Geography Reference
In-Depth Information
The need to modify a geographic data set when it is being used by another program crops up frequently
and is usually frustrating. An error message you will sometimes encounter is “Cannot acquire a schema
lock because of an existing lock . . .” How you correct this depends on circumstances. If you have an
extensive map up and you don't want to remove a data set from it, you can easily save the entire map,
with all its settings, make the modifications to the data set, and then reload (Open) the original map with
the modified data set. However, sometimes you get the “lock” based on having used data sets previously,
but that are no longer in use. Esri is working on the problem of data sets that are locked out because
they were previously used by other programs. One of their current approaches is the ArcCatalog sidebar in
ArcMap.
13.
In ArcCatalog, refresh the Catalog Tree (F5 will do that if Folder Connections is highlighted)
and then alternately select the previews of TextToFeature_Lines and TextToFeature_Polygons.
Now look at the attribute table of each feature class. Note that the polygon feature class table
indicates area. What is the area of the largest polygon? _____________.
Areas and Perimeters Examined
If you have trouble understanding the areas and perimeters, recall our rural analogy: Suppose that each
polygon represents a field of a farm in which different animals are kept. The questions to be answered
are these: How much area does each animal have to graze in, and how much fence is required to keep
each animal where it belongs? In the steps that follow, you will use the labeling capabilities of ArcMap to
display the areas and perimeters of each of the three polygons.
14.
Go back to ArcMap (restart it if necessary and open TextToFeatures.mxd) and add
TextToFeature_Polygons. Right-click TextToFeature_Polygons. Click Label Features. What values
appear within the polygons? ______, ______, ______. Verify from Figure 4-17 that these are
the perimeters of the three polygons. Write the perimeter value on Figure 4-17: P=xx. Be sure
you understand where the 40 came from. Going back to our analogy, it would take 40 units of
fencing to keep the animals in that polygon: the sum of the perimeters of the two squares.
15.
Right-click again on the TextToFeature_Polygons entry, then select Properties. Navigate to the
Labels tab if necessary. Change the Label Field to Shape_Area. Click Apply and then OK. Now you
will see the polygons labeled with the area (instead of perimeter as before) ______, ______, ______.
Inside each polygon on the diagram in your textbook, write the area value: A=yy. Verify that each
number you have written in a polygon represents the area of the polygon.
Labeling Features with Selected Attributes
16.
Right-click again on TextToFeature_Polygons. Remove it from the map, then right-click on
TextToFeature_Lines and select Properties. Navigate to the Labels tab if necessary. Change the
Label Field to Shape_Length. Click Apply and then OK. Now you will see the lines labeled with
their lengths. ______, ______, ______. Now label each line with its OBJECTID. ______, ______,
______. Finally label each line again with its File_ID (the values you provided when you created
the features). ______, ______, ______. Dismiss ArcMap without saving the map.
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