Great GCPs (Ground Control Points) are critical to your PPK workflow.
Plan your GCP layout: Before data collection, carefully plan the placement of GCPs across the survey area. Determine the number and distribution of GCPs based on the project requirements, desired accuracy, and the size and terrain of the area.
Choose recognizable features: Select easily identifiable features on the ground, such as man-made objects or natural landmarks, as GCP locations. These features should be visible in both the UAV imagery and on the ground during the survey.
Establish ground truth coordinates: Use a GPS receiver or a survey-grade GNSS device to accurately measure the coordinates (latitude, longitude, and elevation) of each GCP. Ensure that the GNSS device is properly calibrated and configured for the desired coordinate system and reference frame.
Record GCP information: Maintain a record of each GCP's identifier, coordinates, and any additional relevant information, such as the type of GCP or its physical characteristics. This record will be useful during data processing and analysis.
Mark GCP locations: On-site, physically mark the GCP locations using highly visible targets or markers. These markers should be easy to identify in the UAV imagery and distinguishable from other objects in the scene.
Capture UAV imagery: Fly the UAV over the survey area, capturing the necessary imagery using the predetermined flight plan. Ensure appropriate overlap between images and sufficient coverage of the GCPs in the captured imagery.
Include GCPs in imagery: While capturing the UAV imagery, ensure that the GCPs are included in the photographs. Aim to have at least three GCPs visible in each image to enable accurate georeferencing and orthorectification.
Post-process GCP coordinates: After the UAV flight, post-process the measured GCP coordinates using appropriate geodetic software or tools. This step improves the accuracy of the GCP coordinates by accounting for factors such as atmospheric conditions and satellite signal corrections.
Georeference and orthorectify the imagery: Use the accurately measured GCP coordinates to perform georeferencing and orthorectification of the UAV imagery. This process aligns the imagery with the correct geographic coordinates, removing distortions caused by terrain relief, camera tilt, and other factors.
Assess accuracy: Evaluate the accuracy of the georeferenced and orthorectified imagery by comparing the positions of additional checkpoints (independent ground reference points) with their known coordinates. This assessment helps validate the accuracy of the entire workflow.
By following these steps, you can establish a robust workflow for setting ground control points in UAV remote sensing, ensuring accurate georeferencing and precise data analysis for your projects.
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