All of the rich GIS behavior for representing and managing geographic information is based on three fundamental representations or expressions of geographic information:
Each type is described here.
Geographic features are representations of things located on or near the surface of the earth. Geographic features can occur naturally (such as rivers and vegetation), can be constructions (such as roads, pipelines, wells, and buildings), and can be subdivisions of land (such as counties, political divisions, and land parcels).
Although there are a number of additional feature types, geographic features are most commonly represented as points, lines, or polygons.
Maps convey descriptive information through map symbols, colors, and labels. For example:
In a GIS, descriptive attributes are managed in tables, which are based on a series of essential relational database concepts. Attribute tables provide a simple, universal data model for storing and working with attribute information. They are inherently open because their simplicity and flexibility enables support for a broad range of applications. Key concepts include the following:
Imagery in GIS often refers to a number of types of cell- or pixel-based data sources—for satellites, aerial photography, digital elevation models, raster datasets, and so on.
Imagery is managed as a raster data type composed of cells organized in a grid of rows and columns. In addition to the map projection, the coordinate system for a raster dataset includes its cell size and a reference coordinate (usually the upper left or lower left corner of the grid).
These properties enable a raster dataset to be described by a series of cell values starting in the upper left row.
Each cell location can be automatically located using a reference coordinate for the origin, the cell size, and the number of rows and columns.
Typical image sources include cameras capable of capturing aerial photographs that can be georeferenced and corrected to ground locations (such as digital orthophotography).
Imagery is also used to collect data in both the visible and nonvisible portions of the electromagnetic spectrum. One system is the multispectral scanner carried in Landsat satellites that records imagery in seven bands (or ranges) along the electromagnetic spectrum. The measures for each band are recorded in a separate grid. The stack of seven grids makes up a multiband image.