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GCPs and stitching errors

I recently made a georeferenced orthophoto with MME. I georeferenced it with 7 GCPs. (3 anchor points each selected from two input photos and a fourth point selected from one input photo). Coordinates of GCPs were from RTK GPS and should be within a cm or so. Targets are distinct and can be selected within a pixel or so. In addition to the GCPs, I have RTK GPS coordinates on about 20 other targets which can distinctly be seen in the orthophoto.

My CAD program will not place the image correctly using the world file, but if I import the orthophoto into CAD and align (translate, rotate, and scale) it with the two GCPs most distant from each other, then the other GCPs and targets are somewhere between 2 and 4 feet off of the RTK GPS coordinates. The error is not in any one direction. I also tried aligning using a least squares fit of a dozen points and residuals average about 1.6 feet. So I think the error is in stitching, not in georeferencing. This seems like a lot of error. Are others getting similar results or am I doing something wrong?

I double checked the GCP coordinates I entered and the coordinates of the pixels.

A few details:
3DR Solo flying MAPIR Survey2 RGB camera. 518 nadir-pointed 16MP photos taken from 235'AGL covering 52 acres. The GCPs are all in the central 20 acres. Lots of overlap (13+ images) in the central 20 acres. Output GSD is 1.01 inch/pixel. Input images are free of jello. Mostly flat terrain with a few buildings and vehicles.

Public link:


Official comment


The Maps Made Easy GCP system was designed for users of non-geotagging cameras to get it to a respectable amount of precision, as opposed to not be referenced at all. You cannot expect to get point for point accuracy out of 3 GCPs. This work is better done in an offline setting with the finished outputs from Maps Made Easy due to the number of points that are usually needed for quality results. 

The GCPs in our system are not used to stretch the imagery as it is reconstructed. Instead, the 3 GCP pair locations (Anchor Points) are used to "set the table" for the rotation and location of the reconstructed surface. None will line up exactly. The error to the locations is minimized based solely on the image based reconstruction.

If you have solid waypoint, the best way to get those points to line up exactly is by manually georeferencing the GeoTiff in QGIS using a Thin Plate Spline deformation model to stretch the GCP locations to their proper coordinates. This is not done by Maps Made Easy.


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