How GIS Works - Combining Data
One of the greatest advantages of using GIS is its capacity to combine layers of data into a single map. The following graphics illustrate how multiple individual layers (or themes) can be combined for the purpose of performing complex queries.
Theme 1: Surface water intake
These points represent surface water supply locations throughout Pennsylvania.
Theme 2: Rivers
These lines represent Pennsylvania rivers. Theme 3: Counties
Pennsylvania county boundaries are represented by green polygons. Theme 4:Surface water
intakes, Rivers and County boundaries The surface water intakes and rivers have been overlaid
with the county boundaries to produce a single digital map that can be
printed or used as input for both attribute and spatial queries. Layering
data like this in a GIS allows users to analyze features in one theme,
relative to features in another theme. The following example provides an illustration of a geographic query that involves
multiple thematic layers.
