16. Ground Control Points (GCPs)
Ground control points (or GCPs) are the points on the ground whose coordinates are known in the area of interest. Their coordinates were collected from other sources or measured using typical surveying methods (LiDAR, older maps of the area, Web Map Service). GCPs are points that a surveyor can precisely pinpoint in an aerial mapping survey, allowing enormous areas to be accurately mapped with just a few known coordinates. GCPs can also be utilized as checkpoints to ensure the accuracy of the results.
GCP can be anything that can be easily identifiable normally with high contrast colors. GCPs can be added manually or through the software also. If the GCPs are manually located then they should be large enough to be identified from your particular flight altitude.
Aerial Photogrammetry as well as surveying and mapping professionals require precision and accuracy. GCPs are supporting for providing better accuracy for image processing.
If GCP points are absent then there is no information about scale, orientation, or absolute position in the final results. As a result, they can't be used for measuring, overlaying, or comparing to earlier results. The shape of the surveyed region may not be preserved by the 3D reconstruction.
GCPs improve a project's absolute accuracy by precisely positioning the model on the Earth. They have reduced the GPS shift from meters to centimeters.
The GCPs should be distributed uniformly throughout the study region. It is also suggested that one GCP be placed in the center of the region to improve the overall quality of the restoration. It is advised that at least 5 GCPs be used, each of which is detected in 5 images, as this reduces measurement inaccuracies and aids in the detection of faults made when inserting the GCPs.
Place the GCPs away from the marginal areas in such a way that they will be visible in more number of images.
Note: GCPs are Highly Recommended for processing the images without the geolocations.
DroneNaksha provides this facility on the dashboard. Click on the dots in the right upper corner. Select the Manage GCP option as shown below.
All the images available in the required area will be highlighted in blue circles. These blue circles are the center points of these images. These images are captured by a drone. The number of images captured may be very large up to 200 or sometimes more than that also.
The user can import '.csv' file to add the GCP points to the image.
The GCP points can be marked manually on the map. When a GCP point has located, a circle with 100 meters radius will be considered by default as shown. All the images encircled by this radius will now be turned into red-colored dots. GCP Point will be marked with a white dot. You can increase or decrease the radius according to your requirements.
After adding GCP to orthomosaic image you will get a very good and accurate image as shown.
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