and indexing formats for recording the GI representation with information-
processing technology. This section introduces each representation type, dis-
cusses how it used to represent things and events, and explains how, in very
general terms, it is stored in a GIS. This section also introduces topology, a
foundation for vector GIS.
Position-Based Geographic Representation
Most GI is recorded using a position-based representation as points, lines, or
areas (also known as polygons). This type of GI representation corresponds
to the geometric primitives used to draw two-dimensional map elements. It
is a handy and convenient way to create GI based on existing maps and for
people used to working with maps. It is also very useful for many types of
analysis (see Chapters 13-15). Of course, it can be transformed to other GI
Positional GI representations are usually two-dimensional and static.
Events can only be shown in terms of positions and characteristics at a cer-
tain point in time. Measured properties are (1) either recorded as attributes
of a spatial object, (2) are defined by the extent of the property, or (3) are
associated with the measured properties of a predefined area (raster). Rela-
tionships are either defined by associations between attributes or relation-
ships that can be established and analyzed by transformations. The two most
common storage techniques for this type of representation are vector and
raster (see Chapter 2).
Animation can be used to show events with position-based GI represen-
tations, but it is always based on a series of static geographic representations.
Animations that show a series of images, just as frames in a comic, are rela-
tively easy to create and show. However, they may be based on the interpola-
tion of specific changes rather than measurements, which lessen their accu-
Vector GI is stored in a variety of ways. The most common format has
been what people refer to as the “georelational model.” This model is being
Examples of raster and vector geographic information representation types.