Overlay Functions One of the powerful analytical tools of a GIS is the process of combining information by overlaying two data layers. All sorts of boolean logic and mathematical and trigonometric functions can be applied to these overlays. The wonder of it all is that it's a complete topological overlay, so that the new information takes on all the characteristics of the old. Many of these tools probably belong in one of the above categories, but what they have in common is that they all act on the digital equivalent a topographic map - the digital elevation model of DEM . Data models encourage question-driven GIS analysis, identify what data are required, and provide a pathway through the wilderness.
Data dictionaries provide appropriate documentation for digital data.
In fact, a coverage becomes a further sub-directory that contains a number of files related to registration, attributes, and topology of digital data depicted in the map. We cannot access all these files, but three that we can are very important. This file is very important when registering two coverages or when changing map projections. Initially .AAT files contain 'to' and 'from' node numbers, right and left polygon numbers, length, internal and User IDs for each arc feature. This file is also often expanded to include other attributes.