Negative z Values in DEMs & Dense Point Cloud

[KimPenglase]

I'm using drone imagery data of pine plantations to build understory profiles. The drone was flown at 60 AGL. I am analysing with ArcGIS PRO. The resultant DEMs and Dense point Cloud look great. However most of the vertical heights (z values) are negative. Has anyone got any ideas of where I have gone wrong.

[DavidPike]

They may be correct if the height is height above spheroid/ellipsoid (HAS/HAE) or even Geoid.  Examine the settings of your drone's vertical system and also check values  of HAE etc. against the expected ones for your location.

[KimPenglase]

Thanks David, I'll check it out and see. We used Gravity and AHD (Australia/NZ) on ArcGIS PRO but I'm not sure how the drone (Phantom 4 Pro) determines AGL.

Regards,

Kim.

[CodyBenkelman · Accepted solution]

Most drones report HAE but we often see errors in those values.  You need to apply ground control to ensure proper alignment in x, y, and z.  Without ground control, your z values are relative to the drone, and since the ellipsoid height is almost never at the same height as the geoid this results in 3D products appearing to be either above or below ground.

The solution is using ground control - you don't need to use this map, but presuming you have an ArcGIS Online account (to access the World Terrain service) you can query this map http://esriurl.com/Map4EllipsoidalHeights  in your project area to confirm if the ellipsoidal height is below the geoid (orthometric) height. 

Cody B 

[beel]

You haven't done anything wrong: You were using a DJI unit. Our P4Pro and Inspire 1 UAVs regularly have a vertical error in the tens of meters. Probably has something to do with the geoid used by the UAV. If anyone knows specifically how to fix THAT, I'd love to hear it. When I called DJI Support, they didn't know the answer.

You'll find that the barometric height (listed in the image metadata / EXIF file) is usually pretty accurate.

The solution offered re. ground control should work, but if that's not available, you can also edit the elevation using your planned flight elevation. You can use an EXIF editor (such as EXIFtool), or you can fix the values in Pix4D if you are using that software: How to correct for wrong image altitude without GCP – Support (pix4d.com)

Not sure how to correct the values in ArcPro, but I'm sure a bright and helpful user will offer suggestions.

[KimPenglase]

Hi Beel,
Thanks for that. I'll apply your solutions today and see how I go. I'll let you know how I go. Thanks.
Regards,
Kim