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I've noticed that lots of map viewer (especially those nautical chart viewer) will shrink a mercator projected chart while panning towards poles.

Is this some compensation for mercator distortion?

Is this a dedicated projection?

How to implement that?

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you shouldn't crosspost in all possible sites/mailing list's... it is not a correct approach: 1 & 2 –  A.R Feb 4 '13 at 9:41
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Formulas for the scale of a Mercator projection are given in the answer at gis.stackexchange.com/a/40659. Concerning your last question, what platform do you intend to use for the implementation? –  whuber Feb 4 '13 at 14:23
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Here is a little puzzle that will illustrate what google mercator does. Mercator puzzle –  Brad Nesom Feb 4 '13 at 16:19
    
I'm thinking that the shrink is to make the scale always the same along latitude, so , say (px, py) is normally projected coordinate, to get it shrinked, use (x, y) = (pxcos(lat), pycos(lat)) –  feverzsj Feb 5 '13 at 0:44
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If the Mercator implementation supports a standard parallel, they may be changing it to match the screen's central latitude. That will "shrink" a Mercator-based map. –  mkennedy Feb 7 '13 at 21:08

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