summary: about domain
-a domain store the set of possible data "value" that are used by a map, table and/or column. all ilwis data object (vector map, raster map and table) alway have a domain.
-the same domain can be used by a vector map, a raster map and a table.
-the four most important type of domain are: class domain, identifier domain, value domain and the image domain.
-class domain are used for data object that contain class. a class domain may for instance store the name of land use unit or geomorphological unit.
-id domain are used for data object that contain unique identifier. an id domain may for instance store the unique id of city block, or the unique id of rainfall station.
-value domain are used for data object that contain measured, calculated or interpolate value. a value domain may for instance store the value (range and precision) needed to define height or to define concentration.
-the system image domain is used for satellite image or scanned aerial photogragh containing value between 0 ang 255. this domain is a special type of value domain, as the image domain cannot contain undefined value.
-class and id domain are alway user-defined. value domain can be user-defined, or system-defined.