ESRIC – Esri Compact Cache¶
Driver short name
Driver built-in by default
This driver is built-in by default
Read Esri Compact Cache V2 as a single raster
This driver supports georeferencing
This driver supports virtual I/O operations (/vsimem/, etc.)
Esri Compact Cache V2¶
A reader for Esri Compact Cache V2. The cache is stored in multiple files located in a specific folder structure. From the point of view of this driver the raster is represented by the file named conf.xml, which resides in the root folder of the cache. The exact content of this XML file is not fully documented by Esri, and is subject to change. This driver uses only a few of the XML fields, as necessary to read the raster.
If the /path/Layers contains an Esri Compact Cache in V2 format in the normal Web Mercator tile grid, this command will copy the level 2 content into a TIF file.
gdal_translate -outsize 1024 1024 path/Layers/conf.xml output.tif
Features and Limitations¶
Only V2 Compact cache is supported. This format is identified by the value esriMapCacheStorageModeCompactV2 in the CacheInfo.TileCacheInfo.StorageFormat element of the conf.xml file. Other legacy Esri cache formats are not supported.
Only caches with tiles of 256 by 256 pixels are supported. These are stored as the values of the CacheInfo.TileCacheInfo.TileRows and CacheInfo.TileCacheInfo.TileCols elements.
Tiles in the cache can be in different formats, including JPEG and PNG. The most common are 8 bit color or grayscale JPEG and color PNG with or without opacity. These caches will have the value JPEG, PNG or MIXED in the CacheInfo.TileImageInfo.CacheTileFormat element. Other tile formats, such as PNG8 are not supported. The raster will be promoted to RGB or RGBA by this driver, even if the source is grayscale.
Only caches with 128 by 128 tiles in a bundle file are supported. This value, the linear number of tiles in a bundle, is contained in the CacheInfo.CacheStorageInfo.PacketSize element.
The spatial reference is obtained from the CacheInfo.TileCacheInfo.SpatialReference.WKT field. If this field is not present, the default value of EPSG:4326 is assumed.
As defined in the standard conf.xml file, the cache tile grid has an origin location, stored in the CacheInfo.TileCacheInfo.TileOrigin element, which has X and Y components. The cache size is not explicilty defined. The ESRIC format driver will assume a symetric area around the 0,0 coordinates, in the reference system coordinates. This is true for the standard Web Mercator and GCS reference systems. If this assumption is not valid, the conf.xml file can be modified, adding a non-standard CacheInfo.TileCacheInfo.TileEnd element with the appropriate values for the X and Y components, similar to the TileOrigin. Note that the grid origin and size is different from the raster data extent. For example, it is common to generate a cache in the standard Web Mercator tile grid, which is defined for the whole Earth, even if data is only available for a small region of the globe.
A bundled cache has multiple resolution levels, encoded in a series of CacheInfo.TileCacheInfo.LODInfos.LODInfo nodes. The LODInfo XML nodes are identified by the LevelID value, which have to be successive, starting with 0 for the level with the largest value for the Resolution and increasing towards the LODInfo node with smallest Resolution value. The level convention is similar to other Level, Row and Column tile addressing schemes used in WMTS for example. While common, it is not a requirement to have the resolutions of two successive levels differ by a factor of two. Each cache level will be read as an overview, at the appropriate resolution. The resolution values are in spatial reference distance units per pixel.
Many caches are built level by level from different sources, similar to many web map tile protocols supported by the GDAL WMS driver. This means that the content of a specific level might be from a different source than an adjacent one, or content for a specific level can be missing altogether. This driver will return opaque black when reading areas which do not have tiles in the cache at a given resolution level, even if data does exist at other levels at the same location.
A cache can exceede the maximum size supported by GDAL, which is INT32_MAX, in either dimension. This driver will generate an error when opening such caches. Removing the LODInfo nodes with the highest LevelID from the conf.xml file until the raster size drops below INT32_MAX is a possible workaround, but the highest resolution levels will not be read.