I hoped I had cracked this issue, but I have not! I had someone check the file that I created and I found out the 3d realization is incorrect. It should have been a simple rectangular site. I haven't had a chance to work on this again, but I am open to any additional suggestions.
If you are able to create a mesh from the file, but the mesh is not rectangular, then the points in that file are not just for the rectangular site.
I have tried with files from surveyors in the past, but they also include many points outside of the site boundary, like roads, kerbs, manholes, etc., so you are not getting just the site boundaries and contours as you would expect.
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