# Geodetic Distance Types

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 Geod 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

## Geodesics vs Rhumb Lines (Loxodromes)

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 Geodisics Lines vs Rhumb Lines.

## Specifying a Geodetic Distance Type

Advanced Traverse View Format dialog this dialog lets you specify a geodetic distance type for a particular traverse. See Traverse View Distance Factors.

Miscellaneous Drawing Settings dialog - -  you can choose which geodetic distance to use for line labels.

COGO dialogs - some COGO dialogs allow you to select which geodetic or coordinate distance to display.

## Geodetic Distance Type Definitions

TPC uses the following geodetic distance types throughout the program.

### Geodetic Distance (at ellipsoid surface)

Geodetic distance (at ellipsoid surface) = ground distance measured at ellipsoid surface

The Traverse View status bar displays  “Ellipsoid surface”

### Geodetic distance (at project elevation = 1,500')

Geodetic distance (at ellipsoid surface) = ground distance measured at project elevation x (project elevation factor)

The Traverse View status bar displays  “Elevation = 1,500'”

### Geodetic distance (mean course elevation)

Geodetic distance (at ellipsoid surface) = ground distance measured at mean course elevation x (mean course elevation factor)

The Traverse View status bar displays  “Mean course elevation”

## Geoid Separation

TPC does not currently model geoid separation. It assumes that the geoid and the ellipsoid you have selected as part of your CRS are coincident over the area of your survey. As such, the ellipsoid surface is considered to be Mean Sea Level.