Using Check shot GCPs

dpitman

Hi Folks,

I don't yet have GM.

In GM, how do you examine the coordinates of check shots to compare them to surveyed values?

Thank you,
Dave

DerrickW

dpitman said:

Hi Folks,

I don't yet have GM.

In GM, how do you examine the coordinates of check shots to compare them to surveyed values?

Thank you,
Dave

Like a point to point inverse?  Not sure how to do that in global mapper, as I normally do that in CAD.  

Since you're using the term "GCPs", I'm assuming that you're a drone user.   The way I compare checkshots to my drone-derived DSM/DTM is: 
1: Import GCPs
2. Use calculate/copy attributes tool to copy "ELEVATION" attribute to a new attribute called "Observed"
3. Turn on DSM/DTM layer if it's not already on
4. Select all checkpoints and right-click--->analysis---> apply elevations to features
5. Use calculate/copy attributes tool to copy "ELEVATION" attribute to a new attribute called "Predicted"
6. Use calculate/copy attributes tool with the formula OBSERVED - PREDICTED, and write it to an attribute called "Delta"
7. change display labels to "Delta" attribute.  Export as CSV to calc RMSE, if needed. 

You can also convert the DSM/DTM to a point cloud (or "Create points at elevation grid cell centers") and use the lidar QA/QC tool.   The lidar tool will sample nearby points.  The workflow above will sample a single cell/pixel at the same XY coordinates as your control point. 

dpitman

Yes, I am referring to using the Pixels to Points tool to process images into a point cloud by including surveyed ground control as part of the process, but not including surveyed check shots.  Then after the cloud is processed, sampling the cloud at targeted check shots and comparing (proving) what the cloud says with what the actual coordinate of the point is as surveyed.

Would you suggest the lidar QA/QC tool for this?
(thanks for the comments)

DerrickW said:

Since you're using the term "GCPs", I'm assuming that you're a drone user.   The way I compare checkshots to my drone-derived DSM/DTM is:

You can also convert the DSM/DTM to a point cloud (or "Create points at elevation grid cell centers") and use the lidar QA/QC tool.   The lidar tool will sample nearby points.  The workflow above will sample a single cell/pixel at the same XY coordinates as your control point. 

  

TRB426

Another option is to open your checkshots file as Lidar points:  Open the file with GM and select the radio button "Lidar Point Cloud.." under "Import Type"  Then you can see them in the Path Profile tool. I usually Set the Draw Mode under options to "Color by Intensity" and draw them at a fixed size of 3 to 5 pixels so they show up better in the path profile.