 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
color value,
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 9.3 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.
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