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Vertical Datum of GCP in NAV88 - Which Vertical Coordinate System should I use

Hi,

I am processing currently a project for which I have received GCPs from a surveyor. The Horizontal Coordinate System of the GCPs is NAD83 and Vertical Coordinate System is NAV88. Now I can select NAD83 successfully as the Horizontal Coordinate System for the Output, but I don’t have the option to select the NAV88 system. The only vertical coordinate systems available are egm84, egm96, egm2008. 

Could you let me know if I should enter a different coordinate system or will the processing in egm96 be similar to NAV88 if the coordinates are inserted? The total vertical altitude is about 1.4ft off but is the curvature also different? If not, the use of GCPs should correct the altitude. I could also convert my GCPs in egm96, but I have to deliver results in NAV88 - so converting wouldn’t solve the problem. 

Thanks in advance for your support on this question.

 

Hi David,

This is similar to this post: Geoid 12B

I copy here the most important information:

Currently Pix4Dmapper can do some altitude conversions for known geoids (egm84, egm96, egm2008)

For other geoids, we can only apply 1 constant difference (Geoid Height Above a known ellispoid): https://support.pix4d.com/hc/en-us/articles/202558239-Select-Image-GCP-Output-Coordinate-System-#label12

(You need to know the difference between the NAV88 and a known ellipsoid - the one related to your horizontal coordinate system)

If the geoid varies a lot (not a constant difference), then the only solution is to have GCPs with the desired altitudes (in your case NAV88 altitudes).

So, indeed, if you add your GCPs with NAV88 altitudes, you can set  the altitude as Arbitrary:

the software will give you the results in NAV88 (interpolating). This does not work perfectly (as the curvature and scale issues will not be taken into account) but it is a good approximation (especially for small areas).

In the future, we want to offer more options to our users: The user should be able to import a grid geoid file into Pix4Dmapper that will define the geoid altitudes for his/her area. This is something that our Product Development team will consider for future version of the software.

Hope it helps :slight_smile:

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