Working with Raster Data - Desktop GIS: Mapping the Planet with Open Source Tools - page 65

4.3 Intelligent Rasters

All rasters are intelligent—at least in the sense that they convey infor-
mation. So far we have looked at rasters (DRGs and our world mosaic)
that are useful for general viewing or as a background layer. Typically
these rasters have cell values that don't really mean anything—they
are just a value used to determine how each pixel should be colored.
Cell values in a raster (or grid as these are often called) can be either
integer or floating point, depending on the capabilities of your software.
You might be wondering what sort of data can be represented by a
raster. The answer is anything you can count, measure, or identify for
a defined area on the ground. Some examples include the following:

• Rock or soil types, specified by a unique numeric code for each
• Vegetation types
• Elevations
• Quantity of an element such as gold or silver, determined by sam-
pling and analysis

Why not use a vector layer rather than raster to delineate data by
“type”? Oftentimes it's appropriate to use a vector layer, as we did for
our geologic map in Figure 3.16 , on page 58 . Here each rock type is
represented by a polygon and rendered by value. It really depends on
what you need to accomplish with the data. In the case of a grid con-
taining quantities that we want to use in an analysis, the raster model
is the right choice.

Let's look at a couple of examples of what we might term a “smarter
raster,” in this case a Digital Elevation Model (DEM) and a grid contain-
ing measured quantities of silver.

Digital Elevation Models
The cells in a DEM contain an elevation value. For any location on the
DEM, we can determine the elevation by examining the value of the
cell. The smaller the size of the cells, the more resolution you will get
from the DEM. We can do a number of interesting things with a DEM,
including the following:

• Display it as is for a backdrop.
• Create a shaded relief to display terrain.
• Perform arithmetic operations on the individual cells to create new
values.
• Create contours from the elevations.