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

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

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: 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 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@pix4d.com.

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 function: Menu Project > Select Output Coordinate System.... By 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 function: 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). For more information about the Select Image Coordinate System function: Menu Project > Select Output Coordinate System....
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 function: Menu Project > Select Output Coordinate System....
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

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### Article feedback (for troubleshooting, post here)

• De Villiers Visser

Hi,

I have asked this more than once before.

When are you going to supply the South African Geoid model, SA2010?

Regards

Div

• Daniel (Pix4D)

Hello,

Pix4D does not support any geoid model except for the EGM96, EGM84, and EGM2008.

The recommendation is either compute the geotags with geoid altitudes before importing them into Pix4D, or use GCPs with geoid heights so the project will be tied to them.

Regards.

• Luke Chidzey

You say to use GCPs with geoid heights in this case but in my experience in recent projects the points surrounding and on the GCP can be as much as 0.5m different in height to the measured GCP height. It sometimes seems as though Pix4d does not tie to them properly. Is there a feature in Pix4d that will flag when this is the case or is a manual check the only way to confirm this?

• Daniel (Pix4D)

Hello Luke,

If you find differences of 0.5 m in the points surrounding the GCPs, that means that something is wrong in your project, most probably lack of matches but each project is different and we would need more information about that project in particular.

The Quality Report which is generated for every computation gives very valuable information where you can analyze what is going on. There is an article in our KB which gives you detailed information about it: Quality Report Help.

Regards.