Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition

Vector vs. Raster Representation

The two preceding figures show the difficulties involved in representing the virtually infinite, three-dimensional environment in the memory of a computer, necessarily using only the most fundamental discrete symbols: 0s and 1s. In vector mode, a stream is represented by one-dimensional lines; the lines have no width, only length. If quantities like flow, width, or velocity are to be included, they must be part of the attribute table.

In raster mode, a stream is represented by a sequence of adjacent cells. These cells are two-dimensional—they cover area. The area each cell covers, in basic hydrologic analysis, is the same, whether a mountain creek or a major river is being represented. Again, the geographic representation is only an approximation; even information about quantities such as width must be carried along separately.

This confluence of vector representation and raster representation in storing and displaying information about streams illustrates the challenges of using a computer to represent natural phenomena. Next, an attempt is made to represent the relative “size” of streams and stream channels.