Incorrect locations

I've run my data using both the onboard gps in the phantom 3 and ground control points. When I drop the images into my cad program (after running it through Qgis to change the projection) everything is scaled and rotated from actual locations. Over a 3 acre site the scale error is around 5'. The rotation error is smaller (maybe 10 minutes), but really annoying. It is possible that I'm doing something wrong with QGIS. But everything appears shifted the same when I compare my GCPs to the KMZ file downloaded from your site in Google Earth.
Also on the same job I've got some large elevation discrepancies. The color DEM shows the ground around a golf cart underpass approximately the same elevation as the road above.

Neil Atkinson August 31, 2015 20:48

Official comment

This is almost certainly due to QGIS. You might want to try to find someone that can do a known good reprojection for you that was made in ArcGIS or something so you can compare it to your QGIS results.

It can be hard to tell relative elevations in an absolute numbers sense from the colorized DEM image since the color bands each take up a couple of meters. To me, it looks like there is roughly a 2 meter difference between the road doing under the tunnel and the road over the tunnel. That is without actually referring to the values in the DEM image itself. We are in the process of adding a elevation lookup mouse over function.

Tudor August 31, 2015 22:02

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4 comments

I'll try running it through global mapper. The elevation lookup tool would be useful.

Neil Atkinson August 31, 2015 22:24 0 votes

I ran the geotiff through Global Mapper and the error is still the same.

Neil Atkinson September 01, 2015 03:29 0 votes

We deliver the files in WGS84 EPSG:4326. Converting them from there to your desired projection is the responsibility of the end user. Maps created using the camera GPS generated geotags can be off by up to 5 meters. The 3 pairs of GCPs that are placed in our basemap and manual workflows serve as the anchors for the model since the model is scaled and rotated to match the specified points. Adding more than 3 Anchor points will make it so none of the points are guaranteed to match but only that the error to the set of points is minimized. The GCPs don't affect the generation of the model at all. The model is the model and the GCPs are used to place it.

Tudor September 02, 2015 01:09 0 votes