Geography Reference
In-Depth Information
a
W
X
b
Y
FIGURE 4-14 Shapefile polygons illustrating problems
with overlaps and gaps. (Polygon W has a curving right
boundary; polygon X has a straight left boundary.)
A particular shapefile is restricted to represent only one of these types: points, multipoints, polylines,
or polygons. With points, each individual point has a record in the relational database. If a number of
points are considered the same object, then that object has only one record in the attribute table. As with
geodatabases, polylines can be composed of one or more paths, connected or disjoint. However, the paths
are allowed to be composed only of straight-line segments.
A polygon in a shapefile bears similarity to a polygon in a geodatabase, but no topology is present and
none can be created. Each polygon is a stand-alone affair. It is delineated completely by one linear entity:
a sequence of segments that starts in one geographic location and returns to that location. There may be
adjacent polygons or not. Other polygons may overlap it. See Figure 4-14.
A problem to which shapefiles are particularly susceptible is that there may be slivers of overlap or slivers
of vacancy (gaps) between intended polygons, and there is nothing you can do about it if the data remains
in shapefile format. In Figure 4-14 the sliver “a” is claimed by both W and X, while sliver “b” is in neither.
A geographic area partitioned into mutually exclusive shapefile polygons will have duplicate
information. Since the total boundary of each polygon is defined for that polygon, any common lines are
“double digitized.” Further, there are two independent boundaries, and you have no assurance that they
are congruent. With geodatabases, you can make topological rules to ensure that polygons do not overlap
or have gaps, but not with shapefiles.
With shapefile polygons, you do not get the area and perimeter as attributes.
The advantages of shapefile representation are simplicity, processing speed, drawing speed, and,
usually, economy of storage. Shapefiles are useful when you do not need sophisticated geoprocessing.
Do be aware that a lot of GIS data sets have been put into shapefile format. There may be considerable
conversion to geodatabase format in your future if you want to use those data sets in geoprocessing.
Shapefiles—Layout in the Computer
The format for a shapefile on a disk drive is also much simpler than that of a coverage or a geodatabase.
Basically, at least three files in a folder are required for a shapefile. If a folder named AMENITIES
contains a shapefile named LAWN_SPRINKLERS, then AMENITIES will contain at least these files:
❏
lawn_sprinklers.shp (contains the geographic information)
❏
lawn_sprinklers.shx (contains the spatial index to the geographic information)
❏
lawn_sprinklers.dbf (contains the dBASE table for the attribute information)
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