Basic concepts

Before creating ILWIS maps and tables

A map contains certain features and refers to the geographic position of those features:

	point maps: a point map contains point features;
	segment maps: a segment map contains line features;
	polygon maps: a polygon map contains closed area features;
	raster maps: a raster map contains pixels. In ILWIS, most spatial operations are performed on raster maps.

When using satellite images, you already have raster maps. Pixels in a satellite image have values ranging from 0 to 255, although not every value may occur in your image. In ILWIS, satellite images have domain Image.

When starting from an analog paper map, you begin with:

digitizing point features of interest with the point editor,
digitizing line features of interest with the segment editor, and/or
digitizing the boundaries of area features of interest with the segment editor; then polygonizing these boundaries to obtain polygons; and editing the polygons with the polygon editor.

Afterwards, point, segment and polygon maps can be rasterized into a raster map.

However, before digitizing, you have to know that features in any map can be identified in the following manner:

-	by ID:	each feature is identified by its own name or number; each feature occurs in general only once in the map;
-	by Classes:	features are identified by their class name; features may occur in several places in the map;
-	by Value:	features are identified by a measurable value.

The following scheme illustrates the above:

Map type:	Features in map:	Identification of features by:	Example:
Point map	Points: indiv. sample points Points: classes of farms Points: height information	IDs Class names Values	1,2,3 small farm, large farm 500, 600, 700 (m)
Segment map	Lines: individual rivers Lines: classes of roads Lines: isolines for height	IDs Class names Values	section1, section2 prim. road, sec. road 500, 600, 700 (m)
Polygon map	Areas: indiv. cadastral plots Areas: land use classes Areas: acid deposition	IDs Class names Values	plot 234a, plot 345b rice, maize, forest 1000, 2000, 5000 (mol/ha/y)
Raster map	Pixels: rasterized points, lines or areas with IDs Pixels: rasterized points, lines, or areas with Class names Pixels: rasterized points, lines or areas with Values Pixels: value outcome of calculations	IDs Class names Values Values	plot 234a, plot 345b rice, maize, forest values any value
Raster map	Pixels (satellite images)	pixel value 0 to 255	TM images, SPOT images

The user-defined collection or set of IDs, class names or values that may be used in a map is called the domain of a map. In ILWIS, we speak of maps with domains of the ID, class or value type, meaning that the elements in that map are either identified by IDs, class names or values.

Only maps with features identified by IDs or class names can have attributes; i.e. extra information about the point, segment, polygons or mapping units, in so-called attribute tables. When creating an attribute table, this table should have the same domain as your ID or class map. Afterwards you can add extra columns with the attribute information. The domain provides here the relational link between the map and the attribute table.

Maps containing values, for example a satellite image or a height map, cannot have attributes in an attribute table.