Importance of Ground Control Points (GCPs)

Ground Control Points (GCPs) are simple markers distributed throughout the field, with accurately measured coordinates used to georeferenced aerial or satellite imagery.

They play a crucial role in achieving accurate spatial positioning and correcting image distortions, enabling precise analysis, mapping, and interpretation of remotely sensed data. In addition, it will significantly facilitate the drawing and copying of annotations, especially regarding the Field trials product.

Given that within such a product the field contains many annotations (depending on the number of plots/stripes), copying annotations from one field to another will be completed in just a few clicks without any correction.

Next, we will describe what the process of setting up ground control points (GCPs) in the field looks like,  as well as uploading data to the Agremo application.


1. GCP marker shape

GCP markers should be in the shape of a square that looks like a chessboard (black and white). It is recommended that the marker material is not shiny, due to the reflection of the sun, it is sometimes difficult to recognize in the field. Also, it is recommended at least 50 pixels covering the markers. 

2. GCP marker position

Number and position have a great impact on geo-positioning and might reduce the re-projection potential. 

For users who perform multiple analyzes in the same area, it is highly recommended to choose permanent GCP positions.

Placing the GCPs, always in the same position, helps to mitigate possible geographic errors.

Here are some guidelines to consider when choosing the positions of your ground control points (GCPs):

  • Corners of the field>>> Place one or two GCPs at the corners of the field to provide reference points for the entire field.
  • Areas with topographic variation>>> Place one or two GCPs in areas with significant topographic variation, such as hills or valleys, to help the photogrammetry software accurately map the terrain.
  • Representative locations>>> Choose one or two GCPs in representative locations that are typical of the field to provide a general reference for the photogrammetry software.

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It is highly recommended to have at least 5 GCPs.

We strongly recommend using at least 6 ground control points (the higher is better, i.e. 10 GCPs), if you use multiple drone types of sensors during the process. 

The following image shows recommended placement for 5 or 6 GCPs.

3. Data collection with centimeter-accurate equipment and GCP

If the user owns a Real-Time Kinematic (RTK; centimeter-accurate equipment), it is possible to collect geo-locations for each GCP from the drone. (e.g. DJI M3M drones).

So the following steps are: 

  • Place all GCPs based on the recommendations above 
  • Map the field
  • Collect RTK position from each GCP (take pictures from the center of these GCPs)
  • Follow the steps from Uploading ground control points

Uploading Ground Control Points to the Agremo application

Agremo stitching platform service supports importing of  ground control points GCPs via CSV file in the following format:

  • Latitude
  • Longitude
  • Altitude (if altitude is not provided it will be assigned from the detected marker)

The CSV file with comma-separated values should be in the same zip data set along with the raw images. Only one file should exist in the dataset. 

Uploading a zip file is the same as the regular process of uploading raw images to the Agremo application.


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