GCP polygon test

Review of the Bowl effect of the Dense Point Cloud.

To demonstrate the importance of placing GCPs in the correct positions and constraining the surface model to the GCPs, a 5 acre site was utilized that is retangular in shape.

Five (5) GCPs were set with one in each corner and one in the middle of the site.

The site is a sloping flat piece of ground with an elevation change of about 7' from the west to the east.

Three scenarios where ran as follows:

Scenario #1 - No GCPs

With no GCPs, the surface model has a bowl effect meaning that it is not a flat sloping surface.

Scenario #2 - 4 GCPs on the outside only

The surface model was constrained to the four (4) outside corner GCPs. The center of the site is still showing the bowl effect. Only the outside corners are at grade with the GCPs.

Scenario #3 - 5 GCPs with one in the middle

By constraining the model to all five (5) GCPs, the bowl effect has been eliminated resulting in a flat grade from west to east that matches the GCPs.

Conclusion

The complex algorithm used by photogrammetric software to build the dense point cloud surface model requires constraints (GCPs) in order to process the imagery so that it reflects the actual ground conditions.

Utilizing GCPs is especially critical for 3D survey mapping so that the surface model is correct.

As demonstrated above, without the proper GCPs in the right location, the results will be distorted and not reflect the actual ground conditions.

I have seen the bowl effect when using several different software programs. The algorithm of these software programs are mathematically similiar using the Least Squares adjustments that require constraints in order to provide good results.

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