Geography Reference
In-Depth Information
Geodatabases—Layout in the Computer
I said earlier that a GIS was the marriage between a (geo)graphical database and an attribute database.
The geodatabase still adheres to this in concept. I also said that usually the attribute database was
housed in a commercial relational database management system (RDBMS) as mentioned earlier. The new
wrinkle is that the entire thing—geographic part and attribute part—is housed in a single RDBMS file.
This means, from the point of view of the software, all of the geographic datasets have been rolled up
with the attribute data into a single file. For ArcSDE databases this file may be located in one of several
commercial relational database systems. To determine which commercial RDBMSs are used by Esri
software, consult the help files for the version you are using.
Personal geodatabases are housed using the Microsoft Access database system. With ArcGIS 10.
Differences between file or personal geodatabases and Esri's more extensive products include the lack of
ability to automatically keep up with versions of the data and restrictions of who can make changes to
the database. Also, single user geodatabases are usually smaller and are run on less powerful machines.
With the single-file implementation of a GIS in a geodatabase, there is, therefore, no temptation to go in
with the operating system to move, delete, or rename things; the components are somewhat hidden from
the user, except through ArcCatalog and ArcMap.
Geodatabases—Logical Construction
Within the single file of a geodatabase, there is the framework for quite a complex hierarchy of elements.
You have had some experience with this hierarchy earlier, but here is a summary, with a bit of additional
information. The description is based on the file geodatabase, which resides within a folder. ArcSDE
geodatabases look somewhat different, but only at the top levels.
The database may consist of the following:
(A)
Freestanding,
and not necessarily related:
Feature classes, resembling the point, line, and polygon classes you have dealt with
❏
Raster datasets, which may represent surfaces (e.g., elevation), areal phenomena (e.g., land
cover), or images (e.g., orthophotoquads, scanned maps)
❏
Triangulated irregular network (TIN) datasets
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❏
Tables, which are referred to as
object classes
, and which may be imbued with “behavior,” as
discussed later in the text.
❏
(B)
Feature datasets
, whose constituents share a common geographic reference (datum, projection, units,
and so on) and that are composed of the following:
All the elements cited above in (A)
❏
A relationship class that is a set of relationships between the features of two feature classes
❏
A geometric network that consists of
❏
A junction feature class
❏
An edge feature class.
❏
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As of this writing, TINs are not included with geodatabases, although terrains are.
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