[http://www.bayo.com/]
Why and how to calibrate a coordinate map? To easily plot a position (obtained from a GPS receiver) on a map, the relationship between geographic coordinates and the corresponding position on the image (whether in paper or electronic form) has to be established. It is assumed that the map (on paper) has a grid system or is linked to a calibration algorithm (in the case of a computer file). Well-designed non-professional software and most professional applications perform that calibration by connecting the image file (bmp, jpg, tif or other extension) to a so-called "header" text file in a standard format with the same name as the image file but with a specific extension (usually bmw, jpw or tfw) and containing following data: 2.500 (pixel size in X direction) 0 0 -2.500 (pixel size in Y direction) 620001.250 ("East" or "X" coordinate of the center of the upper left image pixel) 2449998.750 ("North" or "Y" coordinate of the center of the upper left image pixel) Fieldwork to be performed Coordinates of well-identified details on the map will have to be determined in the field. The number and definition of the objects to be measured (corners of buildings, road or street crossings, lot boundaries, etc.) depend on the map type and on the required accuracy. Specific difficulties The quality of the calibration depends on the quality of the initial document. Header files are only valid for one particular map projection. For accurate applications, the map projection as well as the transformation parameters between the WGS84 and the geodetic system must be known.
Calibrating coordinate maps
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Grid on an IGN map
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