ATM/ANS.OR.A.090 Common reference systems for air navigation

HORIZONTAL REFERENCE SYSTEM — WGS-84

(a)A reference system provides a definition of a coordinate system in terms of the position of an origin in space, the orientation of an orthogonal set of Cartesian axes, and a scale. A terrestrial reference system defines a spatial reference system in which positions of points anchored on the Earth’s solid surface have coordinates. Examples are WGS-84, ITRS/European Terrestrial Reference System (ETRS) and national reference systems.

(b)WGS-84 defines, inter alia, a conventional terrestrial reference system, a reference frame and a reference ellipsoid. WGS-84 is currently the reference system ICAO requires for geo-referencing aeronautical information.

(c)Further explanation and guidance may be found in Annex B (Horizontal reference systems) to EUROCONTROL Specification for the Origination of Aeronautical Data, Volume 2: Guidance material (EUROCONTROL-SPEC-154, Edition 1.0 of 04/02/2013).

TEMPORARY NON-COMPLIANCE OF GEOGRAPHICAL COORDINATES In those particular cases where geographical coordinates have been transformed into WGS-84 coordinates by mathematical means and whose accuracy of original field work does not meet the applicable requirements contained in the aeronautical data catalogue, they should be identified until the time when they can be compliant.

VERTICAL REFERENCE SYSTEM

(a)A service provider should use the Earth Gravitational Model — 1996 (EGM-96), as the global gravity model.

(b)When a geoid model other than the EGM-96 model is used, a description of the model used, including the parameters required for height transformation between the model and EGM-96, should be provided in the aeronautical information publication (AIP).

MEAN SEA LEVEL

(a)The geoid globally most closely approximates mean sea level (MSL). It is defined as the equipotential surface in the gravity field of the Earth which coincides with the undisturbed MSL extended continuously through the continents.

(b)Gravity-related heights (elevations) are also referred to as ‘orthometric heights’, while distances of points above the ellipsoid are referred to as ‘ellipsoidal heights’.

(c)Global and local geoids differ in their origin: global geoids consider only the long- and middle-wave part of the Earth’s gravity field, whilst local geoids also consider the short-wave part of the gravity field. Global geoids are used when consistent orthometric heights, over long distances (continent or earth surveying), are required. Currently, the world’s best global geoid model is EGM 200846. It was determined using satellite tracking, gravity anomalies and satellite altimetry. Its accuracy is in the range of ± 0.05 m (oceans) and ± 0.5 m (on land). This accuracy is higher in flat regions than in topographically mountainous terrain, such as the Alps.

(d)For local engineering applications and cadastre-surveying, global geoids are not as accurate as needed. For such applications, local geoid models are calculated. These can only be developed using local field measurements. They offer centimetre accuracy over several hundred kilometres, with a high resolution. Local geoids are not suitable for height comparison over large distances since they are based on different origins and reference heights (different equipotential levels).

VERTICAL REFERENCE SYSTEM Further explanation and guidance may be found in Annex C (Vertical reference systems) to EUROCONTROL Specification for the Origination of Aeronautical Data, Volume 2 (EUROCONTROL-SPEC-154, Edition 1.0 of 04/02/2013).

TEMPORAL REFERENCE SYSTEM

(a)A value in the time domain is a temporal position measured relative to a temporal reference system.

(b)ISO Standard 8601 specifies the use of the Gregorian calendar and 24-hour local or UTC for information interchange, while ISO Standard 19108 prescribes the Gregorian calendar and UTC as the primary temporal reference system for use with geographic information.