Re: image scale for sky map

Google's GScaleControl for the Earth calculates the scale that applies
to the centre point. Because the maps use a Mercator projection, the
scale at different points is different. At any particular point, the
vertical and horizontal scale is the same. Attempting to calculate the
scale across the entire map is meaningless.

What you can do is calculate the pixel location of the centre point, add
one pixel to the horizontal component and convert back to lat/lng.
Subtracting the lng values tells you how many degrees there are per
pixel. Multiply by 3600 and you get the number of arcseconds per pixel
at the centre.

Repeating that process for height will give you almost exactly the same
result.

--
Mike Williams
http://econym.org.uk/gmap

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