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Calibration of Maps

Accurate calibration of geographic maps is a major challenge, being rather complicated since the earth is not a perfect sphere. In order to keep things simple TGeoMap makes a compromise which provides suitable, calibrated graticules for most maps used in common applications (i.e. as long as the scale of the map is not too high). Along with common projections such as the UTM projection, the conic conformal, or the equirectangular projection, TGeoMap provides a general biquadratic regression model. The lines of the graticule are approximated by parabolic equations, which are accurate to half a screen pixel in most situations. However, the parabolic approximation cannot be used in polar regions with large map scales (where the parallels show up as complete circles or ellipses).

Depending on the calibration model the calibration grid is calculated either by means of linear regression from points along the parallels or the meridians, or by biquadratic regression from at least seven different calibration points (which need not lie on the graticule). The methods AddCalibPoint, RemoveCalibPoint, and CalibData provide supports for creating a graticule. The resulting graticule can be stored by the routines SaveCalDataAsXML or by WriteCalDataToOpenXMLFile. The example program SIMPLEGEOMAP.DPR shows how to calibrate a map programmatically.

Hint: To our experience many maps, especially the older ones are often poor in their precision. It is quite common that some part of a map (e.g. a lake) is displaced by several millimeters (which comes to several kilometers in reality if the scale is sufficiently large). In addition, many maps do not contain accurate graticule lines. So be careful when calibrating maps. If you are in doubt about the precision of a map, you should rely on the data from a GPS (global positioning system) device.

Last Update: 2013-May-14