# When to use the Geoid Height Above the Ellipsoid Function?

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

Warning:
• The Geoid Height Above the Ellipsoid function is NOT needed in 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.
• Especially in the case that the project has GCPs, this function is not needed for the majority of projects. By default, Pix4Dmapper outputs will have 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 is the same.
• The geoid / mean sea level: In this case the two datums are different. For more information: 202559869.
• 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 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 us here.

Case 1

1. When selecting the image coordinate system, leave the Geoid Height Above the Ellipsoid to 0 (default value). For more information about the Select Image Coordinate System function202558099By default, Pix4Dmapper will generate the outputs in UTM zone. UTM zones use as datum the WGS84 ellipsoid. So, no conversion will be done with respect to the heights.
2. On the Select Output Coordinate System window, on the Geoid Height Above the Ellipsoid, set the difference between the WGS84 ellipsoid and the desired geoid. For more information about the Select Output Coordinate System function202558099.

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). For more information about the Select Image Coordinate System function202558099.
By default, Pix4Dmapper will generate the outputs in UTM zone.
2. On the Select Output Coordinate System window, select the desired national coordinate system for the horizontal coordinates. For more information about the Select Output Coordinate System function202558099.
3. On the Select Output Coordinate System window, on the Geoid Height Above the Ellipsoid, set the difference between the ellipsoid that the national coordinate system (step 2) uses and the desired geoid.

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