Geography Reference
In-Depth Information
Other files (lawn_sprinklers.sbx, lawn_sprinklers.sbn, lawn_sprinklers.prj (containing the projection
information) and lawn_sprinklers.shp.xml (containing the metadata) may also be present. So the term
“shapefile” is somewhat a misnomer, if one expects a single computer file. ArcCatalog, of course,
represents as shapefile as a single entity, but to the operating system, it is several files. You could actually
move a shapefile from one folder to another using the operating system. You would simply move all
the constituent files, but this is not a recommended practice; use ArcCatalog instead. (Recall that with
geodatabases there is no issue with moving or renaming because you can't see the constituent feature
classes with the operating system; they are locked away in a single database file.)
When you access a folder containing the files that make up a shapefile in ArcMap or ArcCatalog, you see
only the designation—continuing our example—lawn_sprinklers.shp. The other files are hidden, so you
may think of the shapefile as just one entity.
Shapefiles are important because they have been around for some time and immense numbers of datasets
are in shapefile format. Shapefiles were the data format of choice for Esri's ArcView software (up through
version 3).
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Summarizing Vector Dataset Features
For various reasons relating to technology and history, Esri now has two major ways of storing vector
datasets: geodatabase, and shapefile. Some of the differences relate to functionality and some mostly
to terminology. Shapefiles and geodatabases have a lot in common. Geodatabases and shapefiles allow
multipoints. They both delineate polygons with rings. They use polylines, composed of segments, with
one or more paths. The similarities end there. Geodatabase segments may be circular arcs, elliptical arcs,
or Bézier curves. Geodatabases allow for topological relationships; shapefiles do not. A major feature of
geodatabases is the ability to use a geometric network (composed of junctions and edges that possess
not only attributes, but behavior). Geodatabases calculate the lengths of lines and rings, and the areas
of polygons—and shapefiles do none of that. Geodatabases allow subtypes of features, while shapefiles
do not. So, the semi-witty comparison that a geodatabase is a “shapefile on steroids” greatly understates
the case.
Summary of Logical Structures of Vector-Based GIS
Datasets
In terms of logical structure and layout within the computer, shapefiles are at the surface. The files that
constitute a shapefile may reside anywhere on a hard disk, provided that they are all in the same folder.
Attribute data is always stored in dBASE tables. Only a single feature type—point, multipoint, line, or
polygon—is stored in a given shapefile.
A personal geodatabase is a single file in Microsoft Access. A file or enterprise geodatabase may use any
of several commercially available database management systems.
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Esri has used the term “ArcView” in two fundamentally different ways. ArcView versions 1, 2, and up through
3.3 are computer programs. The ArcView associated with ArcGIS (that is, version 8, 9 and 10) describe a level of
functionality. The term has been dropped with version 10.1 when ArcGIS Basic describes this level of functionality.
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