Tiles and gridsets

Tiles

GeoWebCache caches images retrieved from a WMS. The smallest unit of image cached is known as a tile. All tiles are assumed to be the same dimensions and are typically square (i.e. 256 pixels by 256 pixels). The tiles are stored in a rectangular grid, indexed by (x,y) coordinates. A z coordinate (zero-indexed) is used to denote the zoom level, resulting in each tile being indexed as a triplet (x,y,z).

Gridsets

Gridsets and gridsubsets refer to the spatial reference system of the layers served by GeoWebCache. When GeoWebCache makes a request to a WMS, it uses the gridset and gridsubset information to convert its internal tile index to a spatial request that the WMS will understand.

Composition of a gridset, source: http://3.bp.blogspot.com/_0_xIiXP5xuY/S5pEpCjenaI/AAAAAAAAAKY/PDKTGZ6vzGI/s1600-h/Image_Pyramid.gif
Composition of a gridset, source: http://3.bp.blogspot.com/_0_xIiXP5xuY/S5pEpCjenaI/AAAAAAAAAKY/PDKTGZ6vzGI/s1600-h/Image_Pyramid.gif

A gridset is a global definition (i.e. not layer-specific) specifying:

  • A spatial reference system.
  • A bounding box describing the extent, typically the maximum extent for the above reference system.
  • One of either a list of scale denominators, resolutions, or zoom levels.
  • The tile dimensions in pixels (constant for all zoom levels).

A gridsubset is a layer-specific definition specifying:

  • The gridset for the layer.
  • (Optional) The bounding box for that layer (which must be a subset of the extent of the gridSet).
  • (Optional) A list of zoom levels (which must be a subset of what is defined in the gridSet).

For further instructions have a look at the source of the above explanations, here.

Configure a new gridset

So, our first step will be to create a new gridset:

  • Go to Tile Caching ❭ Gridsets

  • Click Create a new gridset to create a new gridset and use the following options for the creation:
    • Name: momo-4326
    • Coordinate Reference System: Use the find-button to select EPSG:4326
    • Gridset bounds: Click Compute from maximum extent of CRS
    • Tile width in pixels: 512
    • Tile height in pixels: 512
    • Define grids based on: Select Scale denominators
  • Click Add zoom level to create a new zoom level. Enter the scale 300.000.000 and the name 0.
  • Once again click Add zoom level. You will see that the scale value is automatically cut into halves (150.000.000). Just enter the name 1 and repeat this step until you reached a total count of 8 zoom levels. The last scale value should be 2.343.750.

  • Click Save.