Structures for Storing Geographic Data - Introducing Geographic Information Systems with ArcGIS - page 245

Geodatabase segments A path—composed of segments

FIGURE 4-10 A path in a
geodatabase is composed of
segments

Straight
line

Circular
arc

Elliptical
arc

Bézier
arc

FIGURE 4-9 Permitted types of geodatabase segments

Single-path polyline Multipath polyline

FIGURE 4-11 A polyline may be composed of one or more paths

same point—that is, a ring is a closed figure. Since it is a closed figure, the software knows whether any
given arbitrary point is inside the polygon or outside. One effect of using the complete, single entity
(ring) to delineate a polygon is that it divorces a given polygon from its neighbors. (It also means that,
for traditional disjoint—but adjacent—polygon representation, such as ownership parcels, each vertex
and line is stored twice.) So, the traditional coverage topology, which assured that there were no gaps or
overlaps between polygons, is no longer present. This sort of topology has been replaced by a much more
general set of topological checks which the user can invoke to ensure data integrity.

A geodatabase “polygon” may be what is called a multipart polygon. This may be a set of two or more
polygons. Either single-part or multipart polygons may have other polygons nested inside them. So, the
term “polygon” encompasses a multitude of conditions. Please look at the illustrations in Figure 4-12.

Nested Polygons in Geodatabases

It is important to look at the calculations of plane area (called Shape_Area) and perimeter (called Shape_
Length) for nested geodatabase polygons. Look at Figure 4-13, which has an island polygon D that has
dimensions of 2 units by 5 units. Be sure you understand the area and perimeter calculations of polygon
A, particularly with regard to the Shape_Area and Shape_Length.

Geodatabases and Attributes

We said that a GIS was the marriage of a geographic database to an attribute database. In a geodatabase,
each row in the attribute database refers to an “object” that is a point feature, line feature, or polygon feature.

One of the indications of maturity of the GIS field is the growing emphasis on attribute data correctness
and integrity. All large databases, spatial and otherwise, contain errors. With geodatabases, a number