Tile Services

Overview of a Tile Server

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"

Note: For tile services the row starts at the top (north) and increases going south, which is the opposite from the way that coordinates typically go. This is depicted in the following diagram:

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:

The zoom is set to the zoom level. Zero is the most zoomed out level.
The resolution is determined from the zoom level.
The distance of the x and y from the tile origin is determined (top left corner of the maximum extent of the map).
These distances are converted to pixels by dividing them by the resolution
The distances in pixels are then divided by the tile size and rounded down to determine the row and column.

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:

Zoom is 1 (as the least detailed zoom level is zero).
The resolution is 500 (second item in the list of resolutions).
Distance from origin is 400000 for x and 700000 for y (1000000 – 300000).
These distances in pixels are 800 for x and 1400 for y.
The column is therefore 3 (800 / 256, is 3.125 rounded down) and the row is 5.

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"