For online tiled raster layers such as WMTS, TMS, XYZ or OSM, the images are assembled from many small image tiles retrieved across the network.  In order to speed up displaying parts of images that have already been viewed, and to save network bandwidth, these image tiles are cached and re-used if the required tiles are already in the cache.

Once the cache exceeds its configured maximum capacity, the tiles which have not been used for the longest period of time are deleted from the cache.

Note that Tile Grid layers in local data stores are not included in the cache, even if they were originally downloaded from online raster layers.

To change the maximum cache size:

  • Enter the new size (in megabytes) in the ‘Cache Maximum Size (MB)’ field

Some online layers may change from time to time.  If it become necessary to be sure that the latest version of the online data is being viewed and not the data from the cache, the cache can be emptied by tapping the ‘Purge Cache Now’ button.