The last skill showed how to use an overlay analysis to extract the drawing objects within a polygonal area. In the current skill, you will learn how to do the same thing for features. There are three skills in this book that describe different ways that you can extract or isolate features on existing layers:
A typical application of the overlay analysis method is shown in the illustration below. This illustration shows a few of the layers in a large-scale wall map of the city of San Francisco. In this map, we want to show the contour polylines inside the parks and areas of open space in the city, but not anywhere else. To do this, we use an overlay analysis in which the source layer = Contours and the overlay layer = Parks. This type of analysis is called “intersect.” The result is a new layer (an SDF file), containing only the contours within the boundaries of the parks, as shown in the second image below.
This ability to restrict the display of features is very useful. However, the main advantage of using the overlay method is to create a thematic analysis. For example, in the illustration below, the transparent blue polygon represents a flood plain. With an overlay analysis, you can easily isolate the features, in this case the parcels, that are inside, or touching any part of, the flood plain. This type of analysis is called a “union.”
The animation shows a thematic analysis using two polygon layers. In this example, an intersect analysis is used to show all of the parcels in a city that fall within the zones classified as “Retail.”