Motus Reference Manual
Page 16 of 146
Version 1.1
29/11/2019
Latitude and longitude give the 2D point on the surface of the Earth. These are
combined with height to give the 3D position on the Earth.
Height is the height above the WGS84 reference ellipsoid. The WGS84 reference
ellipsoid is a model used to approximate sea level across the Earth. Therefore the
height should be considered approximately relative to sea level. Due to the
approximate nature of the WGS84 model, the WGS84 height will not be the same as
the actual sea level. For example, in Australia, the WGS84 height at sea level is 9
metres at some points.
5.8
NED Co-ordinate Frame
The NED (North East Down) co-ordinate frame is used to express velocities and
relative positions. The origin of the co-ordinate frame can be considered the current
position. From that origin, the north axis points true north and parallel to the line of
longitude at that point. The east axis points perpendicular to the north axis and
parallel to the line of latitude at that point. The down axis points directly down towards
the centre of the Earth. See Illustration 6 for a graphical representation of the NED co-
ordinate frame at a position on the Earth.
Illustration 5: World map showing latitudes and longitudes
Equator
