The essence of this type of calibration is in specifying of an arbitrary number of control points (but not less than three) located in arbitrary places on the tablet of the digitizer. To reduce calibration inaccuracy it is advisable that the points embraced a big part of the working field of the tablet. In practice selection of control points is limited by the necessity to know exact coordinates of these points on the map, that is why either crossings of coordinate lines or landmarks coordinates of which are known should be selected as control points.
When pushing the button Fast in the dialog box “Digitizer adjustment”, the system opens a dialog box “Fast calibration” shown at figure 21-7.
Figure 21-7. Dialog box “Fast calibration”
This dialog box contains a list of specified control points. The pane displays numbers of points and their coordinates in digitizer's native coordinate system. At the beginning of calibration the list will be empty.
To add a control point it is required to push the button Add. A dialog box “Coordinates of control point” will be opened (figure 21-8).
Figure 21-8. Dialog box “Coordinates of control point”
The user should select some control point on the map fastened on the tablet of the digitizer, set the cursor of the digitizer so that selected point was directly in the center of the viewfinder and push the button on the digitizer cursor. After that coordinates of the point in digitizer's native coordinate system will be displayed in the pane Tablet position. If there are doubts in precision of entering of coordinates, it is possible to repeat positioning of the cursor and enter refined coordinates pushing the button on the cursor once again.
Coordinates of control point in the system of map coordinates are entered from the keyboard in the pane Position on map.
The order of entering coordinates is of low importance. If it is more convenient for the user, he or she can enter coordinates of the control point on the map from the keyboard at first, then position the cursor of digitizer on this point and push the button for entering coordinates on the tablet.
After pushing the button OK, the control point will be added to the list. Buttons Change and Remove permit changing coordinates of control point selected in the list or remove the point from the list.
The procedure of fast calibration can be completed by pushing the button OK in the dialog box “Fast calibration”, if the number of specified control points is not less than three. In the case when exactly three points are specified, transformation of coordinates is built so that coordinates of these points in map coordinate system took the values indicated by the user in the course of calibration. However, if there are only three control points available, the system does not dispose of additional information which could allow it to find out and compensate calibration errors, even if they are gross. Such additional information appears, if the user has specified more than three control points. In this case it is impossible, as a rule, to find such affine transformation of coordinates, where all the points got the values of map coordinates indicated in the course of calibration. The system calculates transformation coefficients which minimize the sum of squares of deviations of calculated control points coordinates from their specified values.
When the list contains not less than three points, the lower part of the dialog box “Fast calibration” displays transformation inaccuracy. Supposing the hardware inaccuracy of the digitizer and that of the source cartographical material are rather low, transformation inaccuracy is determined by carefulness of specifying control points by the user. It should not exceed the value of 1% or less. Abnormally high inaccuracy (more than 10%) most probably indicates an error in entering coordinates from the keyboard.
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