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Author Topic: Did my elevations get inverted? (DJI Drone Photogrammetry)  (Read 1906 times)

jazzyj

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Did my elevations get inverted? (DJI Drone Photogrammetry)
« on: April 24, 2018, 10:03:40 PM »
I'm still examining things and it's hard as I'm using remote desktop on a cloud server.  But it looks to me like the elevation data is inverted.  Can someone confirm on the DEM export, is the dark area the lower elevations and the light/white area represents the higher elevation,  If so, then my model is definitely inverted. 

I had a few GCPs but only a few to verify I was in the ballpark.  On processing, after photo alignment, I loaded and marked the 4 GCPs and then unchecked all the cameras and did the camera optimization step, rechecked the cameras and proceeded with processing,  The elevation data for the cameras, despite the rumor it is AGL on the DJI drone, is indeed meters above sea level, as the elevation is about 370m and we flew 400ft AGL and the EXIF is showing 526m elevation.

The only thing I can think of is that the photos were taken from about 4 different launch points on the property but the elevation difference between launch points was only 50ft max, that wouldn't invert the elevation data.  When there is a greater distance between the land and the drone, the AGL would be higher (but again, it would be showing 130m elevation in the exif  data in the reference panel of Photoscan if it were using AGL, not 526m.) 

If the dark part of the DEM is indeed the higher elevation then I am misreading everything so this is all moot though.  Waiting for the 3D rendering to complete as once the photographic imagery is projected on the 3D model I'll quickly see if it is indeed inverted.

Would it be prudent even if you had only 3 or 4 GCPs towards the edge of a huge land area (400 acres) to just use those and uncheck all the cameras if absolute horizontal and vertical position accuracy is not of concern?  That would rule out issues with the EXIF data I suppose.