Geographic Information System

A Geographic Information System, or GIS, is a system for managing, analyzing, and displaying information linked to locations. It connects facts about a place or event with where it is. A public-safety map is one possible output; the underlying system can also compare areas, assign records to boundaries, and calculate geographic relationships.

More than the map on screen

Esri, a GIS software developer, describes GIS as combining location information with descriptive information. Its definition includes creating, managing, analyzing, and mapping data. Esri: What is GIS?.

In a public-safety setting, separate layers might represent incident points, station locations, municipal boundaries, and service areas. A layer is a set of similar geographic features displayed or analyzed together. Keeping layers distinct helps identify the source and meaning of each part of the map.

Example

Hypothetical: A city has 500 incident records with usable coordinates and a map of four service districts. A GIS assigns each point to the district containing it and produces a district summary.

The result depends on the incident locations, the boundary version, and the treatment of points on or near a boundary. If 20 additional records have no usable location, a district map of the 500 located incidents does not represent every record in the original file. The summary should disclose that difference.

Common confusion

GIS is not the same as GPS. GPS is a satellite-based positioning system; GIS manages and analyzes geographic information, which can come from many sources. A mapped coordinate may come from address matching or a generalized public location rather than a GPS measurement.

A location relationship is also not a finding about cause. Showing that incidents fall near a station, roadway, or other feature does not establish why they occurred there. Geographic analysis needs a question and method beyond putting layers together.

Reading a GIS result

Look for the coordinate source, coverage period, boundary date, and location precision. A precise-looking marker can still represent an approximate or masked location. Zooming in changes the display, not the accuracy of the source.

For a calculated result, identify the operation used. Counting points inside a district, measuring straight-line distance, and estimating travel along roads are different tasks. A map should explain which result it presents so a reader does not interpret a geometric distance as an observed response time or a service guarantee.

Related terms

• Geocoding
• Municipal boundary
• Choropleth map
• Metadata

Sources

• Esri: What is GIS? — Primary software-provider explanation of geographic data management, mapping, and analysis. The district-assignment example is hypothetical.