GIS Primer

Geographic Information Systems are combinations of computer hardware and software, designed around digital databases that store, manipulate, capture, analyze, create, and display spatially referenced data. These data are stored as digital maps, with each map theme in its own layer. A layer for roads, a layer for soils, etc. The GIS can then, in a kind of electronic overlay, query, manipulate, and combine the various layers to create maps aimed at a specific purpose (Figure 1).

Based on how they store data, GIS can be divided into two basic formats, vector and raster. Vector GIS store spatial data as points, lines, or polygons, much like traditional paper maps. The superior graphics and database connectivity of vector systems make them ideal for cultural resource management, or when cartographic output is a major concern. Figure 2 shows the different elements in a vector GIS, with soils represented as polygons, drainages as lines, and archaeological sites as points.

Raster GIS have abandoned the point-line-polygon standard for a spreadsheet like format with rows and columns into which numbers are placed. These rows and columns are tied to x, y coordinates with the intersection of each row and column forming a cell which corresponds to a specific area in the real world. These cells contain numbers that can represent anything from elevation, to soils, to archaeological sites. Thus, maps in a raster GIS are like big sheets of numbers that can be added, subtracted, multiplied, etc., to form new maps. Figure 3 shows the same soil map seen in Figure 2, but now in a raster format. Notice the blocky appearance, as the smooth lines of the vector have been replaced by a matrix of cells.

One type of map common in raster systems, and very important to this study, is the digital elevation model, or DEM. Traditionally, maps have indicated changing elevation with topographic contours (Figure 4), but in a raster format elevation change is indicated by changing "z" values in each cell. The result is that elevation in a DEM changes continuously across the surface (Figure 5), creating a more realistic model than can be accomplished with topographic contours. These elevation surfaces can be manipulated to create new types of maps. For this study, maps detailing slope and slope length were derived from the DEM of the Umeiri region.