Geography Reference
In-Depth Information
File Geodatbases: Datasets are stored in folders in a file system. Each dataset is held as a file that can
be as large as one TB (Terabyte—roughly a billion bytes, which would be more than 300 million pages
of text. Esri recommends that if you are starting from scratch, you should use a file geodatabase, rather
than a personal geodatabase. The files are held in a folder; the folder name has extension GDB. It is not
limited to the Windows operating system, and you may choose from several RDBMSs to handle the
database.
Personal and file geodatabases lack the ability to automatically keep up with versions of the data, and
there are restrictions limiting the people who can make changes to the database. Personal geodatabases
are usually smaller, run on less powerful machines, and are intended for only a few users in a working
group.
ArcSDE Geodatabase Technology
ArcSDE geodatabase technology exists in both single-user file geodatabases and multi-user systems. Such
databases are stored in a commercial RDBMS—currently there is a choice of Oracle, Microsoft SQL Server,
IBM DB2, IBM Informix, and PostgreSQL. These geodatabases have virtually unlimited size; they support
many users and simultaneous editing sessions. ArcSDE is a required piece of software. Because of the
requirements to have an additional proprietary RDBMS, we don't discuss ArcSDE Technology (SDE
means “Spatial Data Engine”) further. But you should know it exists and supports GIS in large and/or
complex organizations where several to many people may be viewing and editing the database at the
same time.
For comparisons of the various types of geodatabases, you may examine the help files: use the search tab
to look for geodatabase types. Be sure to check out the link to “Types of geodatabases.”
Polygons within Polygons—Perimeter and Area
Calculations
In representing the natural environment or the human-built world, we frequently want to employ plane
areas that are included in other plane areas: lakes in a county, for example; islands in a lake; wetlands
that are internal to an island; and so on. As mentioned, many polygons are disjoint (that is, if you look
at the area covered by a polygon, you see no other polygons), but others are nested (when you look at
a polygon you see other polygons within it). The areas of all ArcGIS polygons are mutually exclusive.
Each has an identification, an area measurement, and a perimeter measurement. Each has its own set of
attribute values. Geographically, however, they may be arranged in two different ways. Consider Figure 4-7
with feature class representations A and B. The small squares are 1 unit on a side.
Feature class A consists of two lines and two polygons (P and Q). Polygon Q is a nested polygon with
respect to polygon P.
Feature class B consists of three lines and two polygons (R and S). Polygon S is not a nested polygon. It is
simply an area disjoint from polygon R.
The area of polygon P is 8 square units, calculated as ((3*3) - (1*1)); that is also the area of polygon R. The
area of polygon Q is 1 square unit; that is also the area of polygon S.
 
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