When to use the Geoid Height Above the Ellipsoid Function? - PIX4Dmapper

Video: Watch the instructional video related to Altitude and Vertical Coordinate Systems:

Warning:

The Geoid Height Above the Ellipsoid function is NOT needed for the majority of projects. It should be used when the user is interested in

Conversions between ellipsoidal and geoid heights.
Conversions between different geoid heights.

For most projects with GCPs, this function is not necessary. By default, PIX4Dmapper outputs the same height (ellipsoidal or geoid) as the GCPs.

The vertical datum can be independent of the horizontal coordinate system. This means that a project may have:

A datum for the horizontal coordinates (latitude, longitude, or X, Y).
A different datum for the heights (altitude Z).

Example: A project with horizontal coordinates based on the ellipsoid of WGS84 (GRS80) can have vertical coordinates based on:

The ellipsoid of WGS84 (GRS80): In this case, the horizontal and vertical datum are the same.
The geoid / mean sea level: In this case the two datums are different.
For more information: Orthometric vs ellipsoidal height.
The ground: In this case the two datums are different.
Any other local ellipsoid: In this case the two datums are different.

PIX4Dmapper can only do conversions between coordinate systems described by mathematical models such as ellipsoids, spheroids, etc. A geoid can not be described by mathematical models. Therefore, PIX4Dmapper can not do conversions between two geoids or between an ellipsoid and a geoid. The user needs to provide additional information by giving the Geoid Height Above the Ellipsoid.

The following section illustrates some examples of where this function can be used.

Image geolocation height

GCPs

Output horizontal coordinate system

Output height

Case 1

ellipsoidal WGS84 height

no GCPs

UTM zone

geoid

Case 2

ellipsoidal WGS84 height

no GCPs

national coordinate system (based on an ellipsoid different than the WGS84 ellipsoid.)

geoid

Note: There are more cases, where the Geoid Height Above the Ellipsoid function can be used. For more information, contact Support.

Case 1

1. When selecting the image coordinate system, leave the Geoid Height Above the Ellipsoid to 0 (default value). By default, PIX4Dmapper will generate the outputs in the UTM zone. UTM zones use as a datum the WGS84 ellipsoid. So, no conversion will be done with respect to the heights.

2. In the Select Output Coordinate System window, for Geoid Height Above the Ellipsoid, set the difference between the WGS84 ellipsoid and the desired geoid.

For more information: Menu Project > Select Output Coordinate System...

Important: Do not change the Selected Coordinate System.

The outputs will have horizontal coordinates in the UTM zone and geoid height.

Case 2

1. When selecting the image coordinate system, leave the Geoid Height Above the Ellipsoid to 0 (default value). By default, PIX4Dmapper will generate the outputs in the UTM zoneIX2. In the Select Output Coordinate System window, select the desired national coordinate system for the horizontal coordinates.

3. In the Select Output Coordinate System window, for Geoid Height Above the Ellipsoid, set the difference between the ellipsoid that the national coordinate system (step 2) uses and the desired geoid.

For more information: Menu Project > Select Output Coordinate System...

PIX4Dmapper converts the heights from ellipsoid WGS84 to the ellipsoid used for the National coordinate system. Then, PIX4Dmapper converts the new ellipsoidal heights to geoid heights based on the Geoid Height Above the Ellipsoid value.

Figure 1. Conversions between Heights