Web Settings / Web Map Layer Settings |
Tile services provide a grid of uniformly sized images (typically 256x256 pixels) with a different set of images for each zoom level / resolution.
Each tile is uniquely identified by the combination of row (y) and column (x) of that tile in the grid for the zoom level (z).
The URL for a tile is found by taking these values and substituting them into a placeholder in the "url" parameter provided in the map layer setting. For example, if the url in the setting is set to:
"http://stratus.ondemand.eu/connect/gettile?mapcfg=Main&name= Base&level=${z}&row=${y}&col=${x}&output=image/gif"
Then the URL generated for zoom level 3, row 5 and column 7 would be:
"http://stratus.ondemand.eu/connect/gettile?mapcfg=Main&name= Base&level=3&row=5&col=7&output=image/gif"
In order to work out the column, row and zoom of the tile that need to be specified for a particular map coordinate (x,y) it goes through the following process:
For example, if a Tile server had an origin of 0,1000000 a tile size of 256x256 pixels and resolutions 1000,500,250,125 then the row and column for point 400000,300000 at the second most detailed zoom level would be calculated as follows:
In the above example, this would generate the following URL:
"http://stratus.pbondemand.eu/connect/gettile?mapcfg=Main&name= Base&level=1&row=5&col=3&output=image/gif"