Pix4D Survey "squishing" DJI Terra data

Hi there. I’m running into an issue where when I import point clouds made in DJI Terra to Pix4D Survey, the vertical data becomes “squished”.

We captured the data in WGS84 UTM Zone 15N (ESPG:32615), but projected the data into NAD83 State Plane Illinois West (ESPG:6457). The data in DJI Terra looks correct, but when importing into Pix4DSurvey the data looks wrong.

For example, a bridge overpass that measures about ~6m in DJI Terra (which is correct), measures about ~6ft in Pix4DSurvey. I attached some photos below.

Can someone advise on what’s happening here?

Hello Sam, from the DJI Terra screenshot it looks that the horizontal Output Coordinate System is set to “NAD83(2011) / Illinois West (ftUS)” EPSG:6457 and the vertical is not set.

Would it be OK to try setting the vertical CRS in DJI Terra, e.g., to EPSG:8228 / GEOID18?

EPSG:6457 is a 2D CRS that uses “feet” units for horizontal axes, and since the vertical units are not defined it might be that DJI Terra keeps the vertical values in meters. PIX4Dsurvey expects that if the vertical axis unit is not defined then it is the same as the horizontal units, this most probably explains the issue that you experience. Hope it helps

Hi Alexey, thank you for this!

I also have accompanying photogrammetry data that I ran in Pix4D Mapper, but that came out accurate.

I am going to try to retry processing the data with that VCS and I’ll let you know what happens.

That won’t compromise the vertical accuracy of the data correct?

That won’t compromise the vertical accuracy of the data correct?

In my opinion, it won’t compromise the accuracy, but to be confident we would need to know what DJI Terra uses under the hood for the CRS transformations. If you have some control points measured in EPSG:6457 then you can set this CRS as the project CRS in PIX4Dsurvey before importing your point cloud, import the DJI Terra point cloud (if the point cloud is in EPSG:6457+8228 then PIX4Dsurvey will convert the vertical values to the ellipsoidal height), and then check the control points’ elevations.

This said, the conversion from EPSG:32615 to EPSG:6457 might potentially result in significant loss in the horizontal accuracy, up to 2 meters according to EPSG official catalog, so it might be interesting to check the horizontal accuracy as well against the check points.