Discrete entities can be found as fields or objects.
Discrete fields divide the study space in mutually exclusive, bounded parts, with all locations in one part having the same field value. Discrete fields are intermediate between continuous fields and geographic objects: discrete fields and objects both use “bounded” features.
Discrete fields divide the study space in mutually exclusive, bounded parts, with all locations in one part having the same field value. Typical examples are land classifications, for instance, using either geological classes, soil type, land use type, crop type or natural vegetation type.
Discrete fields are intermediate between continuous fields and geographic objects: discrete fields and objects both use “bounded” features. A discrete field, however, assigns a value to every location in the study area, which is not typically the case for geographic objects. These two types of fields differ in the type of cell values. A discrete field such as land use type will store cell values of the type “integer” and is therefore also called an integer raster. Discrete fields can be easily converted to polygons since it is relatively easy to draw a boundary line around a group of cells with the same value. A continuous raster is also called a “floating point” raster.
Geographic objects.
When a geographic phenomenon is not present everywhere in the study area, but somehow “sparsely” populates it, we look at it as a collection of geographic objects. Such objects are usually easily distinguished and named, and their position in space is determined by a combination of one or more of the following parameters:
location (where is it?)
shape (what form does it have?)
size (how big is it?)
orientation (in which direction is it facing?).
How we want to use the information determines which of these four parameters is required to represent the object. For instance, for geographic objects such as petrol stations all that matters in an in-car navigation system is where they are. Thus, in this particular context, location alone is enough, and shape, size and orientation are irrelevant. For roads, however, some notion of location (where does the road begin and end?), shape (how many lanes does it have?), size (how far can one travel on it?) and orientation (in which direction can one travel on it?) seem to be relevant components of information in the same system.