PostGIS Raster : Extending PostgreSQL for The Support of ... - CoDE

4.4.4 Representing Features
Figure 4.13: Raster zones and regions [32].
In raster data, the cell typically represents the predominant phenomenon of the area covered by
a cell, whereas vector data can accurately identify individual features. This is due to the fact that
when representing geographic features using raster data, features are nothing more than collections
of cells with the same attribute values and thus lose their unique identities. Raster data is best used
when the primary problem concerns with the locational relationships of the phenomena represented
by geographic features and not the features themselves.
Discrete Data
Discrete data, which is sometimes called thematic, categorical or discontinuous data, is used to
describe discrete objects and can be expressed in both vector and raster formats. A discrete object has
always precise boundaries. So it is easy to define where the object begins and where it ends as well as
how to compose them from discrete features like points, lines and polygons. A lake is a discrete object
surrounded by landscape. Other examples of discrete objects include buildings, roads and parcels.
Points
Figure 4.14: Discrete data represented in raster format [33].
A point is represented by an explicit x,y coordinate in vector format. However, in raster format,
it is a single cell, the smallest unit of a raster. By definition, a point has no area but will represent
an area when it is converted to a cell. For example, a telephone pole or the location of an endangered
plant will occupy the entire area covered by a cell. Therefore, the smaller the cell size, the smaller the
area and thus the closer the representation of the point feature.
As a raster point has the same size as the size of the cell. So the cell size has to be chosen small
enough to capture sufficient input points for the desired analysis. In the case if two or more points
fall within the same cell, the value of one of these points will be randomly selected when assigning a
value to the cell.
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