data set (codified aeronautical information of lasting character)
• Aerodrome mapping data set
• Instrument flight procedure data set
• (Terrain and obstacle data sets — eTOD)

Within the ASBU framework, eTOD is a foundational enabler of the DAIM thread. The DAIM-B0 module requires quality-managed digital AIS data sets to be in place, including terrain and obstacle data meeting Annex 15 standards. Higher DAIM blocks, SWIM information services, and trajectory-based PBN operations all assume that the underlying eTOD datasets are current, accurate, and distributed.

eTOD also underpins the APTA thread (approach procedure optimisation with vertical guidance), where PBN approach minima are computed using PANS-OPS obstacle clearance methodology that relies on verified Area 2 and Area 4 eTOD.

The three-layer SARPS structure#

The ICAO regulatory structure for eTOD operates across three layers:

1. Annex 15 — defines the coverage Areas, the obligation to provide data, and the mandate that numerical requirements live in PANS-AIM. This provides the treaty-level obligation.

2. PANS-AIM (Doc 10066) — specifies the content, attribute lists (Appendix 6), the collection surface geometry (Appendix 8), and the numerical accuracy/integrity values (Appendix 1 Data Catalogue). This provides the operational specification.

3. Doc 8126 (AIS Manual) — provides guidance on implementation arrangements, data originator relationships, cost recovery, and cross-system consistency (e.g., keeping eTOD aligned with aerodrome mapping data for Area 3).

Why eTOD matters#

The downstream effects of eTOD quality are pervasive:

Safety — TAWS/eGPWS effectiveness. Annex 6 §6.15.2 requires operators to maintain current terrain and obstacle data in their onboard terrain database. The database is built from national eTOD. If a State's Area 2 eTOD is incomplete or inaccurate, every aircraft operating in that State's terminal airspace carries a degraded TAWS terrain picture.

Safety — procedure minima. PANS-OPS (Doc 8168) obstacle clearance methodology computes procedure minima from obstacle surveys in the protected surfaces. Where eTOD Area 2 is not compliant, procedure designers must either use conservative generic assumptions or conduct proprietary surveys, raising minima.

Operational access. Conservative minima derived from poor eTOD quality directly restrict access during low-visibility operations. States with high-quality Area 2 and Area 4 eTOD can publish lower CAT II/III minima, improving operational availability.

Chart accuracy. The Aerodrome Terrain and Obstacle Chart - ICAO (Electronic) and the Precision Approach Terrain Chart - ICAO are derived from eTOD. Inaccurate eTOD produces inaccurate charts.

MSAW and A-SMGCS. Ground-based MSAW systems and A-SMGCS routing functions use eTOD as their underlying terrain reference. Gaps or errors result in false alerts or missed alerts.

Relationship to other topics in this workspace#

• AIM — eTOD is one of the core Digital AIM data products; the DAIM thread depends on State provision of eTOD.
• AIXM — AIXM 5.2 includes obstacle and terrain exchange feature types; eTOD is exchanged using AIXM profiles in SWIM.
• PBN — procedure design for PBN approaches depends on Area 2 and Area 4 obstacle surveys meeting eTOD quality requirements.
• ASBU — eTOD is a DAIM-B0 foundational enabler; inadequate eTOD blocks progression to DAIM-B1 (full Digital AIM via SWIM) and APTA modules requiring improved approach procedures.
• Airspace Design — en-route airspace design and minimum altitude determination use Area 1 terrain data.
• ATFM / CDO / CCO — MSAW terrain alerting in ATM systems uses eTOD as the underlying height reference for sector minimum altitudes.

References#

• Annex 15 (Aeronautical Information Services), Sixteenth Edition (2018), Chapter 5, §5.3.3 — Standards and Recommended Practices for terrain and obstacle data sets; coverage area definitions.
• Annex 15, Foreword, Amendment 33 — introduction of eTOD SARPS in 2004 following GNSSP/4 and OCP recommendations.
• Doc 10066 (PANS-AIM), First Edition (2018), Chapter 5, §5.3.3.2 — terrain and obstacle data set specifications and intended applications.
• Doc 8126 (Aeronautical Information Services Manual), Chapter 3, §3.3.3.4 — eTOD data originators outside the aviation system; arrangements for collection and cost recovery.
• Annex 6, Part I, Chapter 6, §6.15.1 and §6.15.2 — GPWS/TAWS mandate and operator terrain database maintenance obligation.
• Doc 9750 (GANP) and ASBU DAIM thread — eTOD as a DAIM-B0 foundational data product (authoritative source — not in local library; see https://ganpportal.icao.int/).

Building blocks of eTOD#

eTOD comprises two distinct data set types, an organising framework of four coverage Areas, a data quality regime, a data product specification structure, and a metadata layer. Each component has a normative basis in Annex 15 and PANS-AIM.

Terrain data set#

The terrain data set is a continuous grid of elevation values representing the Earth's surface. Per PANS-AIM §5.3.3.2.1:

• A terrain grid shall be angular or linear and of regular or irregular shape.
• Sets include spatial (position and elevation), thematic, and temporal aspects of the Earth's surface, covering naturally occurring features — mountains, hills, ridges, valleys, bodies of water, permanent ice and snow — and explicitly excluding obstacles.
• Depending on the acquisition method, the surface may represent the bare Earth, the top of the vegetation canopy, or something between (the "first reflective surface"). The acquisition method attribute makes this traceable.
• Only one feature type — terrain — appears in a terrain data set; obstacles must be captured in the separate obstacle data set.

Terrain attributes (all mandatory per PANS-AIM Appendix 6, Table A6-1): area of coverage, data originator identifier, data source identifier, acquisition method, post spacing, horizontal reference system, horizontal resolution, horizontal accuracy, horizontal confidence level, horizontal position, elevation, elevation reference, vertical reference system, vertical resolution, vertical accuracy, vertical confidence level, recorded surface, integrity, date and time stamp, and unit of measurement. Optional attributes: surface type, penetration level, known variations.

Obstacle data set#

The obstacle data set captures the three-dimensional extent of fixed or mobile structures that penetrate defined collection surfaces. Per PANS-AIM §5.3.3.2.2:

• Obstacle elements are represented as points, lines, or polygons depending on their horizontal extent.
• All defined obstacle feature types shall be provided in a data set; each described according to mandatory attributes in Appendix 6, Table A6-2.
• Obstacles can be fixed-permanent, fixed-temporary, or mobile. Mobile and temporary types carry additional optional attributes.
• Obstacle data shall not be included in terrain data sets.

Obstacle attributes (mandatory per PANS-AIM Appendix 6, Table A6-2): area of coverage, data originator identifier, data source identifier, obstacle identifier, horizontal accuracy, horizontal confidence level, horizontal position, horizontal resolution, horizontal extent, horizontal reference system, elevation, vertical accuracy, vertical confidence level, vertical resolution, vertical reference system, obstacle type, geometry type, integrity, date and time stamp, unit of measurement, lighting, marking. Optional attributes: height, operations (for mobile types), effectivity (for temporary types).

The obstacle data product specification shall describe, for each aerodrome in the data set, the geographic coordinates and the areas covered: Areas 2a, 2b, 2c, 2d; the take-off flight path area; and the obstacle limitation surfaces.

Four coverage Areas#

The four Areas are the organising framework that matches data fidelity to operational application:

Area 1: Entire State territory Terrain: mandatory Standard Obstacle: mandatory for obstacles >=100 m height above ground

Area 2: Aerodrome vicinity (subdivided 2a, 2b, 2c, 2d) Terrain 2a: mandatory Standard Obstacle 2a: mandatory Standard (surface 3 m above runway) Terrain/Obstacle 2b/2c/2d: Recommended Practice

Area 3: Aerodrome movement area zone Terrain and obstacle: Recommended Practice (required to support aerodrome mapping data consistency)

Area 4: CAT II/III precision approach corridor Terrain and obstacle: mandatory Standard

Data quality attributes#

PANS-AIM §5.3.3.2 and Appendix 1 establish the quality framework for eTOD data elements. The key dimensions are:

Accuracy — for terrain and obstacle data, accuracy requirements are based on a 90 per cent confidence level, distinguishing them from most aeronautical data (95 per cent). Post spacing (grid density) and horizontal/vertical accuracy vary by Area, with Area 4 having the tightest requirements.

Resolution — the minimum difference between values that can be distinguished; applies independently to horizontal and vertical dimensions. Resolution requirements are captured in the Aeronautical Data Catalogue (Appendix 1).

Integrity — the degree of assurance that data has not been lost or altered since origination or authorised amendment. Three classes:

• Routine: 1 x 10^-3 probability of undetected error per flight hour.
• Essential: 1 x 10^-5 probability.
• Critical: 1 x 10^-8 probability. Area 4 data and close-in Area 2 obstacle data carry Critical integrity because corrupted data has a high probability of endangering safe flight during a CAT II/III approach.

Traceability — the chain from field survey through processing to distribution; captured via originator and source identifiers, date and time stamps, and acquisition method attributes.

Timeliness — data must be current; operators and AIS providers must maintain update schedules aligned with survey programmes and aerodrome infrastructure changes.

Data product specification#

Per ISO 19100 series geographic information standards, each eTOD data set is described by a data product specification that defines: the scope (Area, aerodrome, State); the spatial reference system; the data content model; the data quality requirements; the acquisition method; the update policy; and the data distribution format. PANS-AIM Appendix 6 Table A6-1/A6-2 provide the ICAO-specific attribute requirements that populate each specification.

The terrain data product specification for Area 1 is typically a State-wide digital elevation model (DEM). The obstacle data product specifications are aerodrome-specific, covering the defined Areas 2 and 4 for each aerodrome in the data set.

Metadata#

Each data set shall be provided to the next intended user together with at least the minimum set of metadata that ensures traceability (Annex 15 §5.3.1.2). For data sets with geographic information — terrain, obstacle, and aerodrome mapping — metadata may be structured according to ISO Standard 19115: Geographic information — Metadata (Doc 8126 §2.3.3.3). This enables interoperability with geographic information systems (GIS) and AIXM-based exchange services.

Hierarchy of eTOD components#

• eTOD
  • Terrain data set
    • Grid (Area 1: State-wide DEM)
    • Grid (Area 2: aerodrome vicinity DEM, within 10 km of ARP)
    • Grid (Area 3: movement area surface)
    • Grid (Area 4: CAT II/III corridor)
  • Obstacle data set
    • Obstacle records (Area 1: >=100 m obstacles, State-wide)
    • Obstacle records (Area 2a/2b/2c/2d: aerodrome obstacles)
    • Obstacle records (Area 3: movement area penetrations)
    • Obstacle records (Area 4: CAT II/III corridor penetrations)
  • Data quality attributes
    • Accuracy (horizontal and vertical, 90% confidence)
    • Resolution (post spacing, vertical resolution)
    • Integrity (routine / essential / critical)
    • Traceability (originator, source, acquisition method, timestamp)
  • Data product specification (per ISO 19100 series)
  • Metadata (per ISO 19115)

References#

• Annex 15, Chapter 5, §5.3.3 — Standards and Recommended Practices for terrain and obstacle data sets; coverage Area definitions; mandatory and Recommended Practice obligations per Area.
• Doc 10066 (PANS-AIM), Chapter 5, §5.3.3.2.1 — terrain data set specifications; feature type, spatial content, acquisition method recording.
• Doc 10066, Chapter 5, §5.3.3.2.2 — obstacle data set specifications; point/line/polygon representation; product specification area coverage.
• Doc 10066, Appendix 6, Table A6-1 — mandatory and optional terrain attributes.
• Doc 10066, Appendix 6, Table A6-2 — mandatory and optional obstacle attributes.
• Doc 10066, Appendix 1 — Aeronautical Data Catalogue; Tables A1-6 (obstacle) and A1-8 (terrain); numeric accuracy, resolution, and integrity requirements by Area.
• Doc 10066, Definitions — integrity classifications (routine, essential, critical) and their probability thresholds.
• Doc 10066, Chapter 4, §4.1.1 — collection and transmission requirements conforming to accuracy and integrity in Appendix 1.
• Doc 8126 (AIS Manual), Chapter 2, §2.3.3.3 — metadata per ISO 19115 for geographic data sets including terrain and obstacle.

The four Areas as coverage tiers#

For eTOD, the analogue of ASBU Blocks is the four coverage Areas defined in Annex 15 §5.3.3.1. Each Area is a progressive tier of spatial coverage and data fidelity, building from State-level terrain (Area 1) through aerodrome vicinity (Area 2), movement area surface (Area 3), to precision approach corridor (Area 4).

Each Area has a defined:

• Spatial extent — what geography it covers.
• Collection surface — the surface above which obstacles must be captured and below which terrain is gridded.
• Obligation level — Standard (shall) or Recommended Practice (should) from Annex 15.
• Intended applications — the safety systems and procedures that depend on that Area's data.

Area nesting diagram#

Area 1 — State territory#

Spatial extent. The entire territory of a State.

Terrain obligation. Mandatory Standard. Terrain data shall be provided for Area 1 (Annex 15 §5.3.3.3.2). No terrain type is excluded from the obligation; the grid covers the full State boundary.

Obstacle obligation. Mandatory Standard for obstacles whose height above ground is 100 m or higher (Annex 15 §5.3.3.4.3). This threshold captures the principal enroute hazards — tall towers, wind turbine farms, large industrial structures — while excluding the vast majority of low-height features that are managed at the aerodrome level.

Intended applications. En-route MSAW terrain reference; airspace design minimum altitude determination; flight simulator databases; State-level terrain awareness for operators flying in the State.

Typical data source. National mapping agencies, satellite-derived digital elevation models (DEMs), SRTM or ASTER-type sources processed to meet Annex 15 accuracy, reprocessed through an ISO 19115 compliant metadata record citing the acquisition method and date.

Area 2 — Aerodrome vicinity#

Area 2 is the most operationally complex of the four Areas because it is sub-divided and because the required accuracy is substantially higher than Area 1 to support instrument procedure design.

Area 2 as a whole. Covers the vicinity of an aerodrome regularly used by international civil aviation, extending up to 45 km from the aerodrome reference point (ARP), or to the TMA boundary if nearer.

Area 2a#

Spatial extent. The runway strip (as defined in Annex 14 Vol I Chapter 3) plus any clearway that exists.

Terrain obligation. Mandatory Standard. Terrain data shall be provided for Area 2a (Annex 15 §5.3.3.3.3(a)).

Obstacle obligation. Mandatory Standard. Obstacles that penetrate a collection surface 3 m above the nearest runway elevation measured along the runway centreline, and for clearway portions at the nearest runway end elevation (Annex 15 §5.3.3.4.5(a)).

Also mandatory: objects in the take-off flight path area projecting above a 1.2 per cent slope surface (Annex 15 §5.3.3.4.5(b)); penetrations of aerodrome obstacle limitation surfaces (§5.3.3.4.5(c)).

Why it matters. Area 2a data is the direct input to the Obstacle Departure Procedure (ODP) design and determines whether a standard SID can be published or a specific ODP is needed. Errors in Area 2a obstacle data can invalidate existing procedures.

Area 2b#

Spatial extent. Departure corridor extending from the ends of Area 2a, 10 km in length, with a 15 per cent splay to each side.

Obstacle collection surface. 1.2 per cent slope upward from the ends of Area 2a at the elevation of the runway end.

Obligation. Recommended Practice (Annex 15 §5.3.3.4.6). Minimum height threshold: 3 m above ground in Area 2b.

Area 2c#

Spatial extent. Area extending outside Areas 2a and 2b, to not more than 10 km from the boundary of Area 2a.

Obstacle collection surface. 1.2 per cent slope; initial elevation is the elevation of the point of Area 2a at which it commences.

Obligation. Recommended Practice. Minimum height threshold: 15 m above ground in Area 2c.

Area 2d#

Spatial extent. From the boundaries of Areas 2a/2b/2c out to 45 km from the ARP, or to an existing TMA boundary, whichever is nearest.

Obstacle collection surface. Obstacles 100 m above ground height.

Obligation. Recommended Practice. Captures tall structures that penetrate the terminal airspace but lie beyond the closer coverage areas.

Practical note. Area 2b, 2c, 2d data, while Recommended Practice, are required by many PANS-OPS-compliant procedure designs to confirm that no tall structures invalidate the protected surfaces. States that omit them often find that procedure designers must conduct proprietary surveys before publishing instrument approaches.

Area 3 — Aerodrome movement area#

Spatial extent. The area bordering the aerodrome movement area: 90 m horizontally from the runway centreline and 50 m from the edge of all other parts of the movement area.

Collection surface. 0.5 m above the horizontal plane passing through the nearest point on the aerodrome movement area.

Obligation. Recommended Practice for both terrain and obstacle data (Annex 15 §5.3.3.3.7 and §5.3.3.4.9).

Operational rationale. Area 3 eTOD is the foundation for:

• A-SMGCS routing and ground incursion alerting.
• Aerodrome mapping data consistency: PANS-AIM §5.3.3.3.1 states that aerodrome mapping data should be supported by Area 3 eTOD to ensure consistency of all geographical data related to the aerodrome.
• Moving-map displays for surface operations.

Area 3 eTOD and aerodrome mapping data can be originated using common acquisition techniques and managed within a single GIS.

Area 4 — CAT II/III precision approach corridor#

Spatial extent. 900 m prior to the runway threshold and 60 m each side of the extended runway centreline, in the direction of approach, for precision approach runways CAT II or III. Where terrain beyond 900 m is mountainous or otherwise significant, extension up to 2 000 m from threshold is a Recommended Practice.

Obligation. Mandatory Standard for terrain and obstacle data for all CAT II/III runways (Annex 15 §5.3.3.3.8 and §5.3.3.4.10).

Operational rationale. The Precision Approach Terrain Chart - ICAO is derived primarily from Area 4 terrain data and Area 2 obstacle data. Operators use Area 4 data to assess the effect of terrain on decision height determination by use of radio altimeters. Accurate Area 4 data can allow lower published decision heights on CAT III runways.

Numeric requirements summary#

Numerical post-spacing, horizontal accuracy, vertical accuracy, and integrity classification values for each Area are specified in the PANS-AIM Appendix 1 Aeronautical Data Catalogue (Tables A1-6 and A1-8). The tightest requirements apply to Area 4 and Area 2a (critical integrity classification); Area 1 carries routine integrity for terrain but essential integrity for tall obstacles.

Area implementation sequencing#

A practical State eTOD implementation sequence follows this order:

1. Area 1 terrain — covers the whole State for en-route safety. Achievable with national DEM or processed satellite data.
2. Area 1 obstacles — systematic survey of all structures

   =100 m AGL; often the hardest collection task at State level.

3. Area 2a/4 terrain and obstacles at each international aerodrome — highest priority because CAT II/III approvals, procedure design, and TAWS databases all depend on these.
4. Area 2b/2c/2d — extend progressively outward; driven by procedure design demand and height of surrounding terrain.
5. Area 3 — movement area surface data; typically coordinated with aerodrome mapping data programmes.

References#

• Annex 15, Chapter 5, §5.3.3.1 — four Area definitions with spatial extent boundaries.
• Annex 15, Chapter 5, §5.3.3.3.2, §5.3.3.3.3, §5.3.3.3.8 — terrain Standards and Recommended Practices by Area.
• Annex 15, Chapter 5, §5.3.3.4.3 to §5.3.3.4.11 — obstacle Standards and Recommended Practices by Area including height thresholds.
• Annex 15, Chapter 5, §5.3.3.2 — Recommended Practice on Area 4 length extension to 2 000 m for mountainous terrain.
• Doc 10066 (PANS-AIM), Appendix 8, Figures A8-1 to A8-4 — graphical terrain and obstacle collection surface definitions for each Area.
• Doc 10066, Appendix 1, Tables A1-6 and A1-8 — numeric accuracy, resolution, and integrity requirements by Area.
• Doc 10066, Chapter 5, §5.3.3.3.1 — aerodrome mapping data to be supported by Area 3 eTOD for consistency.

The six functional axes of eTOD#

eTOD is not a monolithic dataset. It is delivered through six interdependent functional axes (threads), each of which must be operating for the full benefit to flow to downstream applications.

Thread 1 — Terrain data collection#

This thread covers the acquisition, processing, and archiving of terrain elevation grids for each Area.

Area 1 terrain is typically sourced from:

• National topographic surveys converted to digital format.
• Satellite-derived DEMs: Shuttle Radar Topography Mission (SRTM), ASTER GDEM, or TanDEM-X at 30 m or higher resolution.
• Airborne LiDAR for high-accuracy sub-areas.

The acquisition method is a mandatory attribute that must be recorded in the data set, enabling downstream users to assess data suitability for their application.

Area 2 and Area 4 terrain requires survey-grade accuracy. LiDAR and photogrammetric methods are typical. Post-spacing and vertical accuracy requirements are significantly tighter than Area 1, often requiring dedicated data capture campaigns coordinated with the aerodrome operator.

The recorded surface attribute distinguishes bare earth from canopy or first reflective surface — critical for instrument procedure design where canopy cover may obscure a significant obstacle.

Thread 2 — Obstacle data collection#

This thread covers the systematic survey, cataloguing, and updating of obstacle records for each Area.

Area 1 obstacles (structures >=100 m AGL) require:

• Coordination with national telecommunications and power authorities for towers and transmission lines.
• Coordination with energy authorities for wind turbines (a rapidly growing obstacle category).
• Regular review because tall structures are constructed and decommissioned on timescales shorter than survey cycles.

Area 2 obstacles require:

• Aerodrome-coordinated surveys using airport authority records, crane permit systems, and dedicated field surveys.
• The obstacle data product specification must include geographic coordinates of each aerodrome, the Areas covered (2a, 2b, 2c, 2d), the take-off flight path area, and the obstacle limitation surfaces.
• PANS-AIM §5.3.3.2.2.4 is explicit: the product specification must describe all four 2x sub-areas plus take-off path and OLS.

Geometry representation. Point features for structures with small footprints (masts, chimneys). Line features for power lines, fences. Polygon features for large structures, groups, or wind turbine farms.

Coordination requirement. Annex 15 §5.3.3.4.7 and §5.3.3.4.8 recommend coordination with adjacent aerodromes where coverage areas overlap, and with States sharing borders, to ensure consistent obstacle records for the same physical structure.

Thread 3 — Data quality and integrity#

This thread governs the measurement and assurance of eTOD quality across the data lifecycle.

Quality dimensions per PANS-AIM and the Aeronautical Data Catalogue:

• Accuracy: The closeness of the measured value to the true value. Terrain and obstacle accuracy is at the 90 per cent confidence level, not the 95 per cent used for most aeronautical data.
• Resolution: The smallest recordable difference; expressed for horizontal (post spacing) and vertical (elevation step) dimensions.
• Integrity: Assurance that data has not been corrupted in the chain from survey to end user. Classified as routine, essential, or critical based on the consequence of undetected corruption.
• Traceability: The ability to trace a data value back to its source survey, acquisition method, date, and originator.
• Completeness: All required features and attributes are present in the data set.

Integrity classification by Area (indicative):

• Area 4 obstacle and terrain data: Critical (1 x 10^-8 per flight hour undetected error probability). Corruption has high probability of catastrophic consequence during a CAT II/III approach.
• Area 2a and close-in Area 2 obstacles: Essential to Critical (1 x 10^-5 to 10^-8). Direct input to procedure protected surfaces.
• Area 1 tall obstacles: Essential (1 x 10^-5). En-route safety.
• Area 1 terrain: Routine (1 x 10^-3). Large-scale terrain picture; safety consequence attenuated by design margins.

Quality assurance chain. PANS-AIM Chapter 4 establishes that data shall be collected and transmitted to the AIS in accordance with the accuracy and integrity requirements of Appendix 1. Cryptographic technologies (hash functions, message authentication codes) should be employed to assure integrity through transmission. Application tests, logic checks, semantic and redundancy checks, and digital error detection are additional measures (Doc 8126 §2.2.1, §2.3).

Thread 4 — Collection and maintenance#

This thread covers the institutional arrangements, survey cycles, and update obligations that keep eTOD current.

Data originators. eTOD data often originates from outside the aviation system: national mapping agencies, telecommunications tower owners, wind farm developers, construction firms. Doc 8126 §3.3.3.4 notes that States may need to establish cost recovery policies to fund the collection effort, because the data originators — geographic information service providers, tower operators — are not aviation entities.

Formal arrangements. PANS-AIM §2.1.1.4 states that the Aeronautical Data Catalogue subject list should be used to establish formal arrangements between data originators and the AIS. These arrangements define accuracy requirements, update schedules, formats, and the liability regime for out-of-date obstacle notifications.

Update triggers for obstacles:

• New construction permits: crane operations, new buildings, towers.
• Changes to existing structures: height modifications, dismantling.
• Aerodrome infrastructure changes: runway extensions, new taxiways.
• Changes to aerodrome obstacle limitation surfaces.

Survey cycle. Area 4 and Area 2a obstacle data require the most frequent review, ideally linked to the AIRAC 28-day cycle for publishing changes. Area 1 terrain is more stable but must be reprocessed when survey data is updated by the national mapping authority.

Cross-border coordination. For aerodromes near territorial boundaries, Annex 15 Recommended Practices require arrangements between States to share terrain and obstacle data, preventing the anomaly where the same physical obstacle carries different records in adjacent State databases.

Thread 5 — Exchange and AIXM#

This thread covers the standardised encoding and distribution of eTOD through SWIM-based aeronautical information services.

AIXM 5.2 and eTOD. The Aeronautical Information Exchange Model (AIXM) version 5.2 (current version as at 2025) includes feature types for obstacles and terrain data exchange. eTOD is one of the explicit new capability areas added in AIXM 5.2. AIXM 5.1.1 (the widespread operational predecessor) had limited obstacle exchange support; 5.2 extended this to terrain and obstacle data sets with attributes aligned to PANS-AIM Appendix 6.

SWIM distribution. DAIM-B1 envisions eTOD served as a SWIM information service, accessible to procedure designers, avionics suppliers, chart production systems, and MSAW ground systems via a policy-controlled service endpoint. The data model backing this service is AIXM; the transport uses the SWIM Yellow Profile (AMQP 1.0) or Blue Profile (OGC Web Feature Service) depending on consumer requirements.

Onboard database supply chain. Navigation database vendors (providers of the databases loaded into FMS and GPWS/TAWS systems) consume eTOD from national AIS providers and aggregate it into certified avionics databases. The accuracy and integrity attributes in the eTOD data set feed directly into the vendor's data quality declarations for the avionics database.

RTCA/EUROCAE standards. RTCA DO-291B / EUROCAE ED-119B defines interchange standards for terrain, obstacle, and aerodrome mapping data. Doc 8126 §5.3.3.3.4 references this standard for metadata elements applicable to aerodrome mapping data; the same exchange standards apply to eTOD.

Thread 6 — Downstream applications#

This thread covers the primary consumers of eTOD data, confirming the dependency chain from eTOD quality to operational outcome.

TAWS/eGPWS. The most safety-critical downstream use. The terrain database loaded in the GPWS/TAWS unit is derived from Area 1 terrain and Area 2 obstacle data. Annex 6 §6.15.2 requires the operator to implement database management procedures ensuring timely distribution and update of current terrain and obstacle data to the GPWS. Gap or inaccuracy in State eTOD propagates into TAWS effectiveness.

Instrument procedure design. PANS-OPS (Doc 8168) uses Area 2 obstacle data for the critical obstacle (CO) determination in approach procedure design, and Area 4 terrain for precision approach minima. Undocumented obstacles in Area 2a or Area 4 directly invalidate procedure clearance assumptions.

Minimum safe altitude warning (MSAW). Ground-based ATC safety net systems use terrain elevation data to compute minimum vectoring altitudes and alert controllers when radar-tracked aircraft are below terrain clearance thresholds. Area 1 terrain quality determines MSAW coverage and alert quality.

Aeronautical charts. The Aerodrome Terrain and Obstacle Chart - ICAO (Electronic) and the Precision Approach Terrain Chart - ICAO are direct products of eTOD. Chart accuracy is bounded by eTOD data quality.

En-route drift-down analysis. Multi-engine turbine aircraft compute single-engine service ceiling above terrain using Area 1 data; inaccurate Area 1 terrain forces conservative drift-down profiles or additional safety margins.

Flight simulators and synthetic vision. Secondary uses but commercially significant; simulator database fidelity and synthetic vision approach displays both rely on eTOD source data.

References#

• Annex 15, Chapter 5, §5.3.3 — full Standards and Recommended Practices structure for terrain and obstacle data sets, including coordination obligations.
• Doc 10066 (PANS-AIM), Chapter 5, §5.3.3.2 Note — enumeration of intended applications: TAWS, MSAW, procedure design, drift-down, A-SMGCS, charting.
• Doc 10066, Chapter 2, §2.2.1 — cryptographic integrity assurance for aeronautical data.
• Doc 10066, Chapter 2, §2.1.1.4 — formal arrangements between data originators and the AIS based on the Aeronautical Data Catalogue.
• Doc 10066, Appendix 1 — numeric accuracy, resolution, integrity classification by Area (Tables A1-6 and A1-8).
• Doc 8126 (AIS Manual), Chapter 3, §3.3.3.4 — terrain and obstacle data originating from parties outside aviation; cost recovery and formal arrangements.
• Annex 6, Part I, §6.15.2 — operator obligation to maintain current terrain and obstacle data in the GPWS/TAWS database.
• Doc 8168 (PANS-OPS), Volume II — obstacle clearance methodology consuming Area 2 and Area 4 eTOD (authoritative source — not in local library).
• https://www.aixm.aero/ — AIXM 5.2 model; terrain and obstacle exchange feature types.
• https://www.eurocontrol.int/service/terrain-and-obstacle-data — EUROCONTROL regional eTOD implementation.

The worked module: Area 2 obstacle data collection-to-publication#

This module traces the complete lifecycle of Area 2 obstacle data for a single international aerodrome — from initial field survey through quality assurance, AIS processing, exchange, and downstream use. The aerodrome chosen as the reference scenario is a mid-size hub with a single instrument runway equipped for CAT II/III operations.

Step 1 — Defining the product specification#

Before any survey begins, the aerodrome data originator and the national AIS provider must agree on a data product specification for the aerodrome's Area 2 obstacle data set.

Per PANS-AIM §5.3.3.2.2.4, this specification must describe:

• The geographic coordinates of the aerodrome (ARP).
• The areas covered: 2a, 2b, 2c, 2d.
• The take-off flight path area (per Annex 4 §3.8.2).
• The obstacle limitation surfaces (per Annex 14 Vol I Chapter 4).

Additional elements defined in the product specification:

• The spatial reference system: WGS-84 horizontal, MSL vertical (EGM-96 geoid model or equivalent).
• The integrity classification for each sub-area (critical for 2a obstacles within approach and departure surfaces; essential for 2d).
• The update schedule: triggers (new construction permits, annual review) and the AIRAC cycle alignment for distribution.
• The format: AIXM 5.2 obstacle feature encoding for distribution.

Step 2 — Field survey of obstacles#

The survey team collects the three-dimensional position of each obstacle that penetrates the relevant collection surface in each sub-area.

Area 2a survey. The runway strip and clearway are surveyed for all objects penetrating the 3 m surface above runway centreline elevation. GNSS RTK or total station surveys establish horizontal position to the accuracy required in PANS-AIM Appendix 1. Vertical accuracy requirements at Area 2a are the tightest of the four areas.

Each obstacle is assigned:

• A unique obstacle identifier.
• Horizontal position (WGS-84 latitude and longitude).
• Horizontal extent (footprint as point, line, or polygon).
• Elevation (top of obstacle, referenced to MSL).
• Obstacle type (mast, antenna, building, crane, etc.).
• Geometry type (point / line / polygon).
• Lighting status and marking status.
• Integrity classification (critical for Area 2a close-in obstacles).

Area 2b survey. The departure corridor is surveyed for objects penetrating the 1.2 per cent slope surface. Obstacles less than 3 m above ground are excluded. Survey methods may include photogrammetry or LiDAR supplemented by field verification of specific objects.

Area 2c and 2d surveys. Area 2c uses the 1.2 per cent slope surface and a 15 m exclusion threshold. Area 2d uses the 100 m AGL surface; structures not reaching 100 m are not captured unless they penetrate the OLS.

Step 3 — Quality assurance#

Before the data set is submitted to the AIS provider, the originator applies the quality assurance steps required by PANS-AIM Chapter 4:

• Geometric check. Each obstacle position is verified against the aerodrome coordinate system and against existing infrastructure records (aerodrome layout chart, building register).
• Attribute completeness check. All mandatory attributes per Appendix 6 Table A6-2 are populated; no null values in mandatory fields.
• Accuracy metadata. Horizontal and vertical accuracy values are computed from the survey measurement uncertainty; confidence levels confirmed at 90 per cent.
• Integrity classification. Each obstacle is tagged with the appropriate integrity level based on which collection surface it penetrates and which applications will consume it.
• Cyclic redundancy check (CRC). A CRC or cryptographic hash is computed over the data set to enable downstream integrity verification.
• Cross-check against terrain data. Doc 8126 §5.2.6 notes that obstacle data can be checked against digital terrain data in a 3D viewer to identify obstacles that appear embedded within terrain (height error) or floating above it (geoid/datum mismatch). This is recommended as a final geometric sanity check.

Step 4 — AIS processing and publication#

The verified data set is transmitted to the national AIS provider with the metadata record. The AIS provider:

• Validates the product against the agreed product specification and data catalogue requirements.
• Assigns the dataset a unique data source identifier and records the submission date.
• Publishes the obstacle data set with the next AIRAC effective date if triggered by a new or changed obstacle.
• Provides the obstacle data set to recipients of the AIP as a digital download or via a SWIM-connected service endpoint.

Where available, AIP GEN section cross-references the obstacle data set so that the relevant AIP sections (ENR 5 and AD 2 entries for obstacles) may be omitted in favour of the data set, per PANS-AIM §5.3.3.1.2 Note.

Step 5 — AIXM encoding and SWIM distribution#

The obstacle data set is encoded in AIXM 5.2 obstacle feature format for SWIM distribution. Each obstacle record maps to an AIXM VerticalStructure or ObstacleArea feature with the mandatory attributes from PANS-AIM Appendix 6 Table A6-2 mapped to AIXM property names.

SWIM-connected consumers subscribed to the aerodrome's Area 2 obstacle service receive the update through the SWIM Yellow Profile (AMQP 1.0 message delivery) or the SWIM Blue Profile (OGC WFS for query-pull access), according to their service subscription.

Downstream consumers include:

• Procedure design offices (PANS-OPS compliant design tools that import AIXM obstacle data to run obstacle clearance analysis).
• Navigation database vendors (aggregate national AIXM obstacle data into certified FMS/GPWS terrain databases).
• Chart production systems (Aerodrome Terrain and Obstacle Chart; Precision Approach Terrain Chart).
• MSAW ground systems (update their terrain and obstacle reference on receipt of the new data set version).

Step 6 — Operator database management (onboard TAWS/GPWS)#

Annex 6 §6.15.2 places the database management obligation on the operator. Once the updated obstacle data set is published through the national AIS, the navigation database vendor incorporates the changes in the next database cycle (typically 28-day AIRAC or 56-day dual-cycle depending on vendor schedule).

The operator's maintenance organisation applies the updated database to the fleet, following the aircraft maintenance manual procedure for terrain and obstacle database upload. The effective date of the new database aligns with the AIRAC cycle that introduced the change.

The operator's continuing airworthiness management system records the current database version loaded in each aircraft, enabling the operator to demonstrate compliance with Annex 6 §6.15.2 database currency requirements.

End-to-end latency and the AIRAC boundary#

A key quality metric is the elapsed time between a new obstacle being erected and the corresponding TAWS terrain database being loaded in aircraft operating at the aerodrome. The AIRAC cycle creates a maximum 28-day window; actual latency depends on:

• Survey detection lag (how quickly new construction is reported to the aerodrome data originator).
• AIS processing time (weeks to months without automation).
• Navigation database vendor processing (typically one AIRAC cycle after receipt).
• Operator database cycle (28 or 56 days).

In practice, the total lag can exceed 90 days for new tall structures. EUROCONTROL regional eTOD monitoring data confirms that obstacle databases are a persistent gap in State AIM compliance.

Summary: Area 2 obstacle data module#

References#

• Annex 15, Chapter 5, §5.3.3.4.5 — Area 2a mandatory obstacle provisions: collection surface, take-off path, obstacle limitation surfaces.
• Annex 15, Chapter 5, §5.3.3.4.6 — Recommended Practice for Areas 2b, 2c, 2d obstacles; height thresholds and collection surfaces.
• Doc 10066 (PANS-AIM), Chapter 5, §5.3.3.2.2.4 — obstacle data product specification area coverage requirements.
• Doc 10066, Appendix 6, Table A6-2 — mandatory and optional obstacle attributes.
• Doc 10066, Chapter 4, §4.1.1 — data collection and transmission accuracy and integrity requirements.
• Doc 8126 (AIS Manual), Chapter 5, §5.2.6 — cross-checking obstacle data against terrain data in 3D viewer for geometric sanity.
• Annex 6, Part I, §6.15.2 — operator obligation: terrain and obstacle database management procedures.
• Doc 8168 (PANS-OPS), Volume II — use of Area 2 obstacle data in obstacle clearance analysis (authoritative source — not in local library).

Overview#

eTOD provision requires a convergence of surveying technology, geographic information standards, ICAO regulatory machinery, AIXM data models, institutional coordination, and trained personnel. The absence of any single enabler is sufficient to block State compliance across one or more Areas.

CNS and surveying technology#

GNSS surveying. High-accuracy GNSS receivers (RTK differential or PPP) are the primary tool for Area 2a and Area 4 surveys. Horizontal accuracy better than 0.5 m at 90 per cent confidence and vertical accuracy better than 0.5 m are achievable with modern RTK systems on WGS-84 network corrections. These are prerequisites for the close-tolerance Areas.

LiDAR (Light Detection and Ranging). Airborne LiDAR is the standard method for acquiring Area 2 and Area 4 terrain grids at the required post-spacing. Mobile terrestrial LiDAR is used for Area 3 (movement area). LiDAR delivers simultaneous terrain surface and obstacle point clouds; the separation of terrain from obstacle elements is a post-processing step.

Photogrammetry. Aerial photography processed through photogrammetric workflows yields terrain DEMs and building/structure outlines. Used where LiDAR is cost-prohibitive or for updating existing surveys with targeted re-flights.

Satellite-derived DEMs. For Area 1, national and global DEMs derived from radar interferometry (SRTM, TanDEM-X) or optical stereopairs provide the foundational terrain surface. Post-processing for ICAO compliance includes void filling, datum transformation to WGS-84/EGM-96, and accuracy assessment against ground control points.

Total station and GNSS static surveys. For individual tall obstacle position and elevation in Area 1, conventional total station surveys with GNSS control provide the required accuracy for structures where existing spatial data is inadequate.

Geographic information and ISO standards#

ISO 19100 series. eTOD data sets are geographic information products and must comply with the ISO 19100 geographic information standards family:

• ISO 19115: Metadata for geographic data — the mandatory metadata framework cited in PANS-AIM for terrain, obstacle, and aerodrome mapping data sets.
• ISO 19107: Spatial schema — defines the geometry types (point, curve, surface) used for obstacle feature representation.
• ISO 19131: Data product specifications — the framework for documenting Area-specific eTOD product specifications.

WGS-84 and EGM-96. Horizontal coordinates in eTOD must reference WGS-84. Vertical values must reference MSL using a geoid model such as EGM-96. Coordinates transformed from older national datums must be identified as mathematically transformed, and their accuracy must be assessed against the original survey source.

GIS infrastructure. State-level eTOD management requires a GIS capable of maintaining multiple version-controlled data layers, applying AIRAC cycle dates as temporal attributes, and generating AIXM export. Many States manage this through national AIS automation systems.

AIXM and SWIM exchange enablers#

AIXM 5.2. The current version of the Aeronautical Information Exchange Model (published January 2025) is the normative encoding format for eTOD in SWIM. AIXM 5.2 extended terrain and obstacle exchange capabilities beyond AIXM 5.1.1. States must upgrade their AIS automation to support AIXM 5.2 obstacle and terrain feature types before their eTOD can be distributed through SWIM.

SWIM Yellow Profile (AMQP 1.0). The ICAO SWIM Yellow Profile (AMQP 1.0) is the publish-subscribe transport layer for continuous aeronautical data distribution including eTOD updates. Requires a message broker infrastructure at the national AIS provider.

SWIM Blue Profile (OGC WFS). The ICAO SWIM Blue Profile (OGC Web Feature Service) supports query-pull access to aeronautical data sets. Procedure design offices and navigation database vendors can query specific Area obstacle data by aerodrome identifier.

Navigation database supply chain. Avionics database vendors (supplying FMS, GPWS, TAWS terrain databases) must have a data exchange arrangement with the national AIS provider, or access the data via the SWIM service. The RTCA DO-291B / EUROCAE ED-119B interchange standard defines the format for this supply chain.

Regulatory enablers#

Annex 15, Sixteenth Edition (2018). The treaty-level foundation. States that have not filed differences are bound by the Standards. Many States file differences against Recommended Practice provisions (Areas 2b/2c/2d, Area 3), noting these as programmes in progress.

PANS-AIM, Doc 10066 (2018). Provides the detailed numerical specifications. States must ensure their national AIS regulations and quality management systems reference Doc 10066 Appendix 1 accuracy and integrity requirements.

Annex 6, Part I, §6.15. The operator obligation to maintain current terrain and obstacle data in GPWS/TAWS creates a commercial pull from operators on States to provide timely, accurate eTOD. States with non-compliant eTOD face operator complaints and potential safety findings from ICAO Universal Safety Oversight Audit Programme (USOAP) audits.

USOAP. ICAO's USOAP Continuous Monitoring Approach (CMA) audits State AIS provision, including eTOD. Protocol questions map directly to Annex 15 §5.3.3 obligations. States with low Effective Implementation (EI) scores for AIS often have eTOD gaps as a contributing factor.

Regional supplementary procedures. ICAO regional ANPs (e.g., APAC FASID, EUR FASID) may carry regional eTOD implementation commitments or target dates for States. These are the mechanism through which global standards become regional action items in the Seamless ATM Plan and equivalent regional frameworks.

Training and human factors#

Survey personnel. Obstacle and terrain surveyors require training in GNSS surveying methodology, LiDAR post-processing, WGS-84 datum transformation, and the specific accuracy and integrity requirements of PANS-AIM Appendix 1. ICAO Doc 9137 (Airport Services Manual) and national survey authority training resources are applicable.

AIS data quality staff. AIS personnel processing eTOD submissions must understand the data catalogue attribute requirements of PANS-AIM Appendix 6, the integrity classification criteria, and the CRC/hash verification methods. ICAO training programmes for AIM include eTOD data quality as a curriculum element.

Procedure designers. PANS-OPS compliant procedure designers must be trained to import and use AIXM-encoded obstacle data in obstacle clearance analysis tools. Familiarity with the Area 2 geometry (sub-areas, collection surfaces) is essential for interpreting whether obstacle data coverage is sufficient for a given procedure type.

Aerodrome operators. Airport engineers and safety managers must maintain awareness of the requirement to notify the AIS provider of new or changed obstacles — cranes, construction, antenna changes — within the defined collection areas. The formal arrangement between the aerodrome operator and the AIS provider (required by PANS-AIM §2.1.1.4) should specify the notification protocol.

Institutional arrangements#

AIS-to-originator agreements. PANS-AIM §2.1.1.4 requires formal arrangements between data originators and the AIS, covering the data subjects, properties, accuracy requirements, and update schedules in the Aeronautical Data Catalogue.

Multi-agency coordination. Effective eTOD requires simultaneous engagement with: national mapping agencies (Area 1 terrain); telecommunications authorities and tower operators (Area 1 obstacles); energy regulators and wind farm developers (wind turbines); civil aviation authority aerodrome standards officers (Area 2 and 4); airport operators (Area 2a, 3 surveys); and military authorities (restricted areas with tall structures).

Cost recovery. States must establish cost recovery frameworks to fund eTOD collection from commercial obstacle owners. Doc 8126 §3.3.3.4 acknowledges that requiring data from outside the aviation system may need cost recovery policies.

Regional cooperation. For adjacent aerodromes with overlapping Area 2 coverage, and for aerodromes near territorial borders, Annex 15 Recommended Practices require coordination between affected States and aerodrome operators to ensure consistent data.

References#

• Doc 10066 (PANS-AIM), Chapter 2, §2.1.1.4 — formal arrangements between data originators and the AIS; Aeronautical Data Catalogue as the reference.
• Doc 10066, Chapter 2, §2.2.1 — cryptographic integrity assurance; hash functions and message authentication codes.
• Doc 10066, Appendix 1 — numeric accuracy, resolution, integrity requirements; 90 per cent confidence level for terrain and obstacle data.
• Doc 8126 (AIS Manual), Chapter 3, §3.3.3.4 — terrain and obstacle data from outside aviation; cost recovery policy.
• Annex 15, Chapter 5, §5.3.3.4.7 and §5.3.3.4.8 — coordination with adjacent aerodromes and border States for obstacle data.
• Annex 6, Part I, §6.15.2 — operator terrain database management obligation; commercial pull on AIS quality.
• https://www.aixm.aero/ — AIXM 5.2 model; obstacle and terrain feature type documentation.
• https://www.eurocontrol.int/service/terrain-and-obstacle-data — EUROCONTROL eTOD service and regional implementation support.

The performance lens of eTOD#

eTOD is a data infrastructure programme. Its performance is measured not by traffic throughput or delay reduction, but by data quality metrics that translate directly into safety margins, operational access, and interoperability. The KPA framework from ICAO Doc 9854 and Doc 9883 applies, but the primary drivers are Safety (obstacle clearance assurance) and Access (approach minima and operational availability).

Key Performance Areas for eTOD#

The KPAs most directly affected by eTOD quality are:

Safety. eTOD accuracy determines whether TAWS terrain databases, procedure protected surfaces, and MSAW terrain references correctly represent the terrain and obstacle environment. Errors or gaps in eTOD translate directly into undetected hazards: an obstacle absent from a procedure designer's survey creates an unsafe protected surface; an inaccurate terrain cell creates a false TAWS alert or a missed alert.

Access and equity. The quality and completeness of Area 2 and Area 4 eTOD constrains the minimum altitudes that can be published for instrument approaches. States with high-quality eTOD can publish tighter minima (lower decision altitudes, lower visibility minima) for CAT II/III approaches, increasing operational availability in low-visibility conditions. States with poor-quality or absent Area 4 eTOD must either use generic obstacle clearance assumptions that push minima upward, or forgo CAT II/III operations.

Interoperability. eTOD exchanged in AIXM 5.2 format through SWIM services enables interoperable consumption by procedure design tools, avionics database vendors, chart production systems, and MSAW ground systems across State boundaries. Without standardised exchange, each application must maintain a bespoke data relationship with the national AIS provider.

Cost-effectiveness. Accurate, timely eTOD reduces the cost of instrument procedure design (fewer bespoke surveys), reduces operator costs (current TAWS databases without ad-hoc supplements), and reduces chart production costs (derived from a single authoritative data set).

KPA contribution by eTOD Area#

The matrix scores each KPA by its principal benefit from each Area (1 = some benefit, 2 = clear benefit, 3 = primary driver):

Performance Objectives#

PO-1: Ensure TAWS/eGPWS terrain database currency and accuracy#

KPAs: Safety.

Measured by: Percentage of routes/aerodromes within the State covered by current, ICAO-compliant Area 1 and Area 2 eTOD incorporated in certified avionics terrain databases. Target: 100 per cent of international aerodromes with Area 2a and Area 4 data current.

Delivered by: State eTOD provision programme (Area 1 terrain + Area 2 obstacles), AIRAC-cycle obstacle update process, navigation database vendor update cycle, operator database management per Annex 6 §6.15.2.

PO-2: Enable lowest achievable CAT II/III approach minima#

KPAs: Access, Safety.

Measured by: Availability of Area 4 eTOD for each CAT II/III certified runway; compliance of Area 4 data with PANS-AIM Appendix 1 accuracy and integrity requirements. Target: Area 4 data meeting critical integrity class for all CAT II/III runways.

Delivered by: State eTOD Area 4 surveys, PANS-OPS compliant procedure design using verified obstacle data, aerodrome operator surveying obligation under national civil aviation regulations.

PO-3: Support interoperable SWIM distribution of eTOD#

KPAs: Interoperability, Cost-effectiveness.

Measured by: Proportion of State AIS providers distributing eTOD through SWIM-compliant services (AIXM 5.2 encoding, SWIM Yellow or Blue Profile transport). Target (DAIM-B1 aligned): eTOD as a SWIM information service for all international aerodromes.

Delivered by: AIXM 5.2 adoption, SWIM infrastructure at national AIS provider, SWIM Blue/Yellow Profile compliance, subscription by procedure design offices and navigation database vendors.

PO-4: Maintain obstacle data currency through aerodrome change management#

KPAs: Safety, Access.

Measured by: Mean time from new obstacle erection to publication in the AIRAC-aligned eTOD data set; percentage of published obstacle data sets within AIRAC cycle of the survey that triggered the update.

Delivered by: Formal originator-AIS arrangements per PANS-AIM §2.1.1.4; construction permit notification protocols; aerodrome operator obligations; AIS processing automation reducing latency.

Key Performance Indicators#

Safety KPIs#

• Area 4 terrain and obstacle data integrity classification conformance: percentage of critical-class data elements with verified CRC integrity.
• Time from new construction event to AIS publication (obstacle lag metric): should be within one AIRAC cycle for Area 2a/4 new obstacles.
• TAWS terrain database coverage: percentage of international aerodrome approaches covered by current ICAO-compliant terrain database.
• MSAW terrain coverage: percentage of controlled airspace covered by Area 1 terrain data meeting Annex 15 accuracy requirements.

Access KPIs#

• CAT II/III runway coverage: percentage of CAT II/III runways with compliant Area 4 eTOD published in the national AIS.
• Approach minimum achievement: comparison of published CAT II/III decision altitudes against theoretical achievable minima derived from certified Area 4 data; gap = access restriction attributable to eTOD quality.
• Number of aerodromes with Area 2a compliant obstacle data enabling RNAV/RNP approach procedure design.

Interoperability KPIs#

• SWIM eTOD service availability: percentage of time eTOD SWIM service is available with latency meeting SWIM service level.
• AIXM 5.2 conformance: percentage of published obstacle and terrain data sets encoded in AIXM 5.2.
• Navigation database vendor supply latency: elapsed days from AIRAC eTOD publication to certified avionics database release.

Cost-effectiveness KPIs#

• Proprietary survey cost avoidance: number of instrument procedures designed using national eTOD without bespoke obstacle surveys.
• AIS originator notification compliance rate: percentage of notified new obstacles received within the agreed notification protocol timeframe under the formal originator-AIS arrangements.

How eTOD performance is reported#

USOAP. ICAO's USOAP CMA protocol questions for AIS/AIM cover eTOD provision. Effective Implementation (EI) scores for AIS reflect State compliance with Annex 15 §5.3.3 obligations.

State safety programme. Under the ICAO State Safety Programme (SSP) framework, eTOD data quality is a terrain awareness safety performance indicator. States tracking CFIT rates and TAWS activation rates should correlate these with eTOD currency.

EUROCONTROL regional monitoring. EUROCONTROL maintains regional terrain and obstacle data monitoring for the EUR Region, tracking compliance with ECAC/EUROCONTROL eTOD requirements. This provides the most systematic data point on State eTOD implementation.

APAC regional reporting. Within the APAC Seamless ATM Plan, DAIM thread implementation status tracks eTOD provision as a component of DAIM-B0 and DAIM-B1 readiness. APANPIRG performance reports include AIS/AIM metrics.

References#

• Doc 9854 (Global ATM Operational Concept), Chapter 2 — KPA framework; Safety, Access, Interoperability, Cost-effectiveness as primary ATM performance dimensions.
• Doc 9883 (Manual on Global Performance of the Air Navigation System) — KPA definitions and global measurement framework (authoritative source — not in local library).
• Doc 10066 (PANS-AIM), Chapter 4, §4.1.1 — data quality requirements linked to accuracy and integrity in Appendix 1 as the basis for eTOD performance standards.
• Annex 15, Chapter 5, §5.3.3 — Standards and Recommended Practices whose fulfilment constitutes eTOD performance compliance.
• Annex 6, Part I, §6.15.2 — operator database management obligation; TAWS currency as a directly measurable safety KPI.
• Doc 9750 (GANP), ASBU DAIM thread — DAIM-B0/B1 eTOD provision as a block-level performance deliverable (authoritative source — not in local library; see https://ganpportal.icao.int/).

Historical evolution and key dates#

The eTOD regulatory framework developed over two decades from an initial SARPS innovation in 2004 through successive Annex 15 amendments to the consolidated PANS-AIM specification of 2018.

Key regulatory milestones in detail#

2004 — Amendment 33: eTOD enters the SARPS#

Amendment 33 to Annex 15 introduced the foundational Standards and Recommended Practices for electronic terrain and obstacle data. The amendment was driven by three converging recommendations:

• GNSSP/4 Recommendation 3/1: navigation systems panel recognising the need for consistent terrain data to support GNSS-based approach and departure procedures.
• OCP/12 and OCP/13: obstacle clearance panels documenting the data requirements for PANS-OPS procedure design.

The four coverage Areas (1, 2a/b/c/d, 3, 4) were defined in this amendment, along with the mandatory Standards for Areas 1, 2a, and 4, and the Recommended Practice status for Areas 2b/c/d and 3. This created the basic regulatory obligation that States have been implementing since 2004.

2010-2013 — Amendments 36-37: Integrity and data quality#

Amendments 36 and 37 strengthened the data quality framework, introducing the integrity classification concepts (routine, essential, critical) and tightening the alignment between eTOD provisions and the broader aeronautical data quality framework. Amendment 37 introduced the aerodrome mapping data set provisions that complement eTOD Area 3 obligations.

2018 — Amendment 40 and PANS-AIM: The modern framework#

The Amendment 40 restructure of Annex 15 transitioned the document from an Aeronautical Information Services orientation to an Aeronautical Information Management framework. The eTOD provisions were retained in Chapter 5 but the detailed numerical requirements were delegated to the new PANS-AIM (Doc 10066), Appendices 1 and 8.

This delegation creates the current regulatory architecture: Annex 15 = what must be provided and the Area framework; PANS-AIM = how to provide it numerically (accuracy, resolution, integrity, collection surface geometry).

PANS-AIM Appendix 8 published the first definitive graphical illustrations of the terrain and obstacle collection surfaces for each Area (Figures A8-1 through A8-4), resolving longstanding ambiguities in how collection surface boundaries were interpreted.

Area provision status context#

As of the mid-2020s, the global implementation picture is:

• Area 1 terrain: Most States have Area 1 terrain data, but quality varies significantly. Satellite-derived DEMs meet or approach the required accuracy for many areas; mountainous regions and areas with dense vegetation require LiDAR or photogrammetric supplements.

• Area 1 obstacles: Systematic coverage of obstacles >=100 m AGL remains incomplete in many States, particularly where rapid growth in wind energy and telecommunications tower construction outpaces survey update cycles.

• Area 2a and 4: Progress at international aerodromes has been driven by USOAP findings and operator pressure under Annex 6 §6.15.2. Most major hub aerodromes in developed States have compliant Area 2a data; Area 4 compliance is highest where CAT II/III operations require it.

• Area 2b/2c/2d: Adoption as a Recommended Practice means many States have not systematically implemented these Areas. Procedure designers in those States often conduct bespoke surveys.

• Area 3: The least uniformly implemented Area; adoption is progressing as aerodrome mapping data programmes mature.

References#

• Annex 15 (Aeronautical Information Services), Sixteenth Edition (2018), Foreword, amendment table — Amendments 33 to 41; subject matter and applicable dates for eTOD-relevant amendments.
• Annex 15, Foreword, Amendment 33 entry — introduction of eTOD SARPS following GNSSP/4 Recommendation 3/1 and OCP/12-13; applicable 25 November 2004.
• Annex 15, Foreword, Amendment 40 entry — restructure for AIS-to-AIM transition; delegation of eTOD numerical requirements to PANS-AIM.
• Doc 10066 (PANS-AIM), First Edition (2018), Foreword — approval by Council President 28 August 2018; applicable 8 November 2018.
• Doc 10066, Appendix 8 — Figures A8-1 to A8-4, terrain and obstacle collection surface geometry, introduced in the First Edition.
• https://www.aixm.aero/ — AIXM 5.2 published January 2025; extended terrain and obstacle feature types for SWIM distribution.

Primary ICAO regulatory documents#

• Annex 15 (Aeronautical Information Services), Sixteenth Edition, July 2018 — Master SARPS document for eTOD; Chapter 5, §5.3.3 covers terrain and obstacle data set Standards and Recommended Practices; applicable 8 November 2018.
• Annex 15, Chapter 5, §5.3.3.1 — four coverage Area definitions: Area 1 (State territory), Area 2 (aerodrome vicinity, sub-areas 2a/2b/2c/2d), Area 3 (movement area zone), Area 4 (CAT II/III precision approach corridor).
• Annex 15, Chapter 5, §5.3.3.3 — terrain data set Standards and Recommended Practices by Area; mandatory for Area 1, 2a (take-off path, OLS), and Area 4 CAT II/III runways.
• Annex 15, Chapter 5, §5.3.3.4 — obstacle data set Standards and Recommended Practices; mandatory for Area 1 (>=100 m AGL), Area 2a and take-off path, Area 4; Recommended Practice for Areas 2b, 2c, 2d, and Area 3.
• Annex 15, Foreword, Amendment 33 — introduction of eTOD SARPS, applicable 25 November 2004, following GNSSP/4 Recommendation 3/1 and OCP/12-13.
• Annex 15, Foreword, Amendment 40 — Sixteenth Edition restructure for AIS-to-AIM transition; delegation of numerical requirements to PANS-AIM.
• Doc 10066 (PANS-AIM — Procedures for Air Navigation Services: Aeronautical Information Management), First Edition, 2018 — operational specifications for eTOD; Appendices 1, 6, and 8.
• Doc 10066, Chapter 5, §5.3.3.2.1 — terrain data set specifications; grid types, surface representation, single feature type requirement.
• Doc 10066, Chapter 5, §5.3.3.2.2 — obstacle data set specifications; geometry types, product specification area coverage requirements, PANS-AIM §5.3.3.2.2.4.
• Doc 10066, Appendix 1, Tables A1-6 and A1-8 — Aeronautical Data Catalogue: numeric accuracy, resolution, and integrity requirements for obstacle and terrain data by Area.
• Doc 10066, Appendix 6, Table A6-1 — mandatory and optional terrain attributes; 21 attributes including post spacing, horizontal and vertical accuracy, integrity, recorded surface.
• Doc 10066, Appendix 6, Table A6-2 — mandatory and optional obstacle attributes; 25 attributes including obstacle identifier, horizontal extent, obstacle type, geometry type, lighting, marking.
• Doc 10066, Appendix 8, Figures A8-1 to A8-4 — graphical definitions of terrain and obstacle collection surfaces for Areas 1, 2, 3, and 4.
• Doc 10066, Chapter 4, §4.1.1 — data collection and transmission conformance to accuracy and integrity requirements in Appendix 1.
• Doc 10066, Definitions — integrity classifications (routine: 1 x 10^-3; essential: 1 x 10^-5; critical: 1 x 10^-8 undetected error probability per flight hour).
• Doc 10066, Chapter 5, §5.3.3.2 Note — intended applications: TAWS/eGPWS, MSAW, contingency procedures, aircraft limitations analysis, procedure design, drift-down, A-SMGCS, charting.

Supporting ICAO Annexes and PANS#

• Annex 6, Part I (Operation of Aircraft), Chapter 6, §6.15.1 — GPWS/TAWS mandate for turbine aircraft >5 700 kg or >9 passengers; forward-looking terrain avoidance function required.
• Annex 6, Part I, §6.15.2 — operator obligation to implement terrain and obstacle database management procedures ensuring timely distribution and update to GPWS/TAWS.
• Annex 6, Part I, §6.15.4 — extension of GPWS/TAWS mandate to turbine aircraft <=5 700 kg carrying >5 to <=9 passengers for new certificates of airworthiness from 1 January 2026.
• Annex 14 (Aerodromes), Volume I, Chapter 3 — runway strip dimensions referenced in Area 2a definition (Annex 15 §5.3.3.1).
• Annex 14, Volume I, Chapter 4 — aerodrome obstacle limitation surfaces (OLS) referenced in Area 2a obstacle data requirements.
• Annex 4 (Aeronautical Charts), §3.8.2 — take-off flight path area specification referenced in Area 2a obstacle data requirements.
• Doc 8168 (PANS-OPS), Volume II — obstacle clearance criteria for instrument procedure design; consumes Area 2 and Area 4 eTOD to determine protected surfaces and minima (authoritative source — not in local library).

ICAO guidance documents#

• Doc 8126 (Aeronautical Information Services Manual), Chapter 3, §3.3.3.4 — terrain and obstacle data originating from outside aviation; arrangements for collection; cost recovery policy.
• Doc 8126, Chapter 3, §3.3.2 — digital data sets including terrain and obstacle data as AIM products; relationship to AIS-to-AIM transition.
• Doc 8126, Chapter 2, §2.3.3.3 — ISO 19115 Geographic information Metadata applicable to terrain, obstacle, and aerodrome mapping data sets.
• Doc 8126, Chapter 10 — cross-reference table showing Annex 15 provisions mapped to PANS-AIM; eTOD provisions at rows 10.1.1 through 10.2.x.

Authoritative external references#

• https://www.icao.int/safety/pbn/pages/electronic-terrain-and-obstacle-data.aspx — ICAO safety pages on eTOD (authoritative source — not in local library)
• https://store.icao.int/en/annex-15-aeronautical-information-services — ICAO store: Annex 15, Sixteenth Edition (authoritative source — not in local library)
• https://store.icao.int/en/procedures-for-air-navigation-services-aeronautical-information-management — ICAO store: Doc 10066 PANS-AIM First Edition (authoritative source — not in local library)
• https://www.eurocontrol.int/service/terrain-and-obstacle-data — EUROCONTROL terrain and obstacle data services; EUR region eTOD monitoring and implementation data (authoritative source — not in local library)
• https://www.aixm.aero/ — AIXM 5.2 model documentation; terrain and obstacle exchange feature types for SWIM distribution (authoritative source — not in local library)