Proximity and Overlay Operations
Table of Contents
- Geospatial Analysis Skills: Review and Self-Assessment
- Setting up Your Workspace
- Datums, Projections, and Spatial Reference Systems
- Working with Database Tables
- Adding XY Data
- Proximity and Overlay Operations
As you learned through previous readings, spatial operations tend to fall into two primary categories, proximity and overlay. Proximity operations for spatial analysis share similar characteristics as they do for spatial queries in that they evaluate distances from features. An overlay operation is a spatial operation in which two or more datasets are superimposed on one another to assess the relationship between features that occupy the same geographic space. Overlay operations also share common elements with the other spatial query operations. The primary difference is that overlay operations use a spatial query to create new datasets.
As you learned earlier, buffering is a proximity operation that creates a buffer. A buffer is a zone around the map feature with an area calculated using distance or time. Buffering may be applied to point features, line features, or polygon layers. However, the output will always be a polygon layer. Intersect is an overlay operation that preserves overlapping areas. The output will only contain regions of all included layers that overlap (Figure 1.13). The resulting features retain the attribute information from each map layer.
Skill Drill: Buffer and Intersect
Using the skills you learned previously, create a 500-kilometer buffer around the earthquake with the highest magnitude. Then, intersect the buffer with the Sex by Age layer using the Intersect tool. Save the output to your working folder as a shapefile called earthquake_intersect.shp. Add the shapefile to the map.
Hint: If you forgot how to perform a buffer and an intersect operation, review the skills you learned in the Chapter 7 Tutorial Mapping Food Deserts in Southern California, from the Geospatial Concepts Text.
Uncheck the boxes for the rest of the shapefiles so that the new layer is visible. Take a moment to open the attribute table of the earthquake intersect layer. An intersect operation will alter the area of the output features, but will not update the attribute table. As a result, you need to update the Shape_Area field in the attribute table using Calculate Geometry. Be sure to use square kilometers as the unit.
In your Microsoft Word document, record the answers to the following questions.
20. How many counties are within 500 kilometers of the earthquake with the highest magnitude?
21. On the earthquake intersect layer, use the updated areas within each county to calculate males per square kilometer and females per square kilometer. Which county has the greatest difference between the number of males per square kilometer and the number of females per square kilometer?
22. Which counties have the least difference between the number of males per square kilometer and the number of females per square kilometer?
Hint: Questions 20-22 are the hardest of all. They require a number of different steps to get the answer. However, the skills you use here are a repeat of the skills you mastered earlier in this tutorial.