Understanding GIS. David Smith

can also use the menu to select the city name.

      7)Click Add. Do not click OK yet.

      8)Click the General tab. Delete the layer name Cities, and type Los Angeles.

      9)Click OK.

      On the map, only the city of Los Angeles is shown. The other cities are hidden by the definition query.

      10)In the Contents pane, right-click the Los Angeles layer and click Attribute Table.

      The table shows only the record for Los Angeles.

      11)Close the attribute table.

      The other city features haven’t been deleted. Clearing the definition query would display them again. Layer properties affect the display of data in a map, not the essential properties of the data itself: the number of features, their shapes, their locations, and their attributes.

      Layers and datasets

      A layer points or refers to a dataset stored somewhere on disk (as specified on the Source tab of the Layer Properties dialog box). A layer is not a physical copy of the data. A layer is a representation or rendering of the data. The dataset stores the shapes, locations, and attributes of features; the layer stores display properties, including what the layer is named, which of its features are shown or selected, how those features are symbolized, and whether they are labeled. Changes to layer properties do not affect the dataset that the layer refers to. You can make as many layers as you want from the same dataset and give them different properties. These layers can coexist in the same map document or in many different map documents.

       Add a layer of rivers

      Now you can add a layer of local rivers to the map and see where the Los Angeles River fits into the picture.

      1)From the Catalog pane, browse to the Hydro group in the ESRI.gdb and right-click the River layer. Then click Add To Current Map from the menu.

      A layer of rivers is added to the map and placed at the top of the Contents pane. The layer is symbolized with a random color; if the color is difficult to identify, it may be beneficial to change its color using the color palette. (If you are having trouble, refer to the task “Set layer properties,” steps 1 and 2 earlier in this section, for instructions.)

      2)Click on any river to identify it.

      You see the name of the river (many of them don’t have names) and its other attributes.

      By default, the Explore tool identifies features from the topmost layer in the Contents pane. If you miss a river, you’ll identify either the city of Los Angeles or nothing. That’s fine—leave the Explore window open, and click again on a river.

      3)Click on a few more rivers to identify them. Try to identify a segment of the Los Angeles River.

      The river runs west to east across the city, turns south near the city’s eastern edge, and follows a freeway to San Pedro Bay.

      4)Close the pop-up window.

       Make a definition query on the Los Angeles River

      Just as you’re mainly interested in one city, you’re also mainly interested in one river. You’ll make another definition query to show just the Los Angeles River.

      1)In the Contents pane, right-click the River layer and click Properties.

       A shortcut is to double-click the layer name in the Contents pane.

      2)In the Layer Properties: River dialog box, click Definition Query.

      3)Build your query:

      Name is equal to Los Angeles River.

      4)Click the General tab. Change the layer name from River to Los Angeles River.

      5)Click OK.

      The Contents pane reflects the new layer name.

       You can also rename a layer directly in the Contents pane by clicking the name once to highlight it and then clicking it again. (Be careful not to double-click, or you’ll open the layer properties.)

       Change the symbology

      1)Click the color symbol below the Los Angeles River name to modify the symbol. Clicking the symbol opens the Symbology pane on the right side of the map.

      2)Click the Properties tab at the top. Under Appearance, you can change the color and width of the symbol.

      3)Change the color to blue.

      4)Change the line width to 3 pt.

      5)Click the Apply button, and close the Symbology pane.

      On the map, the river is displayed with its new symbology.

       Select Los Angeles River features using a query

      On the map, the river looks like a single feature (just like the city), but it’s not.

      1)In the Contents pane, right-click the Los Angeles River layer and click Attribute Table.

      At the bottom of the table, you see that 0 of 17 records are selected. That means that, in this particular layer, the Los Angeles River is composed of 17 features. Why is that?

      2)Scroll down through the table.

      All the records have the same name value. Most have the same type, but one is an artificial path. There are a few description values. The need to maintain different attribute values for different parts of a geographic object is a common reason that data—especially data representing linear features such as streets and rivers—is constructed this way, consisting of multiple features. We’ll come back to this point in lesson 2.

      3)In the ribbon of tools on the Map tab, in the Selection group, click the Select By Attributes button .

      Having noticed “perennial” (year-round water flow) and “intermittent” (not year-round water flow) values in the description field, you might want to know which parts of the river are which. You can find out using an attribute query. An attribute query is like a definition query in that both types of query single out features in a layer on the basis of attribute values. The difference is that an attribute query highlights (selects) features that satisfy the expression rather than hiding features that don’t.

      4)In the Layer Name or Table View box, confirm that Los Angeles River is selected.

      5)Make sure the Selection type drop-down list is set to New selection.

      6)Click the Add Clause button.

      7)Create a clause for

      Description is equal to perennial.

      8)Click Add.