[Proj] Calculate map resolution / LCC

[Mikael Rittri]

Hello Achim.

I would calculate the resolution like this:

Take the two calibration points that are farthest away;
let's call them P and Q.

1) Compute the true distance in meters between them:

  a) Either take the long/lat coordinates for P and Q,
   and use for example http://geographiclib.sourceforge.net/cgi-bin/Geod

  b) Or else, take the projected coordinates (if you have them),
   and just use the theorem of Pythagoras to calculate the
   distance in meters. (This assumes that the local scale factor of the
   projection is near 1 in the area of interest, which it seems to be.)

2) Compute the image distance between P and Q, in pixels.
    Just use the theorem of Pythagoras on the pixel coordinates.

If the real-world distance between P and Q is s meters,
while the pixel distance is p pixels, then the image resolution
is
       s / p

meters per pixel.

Best regards,

Mikael Rittri
Carmenta
Sweden
http://www.carmenta.com


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