The Time Dimension - Introducing Geographic Information Systems with ArcGIS - page 570

Backup_A and Lex_Backup_B. You will be using topology operations on the Roads database and
anytime you involve topology operations, you need backups of your original data, because those
operations can move objects. Two backups may be overdoing it, but it is unusual to have too
many backups. More frequently, one does not have enough.
5. Preview the Geography of each feature class: Lex_Roads_1994 and Lex_Roads_2002. You can
note, as you flip back and forth, that a fair number of roads were added in the intervening eight
years.
6. Look at the table of each feature class. How many road segments are represented in the 1994
roads file? 12 ________. In the 2002 file? ________.
7. Look at the Description of one of the feature classes. What is the Geographic Coordinate
system name? ________________________. What are the Units? ________________.
We are going to attempt to identify those road segments that are congruent, so we can ignore
them and be left with only those segments that are not part of both datasets. You will recall
that it is inadvisable to ask if two numbers that are computed by a machine are equal. Applying
the same principle, we will ask if two segments are almost congruent—that is, their vertices are
almost identical. We will do this by using topology. As you will recall, a topology may exist on the
feature classes that are within a feature dataset.
8. Highlight the feature dataset Lexington_SP_North_83_Feet. Choose File > New > Topology >
(read the window) > Next. For the name, accept the default. But do not accept the default for
the cluster tolerance. Read the description of the cluster tolerance. Let's assume that two road
segments are coincident if all their vertices fall within 50.0 survey feet of each other. Type that
in. Click Next.
9. In the New Topology window, select both feature classes. Click Next.
It would probably be a safe bet that the 2002 TIGER files are more accurate than the 1994
files. So we should set things up so that it will be the 1994 lines that will move in case they are
not coincident with the 2002 lines.
10. Set the rank for Lex_Roads_1994 to 5. Click Next. Add the rule “Must Not Overlap With ”. 13 In
the Add Rule window use Lex_Roads_1994 for one feature class and Lex_Roads_2002 for the
other. Toggle the Show Errors button a couple of times and observe the Rule Description. Red
indicates the sort of error that the topology rule is looking for. This operation is only for your
information; it has no effect on the functioning of the software. Click OK, then Next. Check the
Summary and click Finish.
11. After the new topology has been created, select Yes to validate it. Be prepared to wait for a bit.
Think of all the road segments the computer has to look at, and maybe reposition. It takes time.
12. Lexington_SP_North_83_Feet_Topology should appear in the Catalog Tree, along with the
Roads feature classes. If you click on it with the Preview tab pressed, you will see a lot of pink,
12 These files were modified somewhat; records in which the FEATURE NAMES were blank were deleted.
13 Note the rule name. Four words!