Calculate lat lon bounds for individual tile

[Gavin]

Hi all,

I have multiple tile sources I'd like to present on a map so when
presenting a tile I need to determine which source to serve it up
from, checking against the bounds.

Anyone know how to calculate the lat lon bounds for a single tile
based purely on zoom, x, and y (from the file structure generated by
gdal2tiles)? BTW: I'm using Google Maps API v3 in case there's a need
to call some functionality from the API to help with calculations.

The reason I'm restricted to purely zoom, x, and y is because the
tiles aren't just getting called by an overlay on the map but also
some custom print functionality that allows printing outside of the
map.

Thanks in advance for any help or suggestions anyone has.

Cheers,
Gavin

[Klokan Petr Přidal]

Hi Gavin,

the math is described at:

http://www.maptiler.org/google-maps-coordinates-tile-bounds-projection/

including the source code for command line utility and an online demonstration.

It is also pretty simple math:

function tile2long(x,z) { return (x/Math.pow(2,z)*360-180); }
function tile2lat(y,z) { var n=Math.PI-2*Math.PI*y/Math.pow(2,z);
return (180/Math.PI*Math.atan(0.5*(Math.exp(n)-Math.exp(-n)))); }

Please be sure you understand difference between XYZ/Google vs TMS in
y axis (discussed several times in this mailing list).

Google Maps API V3 gives you the required functions too via
.fromPointToLatLng() of map.getProjection().
Demonstrated for example at:
http://www.maptiler.org/example-usgs-drg-grand-canyon-gtiff/v3.html

Petr Pridal
Klokan Technologies GmbH
http://www.klokantech.com/

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[Gavin]

Thanks Petr, very much appreciated. And thanks for sharing such useful
tools for the community to use! I've given credit to your work on
http://www.topomap.co.nz

I'd got half way there - I'd managed to convert TMS to Google (for y)
but had run out of time before having a go at converting to lat lon
yesterday.

In case it's useful for someone else - to convert y from TMS to
Google:
y = (1 << zoom) - y - 1;

One I've got the lat lon I know there's some v3 API methods to get
pixels, which I can then add 256 pixels to in each dimension and then
convert to another lat lon point to use for my tile bounds.

Thank again!

On Nov 28, 6:07 am, Klokan Petr Přidal <klo...@klokan.cz> wrote:
> Hi Gavin,
>
> the math is described at:
>
> http://www.maptiler.org/google-maps-coordinates-tile-bounds-projection/
>
> including the source code for command line utility and an online demonstration.
>
> It is also pretty simple math:
>
> function tile2long(x,z) { return (x/Math.pow(2,z)*360-180); }
> function tile2lat(y,z) { var n=Math.PI-2*Math.PI*y/Math.pow(2,z);
> return (180/Math.PI*Math.atan(0.5*(Math.exp(n)-Math.exp(-n)))); }
>
> Please be sure you understand difference between XYZ/Google vs TMS in
> y axis (discussed several times in this mailing list).
>
> Google Maps API V3 gives you the required functions too via
> .fromPointToLatLng() of map.getProjection().
> Demonstrated for example at:http://www.maptiler.org/example-usgs-drg-grand-canyon-gtiff/v3.html
>
> Petr Pridal

> Klokan Technologies GmbHhttp://www.klokantech.com/