What problems occur when working with A.TOC files in ArcGIS?

Answer

Note:
This article pertains to ArcGIS versions 8.x and 9.x. Later versions of ArcGIS may contain different functionality, as well as different names and locations for menus, commands and geoprocessing tools.

Although A.TOC files are image catalogs in nature and construction, they are handled as a single raster dataset in ArcMap. Under this functional assumption the A.TOC file is expected to possess a consistent set of properties among all the data it references. This means all of the A.TOC file’s contents are expected to have the same cell size and color map. However, A.TOC files do not always contain the same properties for each raster dataset. There are several situations where A.TOC files are not supported by ArcGIS.

• The A.TOC file references data with different cell sizes. This occurs when 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 products. For example, if an A.TOC file references both GNC and JNC CADRG products, they will not work. GNC and JNC, by specification, have different color maps to meet the needs of the intended use.
• The A.TOC file references data with relative paths. If for any reason the data or A.TOC files are moved, the relative paths are broken.

The above situations are the basis for the recommendations that A.TOC files are avoided in production systems. While A.TOC files appear to be convenient solutions, they may cause significant problems in a production environment. As such, the most efficient method to utilize the A.TOC file's data would be to directly reference the RPF tiles.

There is also a performance advantage to referencing RPF tiles directly in Image Server. Image Server will search reclusively through the RPF file structure, read the necessary pixels, perform the mosaic, send out the result, and close the file. If the A.TOC is referenced, Image Server will perform the aforementioned operations over the entire A.TOC file; which would result in a performance degradation, because it is less efficient than doing so on the smaller RPF tiles.