Structures for Storing Geographic Data - Introducing Geographic Information Systems with ArcGIS - page 242

3 3

P R

Q

S

Feature
Class A

Feature
Class B

FIGURE 4-7 A nested polygon compared
with an adjacent polygon

The perimeter of polygon P is 16 linear units. This is the sum of the length of the line that defines the
outside of P (length 12) and the length of the line that segregates the nested polygon Q (length 4). The
perimeter of polygon Q is 4 units.

The perimeter of polygon R is 12 linear units, made up of two lines. The perimeter of polygon S is 4 units,
made up of two lines.

Again, polygons may be nested to (almost) any depth. Polygon Q could contain three nested
polygons, one of which might contain five nested polygons, each of which might contain 22 nested
polygons.

In determining areas and perimeters when nested polygons are involved, it might be useful to invoke a
rural analogy. Maybe you are raising llamas and want to know how much area would be available for
grazing. If the animals are to be confined to a given polygon, say X, the polygon is the area an animal
could roam in, which does not include the area of any nested polygons.

Did you ever wonder how the computer “knew” which polygon your mouse cursor was in when you
used the identify tool? You, of course, can look at the cursor and the image and tell which polygon the
cursor is in. Your head contains a remarkable spatial data processing system. But how does the computer
know? If you are interested in an explanation, locate information (on the Internet or elsewhere) on the
“point in polygon” problem.

Multipart Polygons

A polygon feature class consists of a set of polygons; each polygon refers to some surface area on the
Earth. However, an ArcGIS “polygon” may consist of several geometrical polygons. For example,
suppose that you wanted a dataset that depicted of the area in square units of all the states in the United
States. You would find yourself delineating two types of areas. First, obviously, there would be those
defined by traditional state boundaries, which divided the landscape. But you would also find states like
Massachusetts. It consists of a mainland part plus islands. Delineating the surface area of Massachusetts
requires several polygons, since the water around those islands could not be considered land area
belonging to the state.