AGeoP-21 geodetic datums, projections, grids and grid references

What it is

What AGeoP-21 governs

AGeoP-21 is the NATO Allied Geographic Publication that fixes the World Geodetic System 1984 (WGS 84) "as the standard geodetic system for geospatial information used by NATO", and sets out how datums, coordinates, projections, grids and position reporting are applied across NATO's geospatial information. Nations record their agreement to use it in STANAG 2211; AGeoP-21 itself supplies the technical detail. Participating nations agree to a second aim too: "Reduce the number of regional datums and coordinate systems, whenever possible, during revision or recompilation of maps, charts, geodetic data and products for NATO." The publication does not address a supplier or contractor directly at any point: its own terms run to "participating nations", and in practice to the national geographic, geodetic and hydrographic agencies that produce and exchange geospatial information under NATO tasking. Edition A Version 1 was promulgated in February 2016; the document does not state what edition it superseded, and Germany, France, Latvia and Turkey have each filed specific reservations against parts of it.

Preferred datum, coordinates and projections

Geospatial information is referenced to the WGS 84 horizontal and vertical datums "where possible, unless an exception is specified below or a local datum has been designated in accordance with BI-SC Directive 80-4". With the exception of raster, it uses geographic coordinates (latitude, longitude) on the WGS 84 ellipsoid, in decimal degrees; raster data is instead stored in the projected coordinate system intended for its display and use, provided it is based on WGS 84 or an equivalent datum: "This will prevent distortion, improve performance and reduce storage space." Where the preferred coordinate system cannot be adopted, local geodetic datums, ellipsoids, projections and grids "within the NATO area of responsibility (NAOR) will conform to BI-SC Directive 80-4" rather than to AGeoP-21 itself.

No single map projection suits every use, so AGeoP-21 sets a preferred one per domain: UTM for NATO land use; Mercator for dynamic displays needing conformal coverage at any scale short of the poles, and for maritime charts smaller than 1:50,000; Lambert Conformal Conic and Polar Stereographic for higher latitudes and polar coverage; and, for aeronautical charts at 1:500,000 and smaller, Mercator, Lambert Conformal Conic and Polar Stereographic together, arranged in equal bands of latitude. Aeronautical charts in digital form use the Arc-second Raster Chart/map (ARC) system defined by the Defence Geospatial Information Working Group.

Grids and position reporting

The preferred grid is UTM, developed for worldwide use between 80S and 84N; other grids still in use within the NAOR include the British National Grid, the Irish Grid, and the Nord/Sud grids covering Morocco, Algeria and Tunisia, each tied to its own historical ellipsoid. The graticule of latitude and longitude "shall be portrayed on all maps and charts which are used by the NATO Armed Forces", and GEOREF "shall be shown on all aeronautical charts which are used by the NATO Armed Forces". MGRS, built on WGS 84 and UTM, "is the preferred method for position reporting by NATO ground units and ground combat operational forces", and is depicted on aeronautical charts at 1:250,000; where MGRS cannot be used, the local grid reference system applies until WGS 84 and MGRS become available in the area.

The Global Area Reference System (GARS) is carved out deliberately. It is described as "primarily an operational-level, administrative measure" for battle-space deconfliction, and it "shall not be used to define geographic locations, nor shall it be used in a system that requires precise position data" such as weapon systems. Members are also "not be bound to conform to this agreement in respect of maps and charts produced for mission specific, non-standard datasets".

Vertical datums and heights

WGS 84 offers two vertical reference surfaces: the ellipsoid, and an Earth Gravitational Model (EGM) derived geoid. Outside hydrographic use, the geoid "is the preferred vertical reference to replace local and regional mean sea level datums for geospatial information", built from the most recent EGM current at publication. Hydrographic charts are the documented exception: they relate depths to a Chart Datum defined for safe navigation, and AGeoP-21 says plainly that "it is not possible to adopt a WGS 84 EGM derived vertical datum for normal hydrographic applications because the CD/WGS 84 EGM relationship cannot be modeled with sufficient accuracy". Hydrographic agencies are instead directed to model and record the relationship between chart datum and WGS 84 for specialised applications.

Datum transformations

Where information referenced to another datum stays in operational use alongside WGS 84 data in the same area, transformation parameters must be provided, in the note format set by STANAG 3676 for land maps, aeronautical charts and photomaps, and by IHO Special Publication No. 60 for nautical charts. The seven-parameter Helmert (position vector) transformation "shall be applied as the preferred method wherever possible"; a reduced three-parameter Molodenski solution may be used where too few control stations exist for a full seven-parameter fit, but AGeoP-21 warns the parameters from the two methods "must NEVER be used" interchangeably, "as large errors may result". A third method, Multiple Regression Equation transformation, models local distortion with polynomial functions and is "only defined for the specific regions" it was derived for, and "should therefore never be applied outside of these regions". Because transformation itself adds inaccuracy on top of whatever error the original coordinates carried, AGeoP-21 recommends avoiding it "whenever possible, but especially when high accuracy is required such as for targeting", and recommends applying these transformation methods "only to horizontal coordinates", with vertical transformation handled separately.

What it does not cover

AGeoP-21 names no certification body, notified body or audit scheme, and describes no organisation being assessed against it. Where it does set a bar for a technical decision, it routes the choice to a named authority rather than an assessor: selecting a transformation method "must be the responsibility of the NATO Chief Geo Officer in charge", and for higher-accuracy vertical work the document says: "Consult your NATO Geo Officer in charge with questions." Compliance is otherwise a property of the national production process, and of whatever reservation a nation has filed against STANAG 2211, not of AGeoP-21 itself.

AGeoP-21 is published by NATO and listed in the NATO Standardization Document Database. NATO's documents are not sold by ComplyTrain and we host no copies of them.

Standards it references

• STANAG 3676Background