GIS For Dummies

Chapter 16: Comparing Multiple Maps 241

Finding points in polygons

Point features occur in many different places at different times for different reasons. You might want to identify where and to what extent (for example, how many) selected point objects occur within the boundaries of area features. And guess what? You can do just that with ease by using either a raster or vector GIS model. Table 16-1 shows a selection of questions that you might ask to identify whether features you’re interested in occur in the same locations.

You can probably think of a bunch more questions that deal specifically with your business or industry, but I think you get the idea. To answer your questions, you need a piece of software that can keep track of the data you use to make your comparisons.

The GIS (by design) keeps track of point features, their X and Y coordinates, and their attribute information. It also stores the coordinate and attribute information for all the polygons in the locations covered by your database. When you put data into your GIS database, you usually put point and polygon features on separate layers (although you don’t absolutely have to). So, to

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compare these different features, you retrieve the point features (by type or value; see Chapter 11) from one layer and compare the result to polygons — also selectively retrieved by type or value — from another layer.

This process is called point-in-polygon overlay, and you follow these general steps to accomplish it in your GIS:

1.Use your system’s built-in query capabilities to find point features by any attribute you decide.

The retrieved points are your first layer in this instance.

2.Use your system’s built-in query capabilities to find the polygon features that you want to compare with the point features.

The retrieved polygons are your second layer in this instance.

3.Place the point and polygon layers on top of each to create a new map.

Your GIS creates a new visual display that shows both the points and the polygons. It also creates a table that combines the attribute information from each layer.

Creating a new map from your separate layers is important when you want to actually use that map for analysis. Unlike a strictly visual overlay that you might see in a graphics program or many CAD systems, GIS software relates not only the locations you see (the X and Y coordinates), but also the attribute information for every feature you’re comparing.

You use the tables that you get when you perform point-in-polygon overlays to analyze the results. These tables indicate which points occur in which polygons (as shown in Figure 16-1). Using this information, you can count the number of features for each polygon, calculate density by polygon, and perform a host of other analyses.

My discussion of point-in-polygons overlay is based on the vector data model, but you can easily do the same thing by using a raster GIS. When using a raster GIS, the software searches the attributes in the same way that it does in vector and compares the coordinates — but the exact X and Y coordinates are replaced by the grid cell’s position in the matrix.

The point-in-polygon operation is a powerful tool for comparing point objects to area objects, but you might not need to perform this operation if the points and polygons are already on the same map.

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Table 16-2 (continued)

In vector GIS, lines are composed of line segments, each with its own set of beginning and ending coordinate pairs. Most of the time, these beginning and ending locations don’t coincide exactly with the borders of polygons. So, the software has to work a little harder to figure out exactly where the line moves from one polygon to another by determining where the line is going and where it will intersect the coordinates of the boundary for each new polygon (as shown in Figure 16-2). Both raster and vector GIS models can perform line-on-polygon operations, but GIS users more commonly use the vector model for this operation.

Like with point-in-polygon methods (discussed in the preceding section), this overlay operation produces tables that show which lines and polygons intersect so that you can analyze properties like orientation, density, and comparisons of line and polygon types.

Figure 16-2:

Finding the coordinates for points where a line

crosses a (X, Y) polygon’s

borders. (line/polygon intersection)

246 Part IV: Analyzing Geographic Patterns

So, you might search for all polygons that are classed as agriculture in a landuse map and polygons classed as flat lands in your terrain map. This search returns all lands that are both agriculture and flat. The resulting map would combine these two categories into a larger one. Union overlay maintains the categories from each of the input layers, so you can use them later to perform still more comparisons of flat agricultural land with other layers such as soil nutrient values.

Figure 16-3:

Union overlay.

Use union overlay when you want to broaden your search and combine different attributes of your polygons. All the polygons that have any of the search criteria are included in the output.

Using intersection overlay

Sometimes, you want to know which polygon features are shared by each of the polygons you’re searching. In other words, instead of combining pieces of land for sale with lands that are agricultural, you only want to know which agricultural lands you can buy. You can get this information by using a logical AND search, which needs both categories to exist before it returns a value to the output map. This search typically reduces the size of the area retrieved, instead of increasing it like union overlay does (see Figure 16-4).

Use intersection overlay when you want to narrow your search to satisfy a number of criteria at the same time. This technique can help you find the most appropriate places for some activity.

Chapter 16: Comparing Multiple Maps 247

Input

Output

Figure 16-4:

Intersection

overlay.

Intersect

feature

Understanding complement or symmetrical difference overlay

Sometimes, when you try to perform a search, you may find it easier to eliminate things that don’t match all your criteria, instead of identifying those that do match. This search, called complement or symmetrical difference overlay, computes the geometric intersections of all polygon types and categories that don’t have certain attributes in common (see Figure 16-5). You might want to do this kind of search if you plan to later perform an analysis that relaxes the criteria you originally established.

Figure 16-5:

Complement or symmetrical difference overlay.

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Comparing geometry with clip overlay

You can use clip overlay to compare the geometries of different areas. You simply clip one portion of a map based on the size and configuration of another (see Figure 16-7). Think of clip overlay a bit like a cookie cutter. You might use this overlay if you want to combine information from a small subportion of your study area with the entire study area. For example, if you’ve done some extensive field research on a small part of a larger forest and want to compare these new data to the original whole-forest layer. Obviously, you need only the small sample area to do your analysis. You don’t want the computer to do all that time-consuming graphics work for parts of the database that you don’t need.

When you use clip overlay, the attributes of the polygon you clip aren’t a factor in determining the size and shape of the resulting layer. Instead, you gather all the information from all layers that you want — but for only a small portion of the map.

Figure 16-7:

Clip overlay.

I found the clip overlay useful when I was working with a group of researchers studying land-use change during four different time periods. Because we used un-rectified aerial photographs, each of our study areas was slightly different from the others. To fix this discrepancy, we chose the study area that had the smallest footprint (covered the smallest area) as a cookie cutter and clipped all the other layers to match it.

Use the clip overlay when you want two or more layers to have the same geometry (size and shape), which allows you to have all the attributes that you want to evaluate, but for a selected portion of the study area.