Structures for Storing Geographic Data - Introducing Geographic Information Systems with ArcGIS - page 244

Geometric networks are useful in a variety of areas, such as routing school buses over a road network
and keeping track of electrical or piping systems. While I have avoided trying to divide GIS applications
into categories, you could consider that spatial problems that involve flows of entities through conduits
to be a major subclass of GIS problems—making GIS of major interest to utility companies. Geometric
networks support this sort of activity.
Geodatabases—Feature Shape
The concept that a row in a table contains the attribute values of a single feature remains the same,
but geodatabases allow great variety in what constitutes a feature. Specifically, geodatabases allow
multipoints, multipart lines, and multipart polygons.
Points
In storing point features, geodatabases allow “multipoints.” A multipoint is a collection of points
associated with only a single row in the database, so all the attribute values in that row apply to all the
points. An ecologist may have mapped gopher holes in an area. The only recorded difference between
them is location. So, they may be stored together as a single feature. See Figure 4-8 which shows two
features—one multipoint feature depicted with dots and another shown with x's.
Two multipoint features
FIGURE 4-8 Two multipoint features: one dots,
the other x's
Lines
Linear features are represented by polylines. A polyline is composed of one path or several paths. A path
is composed of sequentially connected segments that may be straight lines, but also geometric curves.
You may use a portion of a circle or an ellipse (which are, mathematically, the plots of second-order
equations), or you may use a type of spline, called a third-order Bézier curve. The path is a sequence
of segments, and it has a left side and a right side. If a polyline is a multipart polyline, then the paths
that compose it may be connected, disjoint, or some of each. Look at Figures 4-9, 4-10, and 4-11 for an
understanding of segments, paths, and polylines.
Polygons
A single-part geodatabase polygon, without any island polygons within it, is simply enclosed by a
single ring. A ring might be thought of as a single path (see Lines above) that starts and ends at the