Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition

Make a New Feature Class from a Subset of Polygons: Extract

As you saw, the Union of the feature classes produced many output polygons from a few input polygons. As you also saw, those polygons could be identified by their attributes. Frequently what you want to do after an overlay is to select a subset of the output polygons and make a new feature class. For example, to solve the Wildcat Boat problem, you would want to extract the polygons that met the requirements of soil suitability, land cover, and so on. To do this, you build a query in what is called the Structured Query Language (SQL). There is a lot of similarity between using SQL and what you did above in selecting polygons. The difference is that the result of the SQL process is a new feature class that contains the selected features.

Suppose now that we want to make a new feature class named BOTH_TOP_and_LEFT, consisting of those polygons on the top plus those polygons at the left.

____ 16. Start ArcMap by opening Stalks_Bars_no_gaps.mxd. Make the only entry in the T/C Stalks_Bars_spp by removing any others. From ArcToolbox: Analysis Tools > Extract > Select. Start the Select tool. Read the Help panel. Then read the several different Help discussions, obtained by clicking various places on the Select window—especially Expression. Look at the Tool Help.

When you begin to “build a query,” it is assumed that all features (and therefore records) are selected. This is different from the previous assumption you have been ...