In this tutorial we review the benefits of GCPs (Ground control Points) and how to upload and tag them in Alteia as a self-service.

1. Why using GCPs?

In order to ensure the highest accuracy processing possible, even in RTK/PPK situations, one or more GCPs is recommended as it can greatly increase accuracy.

For this reason, during Standard Processing, the system will expect a GCP file even in RTK/PPK situations.

While we recommend following this protocol, if no GCPs are available you may upload our GCP template file.

• Select Add ground control points (GCP) during a new upload process. With a Self-service execution of photogrammetry selected, user will have to click the GCPs during next steps.
• Selecting both the Advanced photogrammetry & Add ground control points (GCP) options will alternatively result in the user not to click the GCPs and have it managed by Delair.
  

2. GCP Upload

Drag & drop your GCP file in the dedicated area or press Browse file to select the file manually.

The file should be in .CSV or .TXT format, no header row and only delimited with a comma

The file must be in the following format :

1. Name/reference of the GCP
2. X coordinate
3. Y coordinate
4. Z value
5. X accuracy (optional)
6. Y accuracy (optional)

You can find bellow an example of a GCP file :

GCP_EPSG32631.txt

Once the GCP file has been uploaded, verify that the x/y/z columns appear in the correct order.

Press Edit CRS to select the projection of the GCPs. The GCP CRS will take into consideration the CRS selected at Site creation. It must be selected. If it is not the same, reconvert the GCP file to the same Project CRS. If the selected CRS & GCP file coordinate system match, the GCPs should appear on the map as orange dots within the survey boundary.

If nothing appears, then it may be possible that the CRS is incorrect or the X and Y coordinates are reversed.

To retry, deselect Add ground controel points (GCP), fix the file, and then reselect Add ground control points (GCP) and try again.

3. GCP Tagging

When you enter the GCP tagging interface, uploaded GCPs will be listed on left side of window and corresponding orange dots will appear on right side map background:

Please note that you have the option at this step to define the target as a plain Check Point instead of Ground Control Point, i.e. to help in model absolute accuracy estimation instead of being used for georeferencing in the photogrammetry processing :

Clicking on a GCP (on the map) will select its neighboring images (now in dark grey) and display them in a gallery :

You can choose to unselect these images if necessary in the bottom section of this gallery:

Proceed to GCP tagging per image : a green cross marks the clicked position:

Green dots indicate tagged images ; a minimum of 3 images is required per GCP :

In the GCP tagging gallery, GCP-tagged pictures will equally be highlighted by a green frame :

Press Back button to read GCP tagging status list:

Continue your tagging process if necessary. Once you’ve finished, press Back again from the tagging status list to go back to the Launch upload step:

Make sure to click on "Next Band " to tag every band of the datasets.

4. About Coordinates

• Eastings would be X
• Northings would be Y
• X field would be Longitude
• Y field would be Latitude
• Longitude is W or E
• Latitude is N or S

5. More about GCPs

• Learn here about the best practices for installing GCPs on field.
• Learn here about Quality Checks & Statistics on GCP during photogrammetry processing.