GIS For Dummies®

Chapter 16

Comparing Multiple Maps

In This Chapter

Looking into map overlay

Comparing polygons with logical overlay

Understanding the advantages of raster overlay

Using selective overlay to compare features

Geographers have been aware for generations that spatial patterns sometimes repeat themselves. The usefulness of comparing these spatial patterns has been documented as far back as the Revolutionary War when General George Washington’s French cartographer made multiple hinged maps that showed the locations of both British and American armies in the corresponding geographic area. This idea probably goes back as far as maps themselves.

In the early 1960s, the idea of using computers to make maps brought with it a renewed interest in comparing multiple map patterns. Landscape architects used clear acetate to physically overlay and compare maps manually with a fair degree of success. Urban and regional planners found comparing different maps essential to the observation, quantification, explanation, and eventual exploitation of multiple patterns.

Even before map overlay came into use, people observed patterns occurring in the data contained on various maps for an area. Auto thefts happen in locations that have cars, shopping happens in areas that have stores, roads connect towns, vegetation responds to different soil types and different slopes, cities mostly occur near water, certain wildlife prefer selected vegetation, people of similar income tend to live in the same areas, ...