differences in orthophoto compared to GCPs of 10-15cm

Hello people

I tell you about my situation, many times the “orthomosaic” that I obtain in the processing, differs in its final coordinates with respect to the GCP used in it (small differences of the order of 10-15cm or 25 cm sometimes, pixel size 2-5cm normally). But I don’t understand the reason for these differences if I add all the GCPs correctly and look for the match in each image with each GCP.
How do I detect this difference? using any GIS software like GlobalMapper or Qgis.

Normally I use the following methodology, which I want you to tell me if it is correct:

1- I create a new job, I use the 3D MAPS profile.
2- I change the original configuration and in “keypoint image scale” I use the 1/8 image size function (in order to have a fast processing).
3- when step 1 is finished, assuming all the green check lists in the report, I import the GCPs and look for the match in each of the images using raycloud.
4- When I finish loading all the GCPS, I re-edit the “keypoint image scale” setting to FULL… and then I re-run step 1.
5- Assuming that the report gave everything correctly, I continue with steps 2 and 3.

Is my way of proceeding correct? I do this because I consider it to be the fastest way to load the GCPs and then leave the whole process running.

Hi Lui,

Welcome to our Community!

May I ask after you marked GCPs using raycloud, did you reoptimize it? For most cases, this is the recommended workflow. However, this method is not suited for “overnight” processing during which the different processing steps start automatically in a row and do not require any supervision by the user.

I would recommend after marking the GCPs you Reoptimize, then you can run step 2 and 3. There is no need to run Step 1 again. If you wish to have faster but still decent enough results when running step 1 please try 1/4 or 1/2 for the Keypoint Image Scale.

You can find the step-by-step instructions here: How to include GCPs in the project.

One more thing to keep in mind is to make sure you have selected the correct coordinate system for your GCPs.

I hope this could be helpful. Feel free to share with us if it helps. :slight_smile: If the GCPs error persists, please share with us the quality report and the log file.

▸ Quality report (.pdf format): …\project_name\1_initial\report\project_name_report.pdf.
▸ Project log file (text file): …\project_name\project_name.log.

Best regards,

it’s okay.

what I try to do is to minimize the time needed at the beginning of the process, that is, to the point where user intervention is needed. That is why I do a “step 1” with the fastest possible configuration for the sole purpose of having the “raycloud” enabled to add and load the GCPs.
Once that’s done, I set “step 1” back to full image size and do the 3 consecutive “1, 2, 3” steps all night.

You tell me that, after loading the GCPs in all the images, I necessarily have to do a reoptimization even if I then re-execute step 1 completely?

Excuse my English, I’m using the google translator to be able to express myself better.

Hi Lui,

No worries about the language.

Yes, reoptimization should be used when changes have been applied to the project after step 1. Initial Processing has been completed.

Such changes can be:

  • Adding Ground control points.
  • Adding Manual Tie points.
  • Adding Checkpoints.
  • Changing coordinate systems.
  • Disabling images.

This step does not compute more matches between the images, therefore, it is a fast step that improves the accuracy of the project. It reoptimizes the camera positions and the internal camera parameters.

For more information feel free to have a look at: Difference between “Reoptimize” and “Rematch and Optimize”