Three fundamental representations of geographic information layers
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.
Features—points, lines, and polygons
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.
- Points define discrete locations of geographic features too small to be depicted as lines or areas, such as well locations, telephone poles, and stream gauges. Points can also represent address locations, GPS coordinates, or mountain peaks.
- Lines represent the shape and location of geographic objects too narrow to depict as areas (such as street centerlines and streams). Lines are also used to represent features that have length but no area, such as contour lines and administrative boundaries. (Contours are interesting, as you'll read later on, because they provide one of a number of alternatives for representing continuous surfaces.)
- Polygons are enclosed areas (many-sided figures) that represent the shape and location of homogeneous features such as states, counties, parcels, soil types, and land-use zones. In the example below, the polygons represent land parcels.
Maps convey descriptive information through map symbols, colors, and labels. For example:
- Roads are displayed based on their road class (such as line symbols representing divided highways, main streets, residential streets, unpaved roads, and trails).
- Streams and water bodies are drawn in blue to indicate water.
- City streets are labeled with their names and often some address range information.
- Special point and line symbols denote specific features such as rail lines, airports, schools, hospitals, and incidents of various types.
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:
- Descriptive data is organized into tables.
- Tables contain rows.
- All rows in a table have the same columns.
- Each column has a type, such as integer, decimal number, character, and date.
- Within relational databases, these concepts are extended to include a series of relational functions and operators that can be used to operate on the tables and their data elements. This is known as Structured Query Language or SQL.
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.
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.