CTH 9 Second Digital Elevation Model of Australia
Digital Elevation Model data record the terrain height variations from the processed point-located data recorded on an airborne geophysical survey. The aircraft altimeter data records the height of the aircraft above the ground and the aircraft GPS records the height of the aircraft above the ellipsoid. Subtracting the two values enables the height of the terrain beneath the aircraft relative to the ellipsoid to be calculated. This ellipsoidal terrain height is corrected for the variation between the ellipsoid and the geoid (the n-value correction) to produce terrain heights relative to sea level.