Database Reference
In-Depth Information
FIGURE 3.1
An example raster map.
hue, brightness, and saturation; or area representations where pixels encode specific
aspects of the area, such as land cover with pixel values perhaps indicating trees,
scrub, water, and so on; or a height, where the pixels may represent heights. The latter
form of raster is also known as a digital elevation model (DEM). Digitized maps are
also represented as rasters. Figure 3.1 shows a typical raster map for which the image
becomes pixelated if enlarged.
Because the pixels are spatially related, it is possible to perform sophisticated
spatial analysis on raster data. Attempting to perform analysis on many raster maps
is more problematic as the maps have often been generalized and do not accurately
relate to the surface of the earth.
3.3.1.2 Vector
Vector data represents GI as points, lines, and polygons. Although other representa-
tions are possible, these three are the fundamental vector forms and the most common.
These vector forms are mapped to Earth's surface using coordinates that also have
attribute information associated with them to describe the object being represented.
The position of an archaeological find could be represented as a coordinate with
associated attribute data identifying the find and perhaps the date of discovery and
the estimated age of the object. Real-world objects
2
can be represented by one or more
vector objects and forms depending on the complexity and resolution of the model.
A building could be represented as a single point, one or more lines, or one or more
polygons and indeed any combination of point, line, and polygon. An example of how
a simple building may be represented at various levels of detail is shown in Figure 3.2.
Point
Four Lines
Polygon
Complex Polygon
FIGURE 3.2
Building represented by a point, lines, and polygons.
