GCP workflow issue

I have processed models in the past using post-process GCP workflow. On this map, while using my GCPs after re-processing the positioning (x/y) is working great. But the new elevations are both off relatively and globally.

I reprocessed the map twice with the same results. Please give me some ideas remedy my issue.

WTM C Creations March 17, 2021 16:58

Official comment

How were the coordinates for your GCPs measured? What are you comparing it to?

The adjustment we do is primarily in X and Y so assuming your have good input coordinates the outputs will be pretty darn close. The heights will depend on the accuracy of the initial processing and the accuracy of your GCPs. The initial model is tipped and tilted and raised or lowered to minimize the error between the points you define. It looks like there is some tilt in the remapped version from SW to NE.

Choosing GCP locations that are further out towards the edges will minimize the measurement error that gets introduced.

It looks like all the elevation values are pretty close on your GCPs.

Zane March 17, 2021 23:13

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4 comments

Upon further inspection, the X,Y is off 3 or so feet in some places. This is a very small area and I included 5 GCPs. I have no clue why my model is so off.

WTM C Creations March 17, 2021 17:17 0 votes

My GCPs where measured using survey grade equipment(Topcon Nokia rover and base) also additionally ran a level loop through them to verify.

If you look at the GCP just north west of the gas station building for instance. on my corrected model it is reading around 239ft. It is supposed to be 236ft.

WTM C Creations March 18, 2021 15:35 0 votes

You are likely trying to get too much out of our realignment process for the elevations. For any of our models the relative accuracy is going to be within 2-3 GSD units. So if the point you measured was a bit high then that error would get amplified out towards the edges of the area. That is why I was recommending to do your GCPs out towards the edges if elevation accuracy is what you are going for.

We fully admit though that our system is not optimized for elevation accuracy. There are things you can do to make it a best case situation but since we are not adjusting the model height values at all in a relative sense it is likely going to have discrepancies like this.

Zane March 18, 2021 16:35 0 votes