Re: Converting Pixels to LatLong

That depends on the projection used to obtain the map image.

This post is wrong from this point on.

If you draw a three points on the map such that
A_y == B_y && A_x != B_x
A_x == C_x && A_y != C_y

+=====================+
#                     #
#   A-------------B   #
#   |                 #
#   |                 #
#   C                 #
#                     #
+=====================+
[download]

Is A_lat == B_lat && A_long == C_long?

If so, every pixel represents the same long and lat difference.
Pixel_lat = (B_lat - A_lat) / (B_x - A_x)
Pixel_long = (C_long - A_long) / (C_y - A_y)

If not, the math gets a bit more complicated, but it's still pretty straightforward. You'll probably need a fourth point D such that
D_x == B_x && D_y == C_y

+=====================+
#                     #
#   A-------------B   #
#   |             |   #
#   |             |   #
#   C-------------D   #
#                     #
+=====================+
[download]

There surely exists modules to help you here.

Comment on Re: Converting Pixels to LatLong Select or Download Code

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Re^2: Converting Pixels to LatLong by akho (Hermit) on Feb 14, 2007 at 19:19 UTC
No. If the Mercator projection was used, then A_lat == B_lat && A_long == C_long, but pixels near the poles and pixels on the equator represent very different long values. It's hard to find out which projection was used if you only look at a finite number of points. Finding a map with a known projection for use in the program seems a better way to do it.	[reply]
Re^3: Converting Pixels to LatLong by ikegami (Patriarch) on Feb 14, 2007 at 19:26 UTC
Shoot! I mixed up distances with long/lat somewhere along the way.	[reply]