"Piercing Needle" Approach

Point by point overlay analysis is referred to as the"piercing needle"approach. When multiple themes of data -- such as forests, water, soils, elevation, etc. -- are in relative position with one another, inserting a "digital pin" through the stack of overlayed data, as represented in the graphic below, allows questions to be answered concerning everything occurring at a particular location. However, the point by point view of the world limits focus to a particular point which precludes wide-area landscape analysis.

Region Wide: "Cookie Cutter Approach"

The region wide, or "cookie cutter," approach to overlay analysis allows natural features, such as forest stand boundaries or soil polygons, to become the spatial area(s) which will be analyzed on another map.

Example:

Given two data sets, forest patches and slope, what is the area-weighted average slope within each separate patch of forest?



To answer this question, the GIS overlays each patch of forest from the forest patch data set onto the slope map and then calculates the area-weighted average slope for each individual forest patch.