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Rasters divide space into uniform cells or pixels and use a 2-D matrix to store sampled values for each cell.
The cell width and height are of a uniform size; however, the cell height may be
different than the cell width. The cells
of a raster depict a variety of data, such as light reflectance captured by a satellite, a photograph's
a thematic attribute such as vegetation type, a surface value, or elevation. Rasters can represent an imaged map, a surface, an environmental attribute sampled
on a grid, or photographs of objects referenced to features.
In the context of GIS applications, raster data is an abstraction of the real world to which spatial data is expressed as a matrix of cells or pixels, with spatial position implicit in the ordering of the pixels. With the raster data model, spatial data is not continuous but divided into discrete units. This makes raster data particularly suitable for certain types of spatial operations, such as overlays or area calculations. Unlike vector data, however, there are no implicit topological relationships.
The ArcSDE raster API provides a mechanism for loading raster data from an application onto the ArcSDE server and retrieving it. The API is a low-level interface for basic read-write raster data access to the ArcSDE server. Thus, it enables organizations to store their raster data in one of the supported commercial (IBM DB2, Informix, Microsoft SQL Server, and Oracle) or open-source (PostgreSQL) relational DBMS for fast, online, multiuser access to continuous raster data.
Values stored in a raster either represent discrete or continuous data. Discrete raster data represent geographic features that have definable boundaries. Discrete geographic features include buildings, wells, and land parcels. Continuous raster data represent geographic information that can vary at every location, like elevation, temperature, or radio signal strength. Continuous geographic phenomenon do not have distinct boundaries.
With raster data, each pixel is independent of the other pixels, recording information only about the area it covers. For example, a pixel does not know that the pixel beside it might be part of the same tree or building.
A raster is sometimes also called a scan line. A raster image, therefore, is an image made of such scan lines.