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Use Ground Control Points

Ground control points can be used by uploading an additional text file along with the input images, named gcp_list.txt. Below we will cover the creation of such file using the GCP Interface.

note

You must have purchased at least 1,800 or more credits within a year or be signed up with a monthly plan to access the GCP Interface.

In this guide we will assume that you have already captured your imagery with at least 5 GCP targets in sight and you measured those targets. See Getting Ground Control Points.

Import GCP Measurements​

Using the CSV examples downloaded from the GCP Interface, you should now have a CSV file that looks like this:

GCP Label,Northing,Easting,Elevation (m)
GCP 1,235269.88,3811198.11,0.20
GCP 2,235246.25,3811220.60,0.25
[ ... more points ... ]

These are the locations of the GCP targets. We also need to know the coordinate reference system (CRS) that we used. In our example, we used WGS 84 / UTM zone 11N, which corresponds to the EPSG code 32611.

From the GCP Interface, select your CSV file and type the EPSG code (or PROJ.4 definition) of your coordinate system. Then press Go to next step.

tip

If the interface doesn't recognize an EPSG code, the Go to next step option will be disabled. In such cases, use a PROJ.4 string definition, which you can find in a database like epsg.io.

tip

If the map doesn't load in the correct position, verify that you haven't swapped the Northing/Easting (X/Y) coordinates and ensure you've used the correct coordinate system.

Tag Images​

Select a GCP location on the map and click the Tag button:

Click Browse and select the images of your project. If your images are georeferenced, the GCP interface will automatically filter/sort the images by distance from the GCP. If your images are not georeferenced, you will need to manually select them.

  • To tag an image, simply click the location of the target. The border of the image will turn green.
  • To zoom in on an image, hold the Shift key and use your mouse wheel.
  • To remove a tag, click the X button.
  • To undo all changes to a GCP, press the Cancel button at the bottom of the page.

Once you are done tagging all the images that contain the GCP:

  • Press the Save Changes button at the bottom of the page.
  • Select the next GCP from the map and repeat until all GCPs have been tagged with at least 3 images.
note

You can use a GCP even if it shows only in 2 images rather than 3, but this is not ideal.

Export GCP File​

From the Map view, press the Go to next step button. Review that the information looks correct.

Your GCP file can be exported by pressing the Download gcp_list.txt button.

tip

You can resume work on a GCP file by dragging and dropping the gcp_list.txt file along with the project's images by clicking the Resume Work on GCP File button from the home screen of the GCP interface.

Use the GCP File​

To use the ground control point file, simply upload it along with the images when creating a task in WebODM Lightning.