Coordinates of position of digitizer cursor on the tablet enter the computer in digitizer's native coordinate system. Calibration of digitizer is required so that the system could transform them into the map coordinate system. The main aim of calibration is to calculate the coefficients of affine transformation which converts coordinates of the tablet of the digitizer into map coordinates.
This task is similar to the task of transition to another system of map coordinates described above (chapter 20 “Transformation of coordinates”, subsection “Coordinate transformation types”). The difference is that coordinate system of the digitizer is used as a source coordinate system.
Before starting the process of calibration it is necessary to make sure that the digitizer is ready for work and that the map is safely fastened on its tablet. The user should take into account the fact that even a small displacement of the map on the tablet causes the necessity to perform digitizer calibration anew. That is why it is recommended to fix the map safely till completing its digitalization.
To determine affine transformation it is enough that coordinates for three points of the map (control points) were specified in digitizer coordinate system and in map coordinate system.
When entering coordinates of control points it is impossible to avoid inaccuracies caused by such factors as hardware inaccuracy of digitizer, inaccuracies of the source material (map sheet) and of manual positioning of digitizer viewfinder. To reduce calibration inaccuracy it is possible to add more points to the three required ones.
The system allows performing calibration of two types: fast and adaptive. Calibration types differ in selection of control points and in algorithms of calculation of transformation coefficients and of inaccuracy degree.
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