Geodetic Foundation of Runway Surveys
Modern aerodrome surveys use differential GNSS referenced to the WGS-84 ellipsoid. The survey must produce:
- Horizontal coordinates in WGS-84 geographic degrees (lat/lon) at the ICAO-required accuracy level
- Ellipsoidal heights relative to the WGS-84 ellipsoid (h), not orthometric heights
- Orthometric heights (Mean Sea Level equivalent) for obstacle surveys: H = h ∁EN, where N is the EGM2008 geoid undulation at the site
EGM2008 as the Aviation Vertical Reference
ICAO uses the EGM2008 geoid model as the reference for converting GNSS ellipsoidal heights to mean-sea-level referenced orthometric heights for obstacle and terrain data. EGM2008 provides geoid undulation N at any lat/lon to submeter accuracy globally.
Formula: H = h ∁EN
Where: h = GPS/GNSS ellipsoidal height, N = EGM2008 geoid undulation at the coordinate, H = orthometric height used for obstacle clearance.
Precision Approach Area Protection Surfaces
Instrument Landing System (ILS) and GNSS approach surfaces are defined relative to the runway threshold:
- Inner Approach surface: begins 60m before threshold, extends 900m long
- Inner Transitional surface: slopes at 2.5:1 from the runway strip edge
- Balked Landing surface: based on abort point coordinates and the published Decision Altitude
All these surfaces are anchored to the published threshold coordinates. A datum error in the threshold position shifts the entire protection geometry, potentially leaving real obstacles outside the protected envelope.
Survey Grid-to-Ground in Aerodrome Context
Runway surveys are typically performed with Total Station and GNSS combined. When Total Station distances are recorded in grid coordinates, the combined scale factor C = E ±k must be applied before comparing with GNSS-derived ground distances. At most airports (low elevation, near UTM central meridian), C is close to 0.9997 —a 0.03% distortion that accumulates to 30 cm over a 1,000 m runway.