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

Figure 4.30: Pyramids at different levels and scales [36].
The above command generates an sql file that will load all jpegs in current folder into a new
table called raster_table, where each raster record is a 100x100 pixels width/height. The -l will
create an overview table for raster_table with pyramid level 4. The overview table will be called
o_4_raster_table and contains less raster records than the table raster_table has. Each raster
record in o_4_raster_table is a 100x100 pixels width/height but at a lower resolution.
The following script will execute the sql file to load the data into postgisdb:
psql -d postgisdb -f aerials_overview4.sql
4.12 Masks
A mask is a special single band raster where each pixel having either the value of 0 or 1. It is
used to define an irregularly shaped region inside another image. The 1-bits define the interior of the
region and the 0-bits define the exterior of the region.
A mask can be formed from a non blank raster. Each band of a non blank raster can also have a
separate bitmap mask associated with it. Thus, there can be at most n+1 bitmap masks associated
with a non blank raster, where n is the total number of bands of the raster. A bitmap mask can also
be edited or updated independently.
A mask attached to a raster must have the same number of rows and columns as any raster bands
and must precisely cover the same area. It uses the same SRS (the spatial reference system) as that
of the raster itself. Logically, a bitmap mask is not an integral part of the raster image but rather an
additional piece of information. Like pyramid, It is stored as a separate raster and independent the
original raster.
Masks are generally used by applications in the following ways [4]:
• A mask can be used to determine which part of the image is displayed because sometimes users
might want to mask out a part of a raster. An example might be one where a raster layer
describes elevations ranging from below sea level to mountain top. If only certain elevations
above sea level are interested, a mask can be applied to the raster layer to visually simulate
a rise in sea level. A such mask will be computed from the source raster. For example, for
every cell greater than or equal to 100, its value is set to 1, otherwise its value is set to 0. The
output is a new raster containing cells with either a 0 (black) or 1 (while) value. The black areas
(respectively the white areas) represent everything below (above) the elevation of 100 meters.
Combining this mask with a suitable background, the effect of raising sea level is effectively
illustrated as show in Figure 4.31.
• When a mask is used as a noData mask in GIS application, the pixel value corresponding to the
exterior (0-bits) of the mask will be treated as noData value. For this purpose, this value can
be registered as a noData value in the raster band information.
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