The procedure of fast calibration has rather serious disadvantages. Firstly, when determining coefficients of affine transformation by the method of least squares orthogonality of obtained transformation is not guaranteed. In other words, the features which had a squared shape on the source map may become parallelograms with angles differing from right angles on ObjectLand map. Secondly, if control points are irregularly distributed on the tablet sheet, inaccuracy of affine transformation will also be irregular. Transformation will be more precise in the areas where control points lie more densely, in the areas which contain few control points transformation inaccuracy will be higher.
The procedure of adaptive calibration does not have these disadvantages. It requires usage of nine control points placed evenly on a rectangular or square tablet of the digitizer. Besides, calculation of coefficients of affine transformation is performed so that built transformation retained the angles and proportional ratios of segments.
To execute adaptive setting it is necessary to prepare the source map having chosen nine points on it located on crossings of three equidistant horizontal lines and three equidistant vertical lines, as it is shown at figure 21-9.
Figure 21-9. Selection of points for adaptive calibration
The points should be numbered from 1 to 9 in the order shown at the figure, for each point its coordinates in GDB map coordinate system should be known. In practice it is easier to use line crossings of coordinate grid of the source map.
The source map is fastened at the tablet of the digitizer. There is no necessity to stick to strict parallelism between sides of the marked rectangle and sides of the tablet.
After pushing the button Adaptive in the dialog box “Digitizer adjustment” a dialog box “Adaptive calibration” shown at figure 21-10 is opened.
Figure 21-10. Dialog box “Adaptive calibration”
This dialog box differs from the similar dialog box “Fast calibration” (see figure 21-7) in the fact that the list of control points already contains nine numbered points, though their coordinates are not specified. It is impossible to add more points to the existing nine ones, that is why the button Add is absent in this dialog box. To specify coordinates of a control point it is required to select it in the list and then push the button Change (or push the button with number from 1 to 9 in the right part of the dialog box which will lead to the same result). The button Remove cancels coordinate values of the point entered before, but the number of the point in the list is retained.
Specifying of coordinates of control point is performed in the dialog box “Coordinates of control point” exactly in the same way as in fast calibration. The user should, however, pay attention to the fact that nine points selected in advance, as it was mentioned above, were used for calibration and that the position of the point at figure 21-9 corresponded to its number in the list of control points.
The user can specify less than nine control points (but not less than four), however, the less number of points was used for adaptive calibration, the greater statistic inaccuracy in this case will be. In any case, for each control point used its position on the source map should correspond to the number of the point in the list.
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