• What is a GIS?

• Map layers (points, lines, and polygons)

• Geodatabases and attribute tables

• Spatial analysis

What is a GIS?

A GIS, or geographic information system, is an organized collection of computer hardware, software, geographic data, and personnel designed to efficiently capture, store, update, manipulate, analyze, and display all forms of geographically referenced information.

A simpler explanation of a GIS is the joining of a computer software mapping application and geographically enabled information (database) that links spatial information (where things are) with demographic and environmental information (what things are like).

For example, a paper map—such as a road map—can show us what road to take to get from point "a" to point "b," and inform us of the towns and other sites that we will pass as we travel along the route. However, a GIS can do that and much more. For example, it could give us additional information such as the road length, type and whether it is one-way, or it could calculate the time it would take for us to travel from point "a" to "b" based on the time of day of the travel, the speed limits along the route and amount of traffic expected at that time.

Map layers (points, lines, and polygons)

In a GIS, information is organized in layers. Each layer presents a specific type of feature or geographic object. Commonly these are either points, lines, or polygons. Other more sophisticated layers types include coverages, grids, TINs (triagulation), and pixels (imagery). Digital elevation models (DEMs) are special databases that impart 3-dimensional visual characteristics to a map. These are often called hillshade themes.

Point layers represent x and y coordinates for schools, hospitals or other small places on a map. Lines represent such things as roads, railroads and waterways. Polygons represent political and administrative boundaries, lakes, parks or other large areas.  The layers provide a means to separate large amounts of graphic data into smaller groups that tend to be used together. The layers are registered to each other by the common coordinate system they share with the base map.

Geodatabases and attribute tables

A GIS database includes data about the spatial location and shape of geographic features recorded as points, lines, areas, pixels, grid cells, or tins, as well as other attributes organized into map layers. Each layer has an attribute file associated with it. Attribute files are tabular files whose records are linked to and describe the layer features.

Spatial analysis

A GIS can combine many layers of information, analyze one layer based upon another layer, and display only the information needed. Spatial analysis is useful for evaluating suitability and capability, for estimating and predicting, and for interpreting and understanding. There are four traditional types of spatial analysis:

1. topological overlay and contiguity analysis
2. surface analysis
3. linear analysis
4. raster analysis

The Reproductive Health Atlas is an example of the first type — topological overlay and contiguity analysis.

Page last reviewed: 7/28/08
Page last modified: 1/29/07
Content source: Division of Reproductive Health, National Center for Chronic Disease Prevention and Health Promotion