Geography Reference
In-Depth Information
A polygon in ArcGIS, however, is almost always considered a plane figure rather than the line elements
that bound it.
Polygons
—In geometry a polygon is a plane figure with three or more straight-line sides. The sides may
not cross. Perhaps you think of a polygon as a square or hexagon—and you are correct. However, there is
a finite but no small limit to the number of sides a polygon may have, nor do the sides have to be of equal
length, as they are in a
regular polygon
where equal lengths of sides and equal angles are the rule. It is not
unusual for a GIS polygon to have hundreds of sides.
In GIS we take a lot of liberties in the use of the word “polygon.” For one thing, a GIS polygon can
contain other polygons (which can contain other polygons, which can contain other polygons, and so on).
Further, while polygons in shapefiles may only have “sides” that are straight lines, geodatabase polygon
“sides” may be parts of circles or ellipses, or may be Bézier curves. Still further, a GIS polygon may be
several polygons, as described later in the chapter.
In geodatabase and shapefiles, a single polygon (an entity with a single row in a table) is formed by a
collection of one or more rings—as defined previously. If more than one ring is involved, no rings may touch.
Cells
—A (usually) square area that (usually) contains a number related to an entity or condition. Used in
raster or grid data models.
Zone
—A collection of cells that have the same value. Used in raster or grid data models. The cells of a
zone may be adjacent or nonadjacent.
Triangles
—Plane, three-sided polygons used in the TIN data model, where each triangle has a calculated
maximum slope and direction (aspect). Further, each point on or within the triangle has a
z
value, such as
elevation.
Regions
—This term is used in different ways depending on which data model is being considered. In a
raster
or
grid system
, a region is a collection of cells, all of which have the same value (i.e., are all of the
same zone) and which are connected to at least one other cell in the zone.
8
(So that you aren't confused
later: even though we aren't generally concerned with coverages, the term
region
used with the coverage
data model specifies something philosophically different than with the raster model. And geodatabases
handle this concept with what are called multipart polygons.)
Three-Dimensional Entities in a Three-Dimensional Field:
Triangles and Multipatches
Triangles are plane, three-sided polygons used in the TIN data model, where each triangle has a calculated
maximum slope and a direction (aspect). Further, each point on or within the triangle has a “z” value such
as elevation.
Multipatches are the outer surface, or shell, of features that occupy a discrete area or volume in three-
dimensional space. For example, a representation of a building (discussed in Chapter 9).
8
The connection can be specified to be only along edges or to be both edges and corners.
Search WWH ::
Custom Search