Introduction

Geographic Information System (GIS) software allows you to overlay geographic layers to build maps or analyze data that has a spatial (locational) component.  The layers are spatially "registered" so when the user adds them the program can line them up correctly to build a map.  Each category of spatial feature is a separate layer, there are several types of layers as indicated below.  The user has many choices regarding how to depict them and how to analyze the data associated with them.  The first three types of layers are knows as "vector data" as opposed to "raster data."

• Point (e.g., buildings, landmarks).  Zero-dimensional.

• Line, or arc (e.g., roads and streets, streams, railroads, power lines). One-dimensional.

• Polygon (e.g., political entities, census geographies such as tracts). Two-dimensional.

• Raster images (e.g., an aerial photograph or scanned topographic map).  Can be useful as backdrops for overlaying other layers.

Data can be associated with the spatial features, and mapped or analyzed:

• There can be attributes, or spreadsheet data, associated with each feature in a layer (e.g., demographic data for each Census Tract).

• Data tables (e.g., database or spreadsheet files) can be added ("joined") to a layer if there is a common field (e.g., census tract number).  The preferable format to use with ArcGIS software is dBase.

GIS software can also map spatially referenced data files in some spreadsheet and database formats (e.g, if one field contains latitude/longitude coordinates).  Tables that contain address data can be "geocoded" to map the locations on a street layer.  Advanced users can open a non-registered raster image and register it using the program's functions.

You can also add your own information to a map with drawing and writing tools.

The library has the software ArcGIS installed on several computers in the Data Services & GIS cluster on the first floor of Perkins and on the computers in the cluster in the basement connector between the Perkins and Bostock wings of the building.  We have some links to sources of GIS data as well as links to other web sites with more information about GIS.