[OSM-dev] Plotting a point on a tile

Rogier Wolff R.E.Wolff at BitWizard.nl
Fri Aug 15 09:04:19 BST 2008

>   Assume: Latitude = 52.478624 and Longitude= -1.913584 which is the
>   centre of Birmingham, UK.
>   Using the code supplied on
>   http://wiki.openstreetmap.org/index.php/Slippy_map_tilenames#Tiles
>   I can calculate that at zoom level 18, my x and y tiles are 129678 and
>   86020, so I get a tile URL of
>   http://tile.openstreetmap.org/mapnik/18/129678/86020.png
>   Now obviously the map tiles don't line up exactly with my coordinates,
>   so I use the formulas on the wiki to get the tile edges: north
>   east=(52.4794347,-1.9129943) and SW=(52.47859,-1.91436).
>   I think I need to offset the tiles pixel location on screen by a
>   certain amount so that I get my required point on the centre of the
>   map, but this is where I'm getting stuck.  Using the latitudes from
>   these I can calculate that each tile is 92.942metres wide at zoom 18,
>   or 0.363m per pixel - does this help?

Your tile spans .0008477 degrees. 

Your point is .000034 degrees from the south edge, which is 4.025 % of
the range of the tile. which, times 250 (pixels per tile) = 10 pixels. 

Same for east-west. 


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