How GIS Information is Organized

There are two ways to organize information in GIS: by individual element or by data raster.

When organizing information by individual element (such as a road, bridge, or census tract), elements are called features and are represented on the GIS map as either a point (for a traffic count location, for example), a line (for roads, rivers, etc.), or a polygon (for towns, census tracts, etc.).

This information is organized in data layers; each data layer contains a particular type of feature (such as, roads or census tracts or wetlands). In addition to displaying the features, data layers also can contain information on them (such as street names, bridge conditions, roadway functional classifications).

The user of the interactive maps on this site can search (query) the database to find the features or to find detailed information of interest on individual items in a feature (Summer Street, for example), as well as information on all the items (such as which roads are state-owned roads).

In the other way of organizing geographic information, data raster format, the data does not represent individual features. Instead, the area covered by the data is divided into a checkerboard of small squares (a grid), with each square assigned a particular value representing a characteristic of that land area (such as elevation, land cover, or color aerial photographic images). 

The interactive maps also contain metadata, which is information on the original sources of the data, or how the data layer was developed, and on other aspects of the data.

Whether organized by feature or raster, when layers are combined into a map, they are drawn or overlaid on top of each other to create a composite image.