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

Choosing an appropriate cell size is not always simple. For example, the spatial resolution needed
for applications againsts with the requirements for quick displaying, processing time and storages. In
GIS, the choice is made for the least accurate raster data. For example, for a raster data derived from
30-meter resolution Landsat imagery, a digital elevation model (DEM) at a higher resolution, such as
10 meters, may be unnecessary.
Figure 4.20: Raster in different spatial resolutions [11].
However in most of applications, determining an adequate cell size is not a static process and
depends on what data expected to obtain. In the other hand, a raster can always be resampled to
have a larger cell size but any greater detail can not be obtained by resampling the same raster at a
smaller cell size. So, it is useful to store a copy of the raster at its smallest cell size, then it will be
resampled to match the use in application to increase analysis processing speed.
4.6 Spatial Resolution Versus Scale
Spatial resolution refers to the dimension of the cell size that represents an area on the ground. If
the area covered by a cell is 5 x 5 meters, the resolution is 5 meters. The higher the resolution of a
raster, the smaller the cell size and thus the greater the detail. This is opposite to scale.
A scale model represents a copy of an object that is larger or smaller than its actual size. To do
this, a relative proportion (a scale factor) of the physical size of the original object is maintained for
the restitution. The smaller the scale, the less detail shown. So, an ortho photograph displayed at a
scale of 1:2,000 (that means 1 unit on model is equal to 2,000 units on real object) shows more details
than one displayed at a scale of 1:24,000. However, if the same orthophoto has a cell size of 5 meters,
the resolution will remain the same no matter what scale it is displayed at, as the physical cell size
does not change.
Image shown in Figure 4.21 illustrates well these effect. On the left, the scale of the image (1:50,000)
is smaller than the scale of the one on the right (1:2,500) but their spatial resolutions (cell size) are
the same. Another way to display images is to change the spatial resolution and maintain the scale
like the ones in Figure 4.22.
4.7 Bands
PostGIS Raster supports multiple bands. So besides the ordinates row and column, each raster can
have the third ordinate band and is identified as a cell in the space of three dimensions (see Figure
4.23). Band is numbered from 1 to n, where n is the highest band number. Each band has information
about itself such as band data, a pixel type and a nodata value (as described in Figure ??). The pixel
type indicates the number of bits used to express the color of a single pixel as well as the nodata value.
Band data is a two-dimensional matrix of cells. The storage used for band data is band sequential
(BSQ).
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