When only a partial cache is available at a detailed level, you can provide users with a seamless rendering experience using one of the methods below.
Use Data not available tiles
If an area of the map is missing tiles or has not completed caching, you can either choose to create tiles on demand or configure your web server to return Data not available tiles.
For REST-based applications, like those built with the ArcGIS Maps SDK for JavaScript, you can configure Data not available tiles by placing an empty tile (missing.png, missing.jpg, or blank.png) at the root level of the cache. The server will default to this tile if a tile cannot be found in the cache. Follow the steps below to configure a Data not available tile.
- Create an image of the same dimensions and image format as the tiles in your tiling scheme.
- Name this image either missing.png or missing.jpg.
Note:
If you need a clear blank tile for overlay services that require transparency, name the image blank.png. - Save the image you have created in the _alllayers folder for your map service.
The following is an example path: opt/arcgis/server/usr/directories/arcgiscache/MyMapService/Layers/_alllayers/missing.png
Once the image has been saved to the all layers folder, the server will automatically replace missing tiles with the Data not available tile at the root level of the cache.
Caching on demand
Alternatively to using Data not available tiles, you can configure your cache to create tiles when an end user navigates to an un-cached area of the map. Caching on demand is recommended for displaying areas of the map that are less commonly visited that you are not able to cache in full, either due to time or storage constraints.
When you have enabled on-demand caching, the ArcGIS Server site will draw tiles dynamically when a user navigates to an area that isn't cached. Dynamically drawing will take longer than a regular request, as the ArcGIS Server site will need to dynamically generate a group of tiles. This group of tiles will either be 2048x2048 pixels if the map service uses anti-aliasing or 4096x4096 if it does not. The process of dynamically generating caches on demand is a single threaded process and will not use all instances of the caching geoprocessing service. These factors are important to keep in mind when preparing your map to dynamically draw large areas at an acceptable speed.
When implemented well, on-demand caching can save you time and disk space. Maps will often contain areas that are barren, unusable, or uninteresting to the map audience, and as such, caching on demand allows users to still view these areas if they choose to, without you needing to create or store them in your cache. Once tiles have been generated, subsequent map viewers will not have to wait for tiles to be created for these areas.
While on-demand caching is a useful feature, it is important to keep in mind that it can negatively impact performance when not implemented properly or when it is overused.
Note:
Tile content generated using on-demand caching will not be reflected in the cache status of the map or image service. Use the Manage Map Server Cache Status tool with the Manage Mode parameter set to Rebuild cache status to update the cache status.
Resampling
When publishing a tile layer, you can decide not to generate tile caches for areas that are not visited often or for detailed scales to save disk space and reduce the time it takes to generate the cache. In these instances, when a user pans and zooms to these areas, the client application will render resampled content.
Resampling is the process of using existing tile caches to fill areas that have not had tiles generated. The resampled content is based on the closest level of detail (tiles) available for the layer. This process can provide a smoother experience for users but can result in blurry images, particularly when the level of detail being resampled is far from the source level of detail. For example, if the maximum scale where cached tiles are present in a specific area of the map is 1:5000 and a user zooms to 1:100, the resampled cache will loose clarity and accuracy.
If you find that users regularly zoom past the minimum and maximum scales you have defined for the tile layer's visible range, it is recommended that you increase the visible range and build the tiles at larger scales.