Understanding GIS. David Smith
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9)Click Run, in the lower-right corner of the pane.
Twelve records are selected in the table. The corresponding features are selected in the map, showing that the river is perennial for most of its length.
It’s not among your guidelines to locate the new park along the river’s perennial stretch, but it’s interesting that fairly simple data exploration may introduce new ways of thinking about your problem.
10)Close the Select Layer By Attribute Geoprocessing pane.
Find the length of the perennial portion of the Los Angeles River
As you learned by looking at the attribute table, the perennial portion of the Los Angeles River is separated into 12 distinct records. To find the total length of this section, you must perform a summation of the FEET fields, using the Summary Statistics tool. When features are selected in the layer, the Summary Statistics tool processes only the selected records as a subset. Because you have perennial streams selected, the tool will sum up only the SUM field of these selected features.
1)In the attribute table of the Los Angeles River layer, right-click the DESCRIPTION field and click Summarize.
2)Rename the output table LARiverPerennial by clicking the Browse button and entering the name.
3)In the Field drop-down menu, click FEET; the Statistic Type menu will default to SUM.
So you’re using the Summary Statistics tool to find the total length (sum of each FEET record) of the perennial portion of the Los Angeles River.
4)In the Case field drop-down menu, confirm that DESCRIPTION is selected.
5)Compare the tool to the figure, and click Run in the lower-right corner of the Geoprocessing window.
6)When the tool is finished running, the resulting table is added to the bottom of the Contents pane. Right-click the table, and click Open to see the results. Confirm that the total feet of perennial streams is 229,947.
7)At the top of the Los Angeles River attribute table panel, click the Clear Selection button
Clearing the record selection clears the feature selection on the map and in the table.
8)Close the attribute tables of Los Angeles River and LARiverPerennial so that the map fills the middle of the application. You can also close the Summary Statistics pane now.
9)Save the project by clicking the Save button
Save your project often as you work through the workbook.
Selecting features
You make selections to work with a subset of features in a layer. You can use a selection to zoom the map to a specific area, make a new layer from the selected subset, get statistical information about the subset, or do many other things.
Selections are also used in queries. Whether you do an attribute query (to find records with a certain attribute value) or a spatial query (to find features with a certain spatial relationship to other features), the records and features that satisfy the query are returned as a selection on the layer.
Change the basemap to Imagery with Labels
Imagery provides a detailed, photorealistic view of the ground, and you’ll rely on it to explore the Los Angeles River in more detail. Imagery also has other important uses, such as providing a background against which to edit features (lesson 5) and “ground truthing” analysis results (lesson 6).
1)On the Map tab, click the Basemap drop-down arrow and click Imagery with Labels.
World Imagery is added to the bottom of the Contents pane and replaces streets. A layer named World Boundaries and Places has also simultaneously been added to the top of the Contents pane. This layer consists of locations, boundaries, and labels and should be visible on the map. (You should see yellow place-names covering the city of Los Angeles feature, for instance.) Your Contents pane and map should look like the figure.
Create a bookmark
Certain views in a map are useful for orientation or reference. You can bookmark a view to make it easy to return to.
1)First, zoom to the Los Angeles layer by right-clicking the Los Angeles layer and clicking Zoom to Layer.
2)On the Map tab, click Bookmarks > New Bookmark.
3)In the Create Bookmark dialog box, replace the default name with City of Los Angeles. Click OK.
4)Use the Zoom In tool by pressing and holding the Shift key, and zoom in on the river’s mouth at the Port of Long Beach.
5)In the lower-left corner of the map, click the Map Scale drop-down arrow and click 1:10,000.
At this scale, one unit of measure on the map is equivalent to 10,000 of the same units on the ground. Loosely, a thing on the map is 10,000 times smaller than its actual size.
6)Use the Zoom and Pan tools to explore the harbor.
The imagery is high resolution, and you’ll see more detail as you keep zooming in. Eventually, you’ll reach a limit in the level of detail, and the image will become pixelated.
7)When you’re ready, on the Map tab, click Bookmarks > City of Los Angeles.
The map view returns to the bookmarked extent.
Change the symbology for Los Angeles
The boundary of Los Angeles is filled in with a solid color, so the imagery underneath is still covered up. Soon you’re going to zoom in and follow the river’s course through the city. It will serve that purpose to resymbolize the city so that you see only its outline.
1)In the Contents pane, click on the color symbol below the name Los Angeles to modify the symbol.
The Symbology pane opens on the right side of the map.
2)Click Properties. If necessary, expand Appearance.
3)Change the color to No Color.
4)Change the outline color to Autunite Yellow.
5)Increase the outline width to 2 pt.
6)Click Apply.
The new symbology is displayed on the map.
7)Close the Symbology pane.
Save the layer as a layer file
It wasn’t hard to make this particular symbol, but it often takes time and effort to create good symbology.