Three subclasses of Coverage are recognized in the base standard: Raster, Field, and PartitionedCoverage. User defined subclasses of Raster are intended to describe typical raster data, where a series of values are assigned to the positions in a grid. The values may indicate reflectance values, vegetation type, or just about anything else. The grid is defined in terms of a regular, multi-dimensional, rectangular, hexagonal or triangular structure. In addition to the values pertaining to the grid positions and the grid itself, a raster may be defined with other attributes as well, such as the height of a satellite, the time of observation, etc.