summary: structure of spatial data in ilwis
in this section you learned the following:
-ilwis use two type of map: vector map and raster map.
-vector map in ilwis are either point map, segment map or polygon map.
-vector map can be obtained either by digitizing analog map, or by importing them from other sources.
-raster map can be obtained either by rasterizing vector map, by importing them from other sources or by performing gis or image processing operation in ilwis.
-raster map, polygon map, segment map, point map, table and columns are called data object. they contain the actual data.
-service object are used by data object; they contain accessories that data object need beside the data itself. domain, representation, coordinate system and georeferance are called service object.
-vector map need a number of service object: a domain, a representation and a coordinate system.
-raster map also need these service object: a domain, a representation and a coordinate system. all raster map contain information in discrete cell or pixel, which are ordered in row and columns. to relate these pixel to map coordinate another service object called a georeferance is needed.