Geography Reference
In-Depth Information
FIGURE 8-33 Stream Channels
shows five stream links. Each cell has a flow accumulation of greater than 7.0. These cells are considered
to make up streams. Each stream segment is uniquely numbered, as shown by the color coding.
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.
Assigning Orders to Stream Links
You can attach an order number (integer value) to each stream segment or link. Generally, streams with
lower numerical values have a size that can carry smaller volumes of water, but this is not always the
case, as you will see.
 
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