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# hrldcpr / mercator.py Public

Python utilities for Google Maps-style Mercator geometry

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These functions are helpful for dealing with Google Maps tile geometry on the serverside, i.e. without the benefit of the Google Maps Javascript API.

For example, to find the lat/lng bounding box of the top-most tile in the standard Google map, which is tile (0,0) at zoom-level 0, you would do:

``````> python
>> import mercator
>> mercator.get_tile_box(0, 0, 0)
(-85.05112877980659, 85.05112877980659, -180.0, 180.0)
``````

So, the entire Google map has the full range of longitudes, but only goes to +/-85.05 degrees latitude, which is to be expected since the Mercator projection maps +/-90 degrees latitude to +/-infinity.

You can verify that this library agrees with Google Maps API using their Javascript API's Projection.fromPointToLatLng().

For example at http://code.google.com/apis/maps/documentation/javascript/examples/map-coordinates.html if you paste this into your location bar:

``````javascript:alert(map.getProjection().fromPointToLatLng(new google.maps.Point(0, 0)))
``````

then you'll see the coordinate of the top-left part of the map, which again is:

``````  (85.0511287798066, -180)
``````

Back in Python, we can see what tile (0, 0) at zoom-level 1 looks like:

``````>> mercator.get_tile_box(1, 0, 0)
(0.0, 85.05112877980659, -180.0, 0.0)
``````

As we would hope, it is the top-left quadrant of the map, since at zoom=1 there are (2^zoom * 2^zoom) == 4 tiles.

Python utilities for Google Maps-style Mercator geometry

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