DESCRIPTION tries to automatically generate a given number of new ground control points (GCPs) by matching the input map to a target map through FFT correlation, based on a few existing ground control points, previously defined by the user, for example through the Ground Control Points Manager. The goal of the module is thus to automaticallty increase the number of control points to enable higher quality geocoding of imagery to a master image (co-registration).


It is recommended to use maps filtered with the DIVERSITY or STDDEV filters of the r.neighbors module, with a window size of 3x3 or 5x5 pixels. However, the algorithm sometimes works well also with the original maps. The produced GCPs can then be used on the original imagery.

The actual number of newly generated ground control points will likely be less than the given maximum number of ground control points because each generated point is filtered using its FFT correlation coefficient, and optionally also by the given RMS threshold. supports the usual transformation orders 1-3 and requires the corresponding number of previously set ground control points: 3 for order 1, 6 for order 2, 10 for order 3.


The GRASS 4 Image Processing manual
Ground Control Points Manager


Ivan Michelazzi
Luca Miori
Markus Metz

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