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

Lines
Figure 4.15: Raster point representation [34].
In vector format, a line is an ordered list of x,y coordinates whereas in raster format it is represented
as a chain of connected cells with the same value. When there is a break between the chain of same
valued cells, it corresponds a break in the line. A break can also be used to define two roads or two
rivers that do not intersect.
Figure 4.16: Raster line representation [34].
Converting a vector line to raster data is similar to converting a vector point to raster data. For
any line that passes over a cell, this cell will receive the attribute value of this line. If multiple lines
pass through a single cell, one of the lines will be randomly selected to represent that cell.
The same as vector point, the width of vector line will become the width of the cell. For example,
if the line that is being converted represents a road and the cell size is one meter, then the road will
be one meter wide in the output raster data.
Polygons
A vector polygon is an enclosed area defined by an ordered list of x,y coordinates in which the first
and last coordinates are the same. In contrast, a raster polygon is a group of contiguous cells with
the same value that most accurately describe the shape of the area. Examples of polygonal features
include buildings, ponds, soils, forests, swamps and fields.
The accuracy of the raster representation depends on the scale of the data and the size of the cell.
The finer the cell resolution and the greater the number of cells, the more accurate the representation.
Continuous Data
Continuous data is used to describe continuous surfaces. A continuous surface represents phenomena
in which each location on the surface has a measure of the concentration level or a value describing its
relationship with a fixed point in space or with an emitting source. Continuous data is also referred
to as field, nondiscrete or surface data.
One type of continuous surface, that is derived from those characteristics that define a surface, is
a surface where each point in the surface is its measured from a fixed reference point. These include
elevation (the fixed reference point is a sea level and the measure at each point is its height above the
36