Structures for Storing Geographic Data - Introducing Geographic Information Systems with ArcGIS - page 276

Moral: If you tell a computer to ask a simple question of its data, such as “Is A equal to B?,”
the answer may be “no” even though A and B are meant to be the same and are very close.
The computer will report that they are not the same, but the reason for the difference may
be that they are calculated in different ways by the computer. A better question would be
something like “Is the absolute value 12 of A minus B less than some appropriate very small
value (such as 0.0005).”
Exercise 4-8 (Project)
Geodatabase Topology
With geodatabases all sorts of conditions of data misbehavior can be tested for. Topology can be
constructed between different data sets, as long as they are in the same personal or file geodatabase
feature data set . (Recall that a geodatabase can contain a feature data set, which can contain feature
classes. Those feature classes all have the same extent, projection, and so on. Topological relationships
can be formed among those feature classes. A geodatabase can also have free-standing feature classes,
which may bear no relationship with each other. Topological relationships may not be formed within or
among those feature classes.)
Let's look at a simple example. Recall that the first geodatabase you looked at in this topic contained
a feature class consisting of fire hydrants in a village. Here you look again at the same data, but it has
been converted from a File Geodatabase (Water_Resources.gdb) to the form of a Personal Geodatabase
named Water_Resources.mdb. Your first step is to copy the data from this Personal Geodatabase into your
personal working area.
1. Start ArcCatalog. Navigate to the folder
[___]IGIS-Arc\Geodatabase_Topology
and highlight it. Select Edit > Copy. Navigate to
___IGIS-Arc_ YourInitials .
Highlight it. Select Edit > Paste.
2. Expand all the entries of
___IGIS-Arc_ YourInitials \Geodatabase_Topology
so that you see all the constituents of the Hydrants geodataset, including Fire_Hydrants and
Water_Lines.
3. Explore Fire_Hydrants in the Preview pane, then look at Water_Lines.
12 The absolute value of a number is, crudely, the number with the algebraic sign stripped off (so it assumed to be
positive). Formally, if “q” is the absolute value of a number “p,” then “q is equal to “p” if “p” is positive, and “q” is
equal to “negative p” if “p” is negative. (Recall that a negative of a negative number is a positive number.)