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

for discrete data while floating-point values are suited for continuous data (such as surfaces). Cells
can also have a noData value to indicate the absence of data.
Depend on represented data, there are two ways to apply a value to a cell. Generally, when the cell
value represents a measure as in the case of elevation, it is applied at the center of the cell. However,
in most cases, the cell value is resulted from a sampling of a phenomenon, so it is applied for the whole
cell square.
Figure 4.7: On the left side: cell values applied at the center point. On the right side: cell values
applied for the whole square.
Raster data is stored as an ordered list of cell values. For example, such a raster in Figure 4.8 will
be stored as a list of 80, 74, 62, 45, 45, 34 and so on.
Figure 4.8: Raster cell values [16].
The area represented by each cell consists of the same width and height and equals a portion of
the entire surface represented by the raster. For example, a raster representing elevation may cover
an area of 100 square kilometers. If there were 100 cells in this raster, each cell would represent one
square kilometer of 1 km in width and 1 km in height.
The dimension of the cell can be as large or as small as needed to represent the surface conveyed
by the raster and the objects within this surface. In other words, the cell size determines how coarse
or fine the objects in the raster will appear. The smaller the cell size is, the smoother the raster will
be. However, when decreasing the cell size to get better image, the number of cell will increase as the
surface size is maintained. A large number of cells will take long time to process and demands more
storage space. In contrast, if a cell size is too large, information may be lost or subtle patterns may
be obscured. For example, if the cell size is larger than the width of a road, the road may not exist
within the raster dataset. In Figure 4.10, it shows how does the polygon look like as it is represented
at various cell sizes.
The location of each cell is defined by the row or column where it is located within the raster
matrix. Essentially, the matrix is represented by a Cartesian coordinate system, in which the rows of
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