Geodetic distances are derived from geodetic positions (Latitude / Longitude) as opposed to coordinate distances which are derived from grid coordinates (Northings / Eastings). Geodetic distances (geodesics) are always an arc distance at some height relative to the ellipsoid surface. As such, there are only three options for the elevation at which they are computed: 1) the mean course elevation or 2) the project elevation or 3) on the ellipsoid surface itself.
A survey can have one Coordinate Reference System (CRS) that relates it's geodetic and rectangular coordinates. The CRS defines the ellipsoid used in the geodetic direct and inverse computations. TPC computes geodetic distances by inversing between two geodetic positions on the ellipsoid surface and applying the appropriate elevation factor (converts between the ellipsoid surface and the ground elevation). Since the CRS relates the coordinates of a point to its geodetic position, TPC can compute both a coordinate distance and a geodetic distance between any two survey points.
Geodetic Distance
TPC refers to all geodetic distances as Geoid Distance or Geodetic Distance and further qualifies the elevation component as shown below.
Geodetic distances use the equation:
geodetic distance on the ellipsoid surface = ground distance x (elevation factor) where: elevation factor converts the ground distance to an equivalent distance on the ellipsoid surface
Most of the time, you will be working in TPC with geodesics (the shortest distance between two points along the ellipsoid). In certain cases, especially when involving East-West computation on the Public Lands Survey System (PLSS), TPC includes the option to use the equivalent latitudinal arc, which is a Rhumb line. See Geodesics Lines vs Rhumb Lines.
