Negative elevation

[mimmoo]

Hi,

My goal is to estimate volume of the log piles in a mill yard. I had acquired 137 images (with 80% overlap at 40m above ground) from a UAV Platform - all in jpeg. I am trying to build 3D point clouds (and eventually DSMs) using Photoscan pro. I have negative elevation values (see attached screen shot ). I am new to this environment and had been following the workflow shown in the tutorials.

1. No markers used but have image centre GPS co-ords from the sensor. I used sensor altitude agl and not gps altitude. Projection WGS84.
2. Aligned the photos using high accuracy and "reference" pair-selection. Optimised cameras after removing points with high reprojection error (over 1 pixel).
3. Built high quality dense point clouds (as in the image), Mesh with  height surface type, decimated the mesh, filled holes to build texture (adaptive ortho mapping mode and mosaic blending).

Unable to figure out why I have negative elevation. Any insight to where I am going wrong is much appreciated?

Thanks!

[stihl]

What was your AGL?

Since you did not use any markers, the absolute accuracy of the DEM can be anything between 5 and 20 meters (if your measured image positions are somewhat valid to begin with)

The elevation shown is relative to WGS heights in that area. The lowest elevation is about 7 meters and the highest about 6, this would make sense if you look at the accuracy of the image positions.

The only thing you can do with your model right now is measure the relative volume in the model itself. What you can't do is measure the volume relative to a given height. For that you'd first need to reference the model by placing markers/GCP's.

[mimmoo]

Thanks for the response!

AGL varied between 36-40m.

I had a mishap with the markers - lost all my GPS waypoints due to a corrupt SD card.

[stihl]

Bummer!

Only thing I can think of now to salvage your project is to go back and measure natural ground control points, ie familiar points you can see back in your model.

For that I'd recommend taking a thorough look at your orthophoto and determine if you can see any of those natural GCP's that you seem fit.