Introduction
Ground control points (GCPs) is one workflow that can be used to achieve high absolute & relative accuracy in a survey. Ground control points are physical markers placed inside the survey area that are then measured using specialized survey equipment to find their positioning globally.
Once you have carried out your drone scan, your GCP markers will be contained in the photos you have taken. Now, we need to combine the location information measured in the field with the photos taken in the drone scan.
This article will show you how to add your GCP coordinates to the platform and mark their positions in the photos of the drone scan.
Prerequisites
An operation that has all photos successfully uploaded (operation status is 'Uploaded')
A list of the coordinates of your ground control points
The EPSG code for the projection system in which your ground control points were measured (for information on EPSG codes for your local region, see: www.epsg.io )
How to set the Coordinate Reference System
Go to the uploaded operation that you would like to tag the GCPs in, and click on the button "GCPs" at the top right.
Update the coordinate reference system (CRS) to match the coordinate reference system of your ground control points. By default, the CRS for the ground control points equals the CRS selected during the operation's creation. Updating the coordinate system here will update the coordinate system of the operation. Once data has processed, the data will be visualized in this coordinate system and the available downloads.
To learn more about which CRS Sitemark supports, you can use this support article.
Provide GCP information
There are 2 options to upload the GCP information into the platform. You can either enter the information into the system manually one by one or upload a file (.csv or .txt) with the information using the "import from file" button on top. You can also download a template for the import from here as well.
Legend
X: Longitude
Y: Latitude
Z: Elevation
If the information has been entered correctly (making sure you have also selected the correct coordinate reference system) the GCP marker should show up in the correct location on the map.
Tagging the ground control points in photos
Click on the GCP marker on the map (it should then turn blue and white to indicate it's been activated)
Select one of the photo markers on the map to begin locating the GCP within the photo.
You will be taken to a screen similar to the image below. Locate the GCP in the photo and use your mouse to place the crosshair in the center of the GCP as accurately as possible.
Once you are happy with the positioning of the crosshair on the GCP you can save your point. When the photo has been successfully linked there will be a green line connecting the photo marker to the GCP marker.
The GCP needs to be tagged in at least 4 photos, however, we recommend that you tag it in at least 5 to get the most accurate results.
Once you have finished tagging all the GCPs, go back to the operation menu and click Process, which will start processing the data. You will receive an email notification once your processed results are available.
Frequently Asked Questions
I have entered/imported my GCP's but don't see them appear on the map?
Most likely this is caused by a wrong coordinate reference system. Ensure you have selected the correct EPSG code.