Structures for Storing Geographic Data - Introducing Geographic Information Systems with ArcGIS - page 280

11. Launch ArcMap with a Blank Map. Add Fire_Hydrants to the map. Now add Water_Lines.
Observe the map. Finally, add Hydrants_Topology. (You will be asked if you want to add all
the feature classes that participate in the topology; since you have already added them, say
No.) Making sure that the T/C is in the “list by drawing order” mode, put the Topology layer
immediately below the Fire_Hydrants layer. You can see that the two hydrants shown in the
red squares are in fact slightly off the east-most lateral. Zoom in and measure the distance
(use feet) from the end of the east lateral to the northeast hydrant. See Figure 4-26. What
is it? ____________ feet. Zoom back to Full Extent.
FIGURE 4-26
12. Use the Identify tool to determine which water pipes are not lined up properly. Their OBJECTID
numbers are ______ and ______.
13. Look further into this matter of misaligned water lines. Add as data WL_copy, which is a copy of
the original Water_Lines feature class. Pull it to the bottom of the Table Of Contents. Make its
size 4 and its symbol a bright green color. Make the symbol for Water_Lines bright red, size 1.
14. Using zooming and panning, look at each of the hydrants on the east lateral, starting with the
southernmost one. What you will find is that the original water line diverges away from the
hydrants as it goes north. However, the water line that participates in the topology covers the
first five hydrants, but not the last two. What has happened here is that the water line came
within 5 feet of the first five hydrants, so that, during validation, the lines were snapped over to
each hydrant. The other two hydrants were further away than 5 feet, so they were reported as
errors, and the water lines were not moved to the hydrants. In fact, moving the lines to five of
the hydrants actually caused the lines to diverge from their original path, pushing them further
away from the last two hydrants, as indicated by the lack of congruity between Water_Lines and
WL_copy.