An auxiliary file (.aux or .aux.xml) accompanies the raster in the same location and stores any additional information that cannot be stored in the raster file itself.
An auxiliary file can store many different types of information:
- Color map
- Statistics, histogram, or table
- Pointer to the pyramid file
- Coordinate system
- Projection information
Statistical information for the raster dataset is stored in the auxiliary file if it cannot be stored internally. It will be created automatically the first time statistics are required to perform a task on your raster layer. It might take longer to create this file for larger raster datasets, because more cells need to be accessed from which to derive the statistics. However, once the auxiliary file has been created, the statistics within it will be reused for future procedures that require statistical information.
The auxiliary file will also store a pointer to the pyramid file (either .rrd or .ovr) if pyramids have been created for your raster dataset. If you use the operating system to move the raster from its directory after pyramids have been built, the software will look in the location the pointer indicates to find the pyramid file. If it cannot find the pyramid file there, it will look in the directory into which you have moved the dataset. It is recommended that you copy or move datasets using the Contents pane to make sure you copy all related files.
The auxiliary file will be created in the same directory as the source dataset if you have write access to the directory in which the dataset resides and to the dataset itself. If you do not have write access to both the dataset and the directory in which it resides, the auxiliary file will be written to the default proxy file.
The information stored in an .aux file is only accessible using a product from Esri, ERDAS, or a third-party product derived from the RDO/ERaster library.