GCP. Using Ground Control Points for DTM/DSM correction.

jcampen

Hi GM Team,  I am aware that with the GM Pixels to Point Tool is able to correct the XYZ in a data point cloud by adding GCPs into the data set in GM. However I am importing an already created Digital Surface Map from Pix4D into Globalmapper and wondering if the Pixels to Point tool is also able to correct the DSM as well. Or how can this be done otherwise?

I am also aware this is a function within Pix4D, however it often gives some distortion and inaccuracies from previous survey data sets done in Pix4D.   The issue with Pix4D is I am also not able to import DFX/DWG site plans and previous generated contour DFX/DWG files into it to improve the XY and Z adjustments I need.  Whereas this is all doable in GM.

From what I know I can generate a DTM from the point cloud in GM, however the resolution is not always as accurate as what I have being generated in Pix4D.  So my objective for accuracy and faster workflow is to be able to adjust the DSM's I generate from Pix4D using Globalmapper and by using GCP or whatever other methods this can be achieved by. 

 

Appreciate any advice how this can be done.

DerrickW

Hiya jcampen.  What you're describing is part of my normal workflow, so I'll run through my process. 
1. Import GCPs & DSM into GM (From a txt or csv file). 
2. Right click GCP layer, calculate/copy attribute values. 
3. Copy the initial ELEVATION value (in this case, the surveyed elevation from the field staff) to a new field called "OBSERVED"
4. Select all GCPs and right click in the map view. Add/update elevations from  terrain layer to GCPs. 
5. Right-click GCP layer again and calculate/copy the updated ELEVATION value (updated from the DSM) to a new field called "PREDICTED"
6. Right-click GCP layer again and calculate/copy a new field called DELTA.  In my case, the calculation is OBSERVED - PREDICTED because I'm going to eventually average the deltas to to compute a uniform Z adjustment (not rubber sheeting). 

Once I have those 3 fields (Mainly DELTA) in my GCP & Checkpoint layer(s), I export them to a CSV and do my averaging and RMSE calcs in excel.   The average adjustment in excel is then applied to the DSM layer in global mapper by double clicking on the layer or layer group and applying an elevation adjustment.  At this point, I'll export out the adjusted DSM, bring it back in, and do another round of post-adjustment QA/QC checks. 

DerrickW

You could also export your DSM Geotiff back out as an LAS/LAZ file and then bring it back in and use the normal lidar module QA/QC tool if you prefer that option. 

DerrickW

Sorry for the triple post. Not seeing an option to edit the original. 

jcampen

Hi Derrick, thank you so much for showing this! I will give it a try and see how it goes. Vey much appreciated!

alienyouth

I've been searching for ways to perform elevation adjustment for DEMs using ground survey data and this is the most concise method I've come across. Thanks for this Derrick.

The only snag i see is that it is only applicable for small areas; i wish there was an all-in-one tool or script that can work for larger areas or iterate over specified grid areas. (GM dev team, pls take note).

Thank you once again.