Best Practices for Ground Control Points (GCPs) and Checkpoints

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Number and Distribution

  • A minimum of 4 GCPs is required for non RTK/PPK maps (See more about our Single GCP Processing for RTK and PPK data here.) 
  • More GCPs for larger areas: More GCPs provide more anchor points for the processing algorithms, ensuring accuracy across larger areas. We generally recommend 1 GCP for every 60 drone images for non-RTK/PPK drones. We recommend 1 GCP for every 200 images for RTK/PPK enabled drones. More GCPs will improve accuracy but with diminishing returns.
  • Checkpoints: Measurements of error given by checkpoints are only indicative of the error in your map in the area where you placed your checkpoint. Errors in your map can change throughout, which is why it is important to have checkpoints in each area of interest. We recommend at least 1 Checkpoint per GCP map. 
  • Even distribution: Ensures consistent accuracy across the map, especially important for larger areas with varied terrain.
  • Strategic placement: Key areas like corners and elevation changes experience more distortion in aerial imagery. Placing GCPs here provides reference points for the processing engine to correct these distortions accurately. If you cannot see the center point of the GCP in your map photos, it cannot be accurately marked during processing. 
  • Buffer zone: Processing algorithms need context around the map edge to properly stitch everything together. A buffer zone of clear space ensures the algorithms have enough data to work with.
  • Properly Label GCPs and Checkpoints (optional): For best results apply an identifier to your target that corresponds to the GCP or Checkpoint label in your .CSV file. 

 

Quality

  • High visibility and High Contrast: The detection algorithm relies on identifying GCPs in the imagery. Bright, contrasting markers stand out, making identification and accurate measurement easier.
  • Surveyed coordinates: Accuracy matters! Use high-accuracy survey equipment to get sub-inch/centimeter-level precision, minimizing errors and ensuring trustworthy measurements.
  • Clear center point: Ambiguity is the enemy of accuracy. A marked center or intersecting line eliminates confusion and ensures everyone's measuring from the same point. Manhole covers are an example of a poor GCP and checkpoint marker
  • Examples:

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Bonus Tips

  • Consistency is key: Using the same methods throughout eliminates variables that could introduce errors and compromise accuracy.
  • High-resolution camera: More detail means better identification of GCPs and surrounding features, leading to more precise measurements.
  • What CRS are we in: Ensuring you know which Coordinate Reference System the GCPs are surveyed in ensures a seamless process when uploading your data.

 

Remember, GCPs are the bridge between your drone imagery and accurate, real-world measurements. By understanding the reasoning behind these best practices, you can make informed choices that optimize your drone mapping projects for exceptional precision and reliability.

 

 

Placement examples: 
In this example, the subjects of interest are the 2 star-shaped buildings so we place our GCP as best we can in the corners and center.
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