Getting information from rasters

From an elevation raster, you can derive new rasters of slope, aspect, and hillshade (surface illumination). You can also derive viewshed rasters, which tell you which parts of the surface can and cannot be seen by an observer stationed at a given location. These same techniques can be used on TIN surfaces, with the output again being a raster.

Reclassification is the systematic replacement of one set of raster values with another set according to your specifications. Reclassification has several uses. One is to replace natural values with preference values. For example, you could replace all slope values between 0 and 10 degrees with values of 1, where 1 is a preference code meaning “suitable for building.” Reclassification is also a good tool for simplifying data that you want to convert to features.

Calculating surfaces

Many new surfaces can be derived mathematically from elevation—slope, aspect, and hillshade are the most common. In ArcGIS® 3D Analyst™, you can create these surfaces from TINs or rasters—the output in either case is a raster.

Contour lines are not strictly surfaces, but rather a traditional vector technique for representing elevation and other phenomena such as temperature and barometric pressure. 3D Analyst can generate contour lines from TINs or rasters.

Reclassification is the replacement of an existing set of raster cell values with a new and generally smaller set of values. The purpose may be to simplify the original surface for further analysis or to evaluate it by some standard (such as assigning preference values to areas).

Calculating surfaces with observers

Like slope, aspect, and hillshade, viewshed is a surface that can be derived from an elevation TIN or raster. Unlike slope and aspect—but again like hillshade—viewshed is a relationship between an elevation surface and another element. In a viewshed analysis, this other element is an observer and the analysis reveals which parts of the surface the observer can see.