Vertical shift between the model and the GCPs / Cannot mark the GCPs in the rayCloud

Error

The rayCloud displays the Ground Control Points (GCPs), in blue, far above or below the model. The GCPs do not align with the images and the model.

shift_model_gcps.png


Description

After running step 1. Initial Processing and importing the GCP coordinates, the points are displayed way above or way below the model. The vertical shift between the images (wrong location) and the GCPs (correct location) is too significant for the points to be linked to the model. Therefore, the software cannot estimate a reliable projection of the points on the images where the image marks are supposed to be. It can then happen that no thumbnail is displayed in the right sidebar when clicking a GCP. As a consequence, marking is not possible in the rayCloud.


Cause

The elevation of the images written in the EXIF data (geotags) are measured by the drone's GPS. These coordinates may be inaccurate and depending on the drone and the project's location. The inaccurate elevation measurements can result in a vertical shift that exceeds 100 meters. To address the error the project must include GCPs. Follow the procedure below to use the rayCloud to incorporate GCPs.

 
Information: The error related to the elevation measurements collected by the drone's GPS only introduces a vertical offset, meaning that project's relative accuracy is not affected.

 

Solution

There are two possible options.

  • Method A: Editing the altitude of the images. This is straightforward but the precision of the vertical coordinates is lost. It is not a problem though, as the GCPs will accurately georeference the model. This does not overwrite the geotags of the EXIF. In addition, it requires to re-run step 1. Initial Processing.

  • Method B: Creating three MTPs before importing the GCPs and match them based on their names. It is a bit more complex but no vertical coordinates are preserved. It is faster as step 1. Initial Processing does not need to be re-run (only reoptimizing the results is necessary).

 

Method A

Once the project is created and after running step 1. Initial Processing:

1. On the menu bar, click Project > Image Properties Editor...
2. Left click anywhere in the first line, scroll down to the bottom of the list, hit and hold the Control and Shift keys together, and click the last line to select all images.
3. Right click in the column Altitude anywhere and click Edit Altitude in Selected Rows.
4. Enter an average corrected value and hit the Enter key.

 
Tip: The value should be estimated as Himage corrected = HGCP + Hflight above ground level where H are averaged elevations in the same coordinate system.

5. Click OK.
6. Re-run step 1. Initial Processing.
7. Mark the other GCPs in the rayCloud following: 202560769.

 

Method B

Once the project is created and after running step 1. Initial Processing:

1. Create at three new Manual Tie Points (MTPs) using the right sidebar of the rayCloud. Each new MTP must correspond to a GCP landmark or feature that is visible in the image content. These MTPs will be converted to GCPs later. For more information: 202560109.

 
Important: For each MTP:
  • In the Label field, write the name of the GCP and make sure it is identical to the GCP name stored in the GCP coordinate file.
  • Do NOT edit or manually enter the GCP coordinates.

2. Import the GCP coordinates using the GCP/MTP Manager. For more information: 202560039.
3. Click Yes to All, so that the MTPs are converted to GCPs and the coordinate values are overwritten with the correct values.



4. On the menu bar, click Process > Reoptimize. The model should adjust to the three GCPs.
5. Mark the other GCPs (if any) in the rayCloud following: 202560769.

 

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