Collecting the right number of ground control points

The following is the minimum number of ground control points (GCPs) to collect, but it is recommended that you collect more than the minimum to ensure accuracy. However, collecting over 20 GCPs per image does not significantly improve the accuracy for most math models. To improve the accuracy, collect GCPs evenly throughout the image at a variety of elevations and in areas where images overlap. Also, the quality of the GCPs impacts the number needed to ensure accuracy.

Table 1. Minimum number of GCPs
Math model Minimum GCPs Recommended
Aerial photography Three or four per project Three per image for highest accuracy
Optical/Radar satellite:
MERIS Not recommended due to image bow-tie effect 
SPOT 1 TO 4  Four per image  Six to 10 per image
SPOT 5, IRS, ASTER, EOC, LANDSAT, QUICKBIRD (Basic), PRISM, AVNIR-2, KOMPSAT-2, KOMPSAT-3, CBERS, DMC, CARTOSAT, ORBVIEW, WorldView-1 (1B product), WorldView-2 (1B product), GeoEye-1, SSOT Six per image  10 to 15 per image 
RADARSAT, ERS, JERS, ASAR, IKONOS, PALSAR, FORMOSAT-2,  EROS, QUICKBIRD (Ortho Ready Standard), TerraSAR-X/TanDEM-X, WorldView-1 (Ortho Ready Standard), WorldView-2 (Ortho Ready Standard) Eight per image 10 to 15 per image
RADARSAT-2 with Toutin's model None Optional. When using Toutin's Model with RADARSAT-2 data, ground control points (GCPs) are not required to achieve accuracy within spacing of one pixel. You can still collect GCPs if you are confident in the accuracy of the GCPs.
Radar-specific modeling:
Radar-specific model GCPs optional Improve accuracy with a minimum of eight GCPs for RADARSAT-1, RADARSAT-2, TerraSAR-X/TanDEM-X, and old JAXA PALSAR data.
Rational functions:
If computed from GCPs Five per image* 19 per image*
If extracted from image file None Optional. Using zero-order RPC adjustment requires a minimum of one GCP/TP, using first-order RPC adjustment requires a minimum of three GCPs/TPs, and using second-order RPC adjustment requires a minimum of six GCPs/TPs.
Thin Plate Spline Three per image More than the minimum will average out errors introduced by inaccurate GCPs or terrain variations
First-order Four per image More than the minimum will average out errors introduced by inaccurate GCPs
Second-order Seven per image
Third-order 11 per image
Fourth-order 16 per image
Fifth-order 22 per image

*Depends on the number of coefficients that you want to use, for more information, see Understanding the rational functions math model.

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