A.TOC files are image catalogs in nature and construction; however, they are handled as a single raster dataset in ArcMap. Under this assumption, the A.TOC is expected to have one set of properties. For example, there can only be one cell size and one color map. But A.TOC files are not always created to contain homogenous raster datasets, therefore, they are not fully supported in ArcGIS.
There are two typical situations where A.TOC files are not supported by ArcGIS:
- The A.TOC file references data with different cell sizes. This occurs when parent directories of RPF data contain subfolders of data in different latitudinal zones. Each latitudinal zone has a different cell size.
- The A.TOC file references two kinds of RPF product. For example, if an A.TOC file references both GNC and JNC CADRG products, it will not work. According to their specifications, GNC and JNC CADRG products have different color maps to meet the needs of the intended use.
Another issue when using A.TOC files is they reference data with relative paths. If for any reason the data or A.TOC files are moved, their connections to the source data will be broken.
Due to these reasons, it is generally recommended that A.TOC files be avoided in a production system. Referencing the RPF tiles directly is the most efficient way to use this raster data source in ArcGIS.
Additionally, when creating an image service, there is a performance advantage to referencing RPF tiles directly. This is because if the image service is referencing one large file, like an A.TOC, instead of a couple small files, like individual RPF tiles, there will be a performance drop.