GCP Accuracy

GCP Accuracy

Ground Control Points (GCP) Accuracies

Measuring precise ground  control point is very essential to getting a precise 3D model.  Below are some recommendation as to how to select and measure GCP"s

  1. Accurately measure the position and elevation of the control points. Carefully measure the position and elevation of control points with an accuracy of better than 2 cm (0.79 inch). Make sure stand on each point for at least one minute to get a good measurement.   Remember, the accuracy of measuring the position and elevation of control points sets the accuracy baseline for all the measurements and drawings related to the images.
  2. Take measurements in two sessions.  When using RTK equipment, it is strongly recommended to make two sessions of measurements with at least one hour gap between them.  Then average the measurements of the two sessions.  GCP's with high deviation between the two sessions should be discarded or re-measured.  
  3. Choose clearly defined control points. Choose control points that have a high contrast to the area around it and which will be unambiguously and accurately identifiable in the images. Use available landmarks or deploy your own prominent objects in the survey area if no appropriate landmarks are available. Good choices for control points are pointed shapes on flat surfaces such as painted street marks, or painted corners, etc. Corners of a building or tips of objects are only good as check points. 
  4. Avoid measuring points on slanted surfaces. This is specifically important when using a Total Station in reflector-less mode.
  5. Spread GCP’s as evenly as possible in the site area
  6. GPC Density: Place GCPS at every 400 feet (120 meters). If you use an RTK drone, have a GCP every 1000 feet (300 meters).
  7. Make a drawing with GCP’s locations and names 
  8. Mark GCPs correctly on the ground and include the name of the GCP



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