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dou wen
2005-04-19
2013-04-17
• dou wen - 2005-04-19

hi, i am a new user of the wonderful qpegps, i must say, it is very cool,and work very well .
but when i study the code to see what i can do some for it , i encounter some problem:
it took me a longtime to study the algorithm of caculating lat. and long.  from xy and vice versa.
i still could not say i understand it completely  now. could you give me  a detial description of it.(or a document about it?)

BTW:
Why not use UTM coord. to calibrate map directly?

Is it possible to give a more stronger calibration algrothim with multi stand point other than only 2 point?

• Ralf Haselmeier - 2005-04-19

1.: the datum describes the a model of the earth (e.g. WGS84...). Qpegps works if the datum of the map is the same as the datum of the positions send by the gps receiver; else qpegps needs to convert the positions ...
2.: the "projection" is the algorithm which is used to display the 3 dimensional world on a 2 dimensional map => for the projections we need the corresponding parameters and either a correct scale or 2 points to show the position on the map.

UTM (universal transverse mercator) is a transverse mercator projection with a special way of writing positions instead of longitude and latitude (zones and distances relative to the zones).

"Calibrating maps" is used in programs which are doing interpolations of several reference points instead of using a projection algorithm.

That is the reason why qpegps works best, if you know the used projection (e.g. Lambert) and its parameters. For small scales "Linear" will work good. ("Fritz" is no real projection algorithm and is based on distances (just a workaround for unknown projections)).