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Dean Arnold


Geo::Mercator - Compute Mercator Projection of latitude/longitude into meters


    use Geo::Mercator;
    #    get projected meters for bounding box coords
    my ($minx, $miny) = mercate($minlat, $minlong);
    my ($maxx, $maxy) = mercate($maxlat, $maxlong);
    #    now we can generate horizontal and vertical 
    #    meters-per-pixel scales
    #    once we've got an image, we might want to
    #    translate back (e.g., an interactive map)
    my ($lat, $long) = demercate($x, $y);


Computes Mercator Projection of input latitude/longitude coordinates (in degrees) into (x, y) coordinates (as distances in meters) from the meridian/equator. Also provides a method to convert back into latitude, longitude.


Use of Mercator projection on latitudes above/below +70/-70 degrees is strongly discouraged, due to the gross distortions of the projection. In fact, any use of the Mercator projection is strongly discouraged...but its the view we've all been programmed to see for 400 years, so its genetic sense memory by now. And if its good enough for Google, its good enough for me.

This module was developed primarily for graphic rendering purposes. The returned distance values should not be used for navigational purposes. They are only useful for certain mapping operations, e.g., rendering a map to scale. For better approximations of actual distances, consider Math::Trig's great_circle_distance() function instead.

There are many other packages available on CPAN to compute various (and probably more accurate) geographic projections, but many rely on external libraries, or are kitchen sink packages. I just needed a simple, pure Perl interface for Mercator Projections, so, in the spirit of "Simple things should be easy.", I whipped up this simple module.


All methods are package methods, and are exported by default.

($x, $y) = mercate($lat, $long)

Converts the input latitude and longitude values (in degrees) into Mercator projected points (in meters) from the equator/ meridian.


  1. Values are returned in opposite order from the input ($x => $long, $y => $lat), due to the difference in common conventions for GIS ordering vs. cartesian ordering.

  2. Both input and output values may be negative. Negative latitude is south of the equator; negative longitude is west of the meridian.

($lat, $long) = demercate($x, $y)

Converts the input Mercator projected distances (in meters) back into latitude and longitude values (in degrees). Note that, like mercate(), the outputs are in opposite order from the inputs, due to GIS vs. cartesian ordering conventions.


The Mercator Projection is described at http://en.wikipedia.org/wiki/Mercator_Projection (and at many other sites as well).

GD::Map::Mercator uses this module to translate WDB data.

Any of the Geo packages at http://search.cpan.org/search?m=all&q=Geo&s=1.


Copyright(C) 2008, Dean Arnold, Presicient Corp.

Permission is granted to use, copy, modify, and redistribute this software under the terms of the Academic Free License Vers. 3.0, available for review at http://www.opensource.org/licenses/afl-3.0.php.