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

One approach to make this intersection operation without PostGIS Raster is to convert those
raster images to shapefiles (a popular geospatial vector data format), import them in PostGIS and do
a traditional intersection query between geometries only. The problem is that converting only one of
those Shuttle Radar Topography Mission (SRTM) files is very difficult. The vector version of only one
of those files exceeds the limit of 2 GB imposed on shapefiles. Since it is mostly impossible to convert
those rasters to vectors, the PostGIS Raster approach is to load them into PostGIS and will vectorize
only ones that are needed to do the requested operation. This approach benefits greatly from the
indexing of raster data in the database.
4.17 Retrieving
As each storing type is specified for a class of application, it results in a strong difference in the way
they retrieve raster data.
4.17.1 In-database Raster Retrieving
There exist two modes when retrieving in-database raster data from PostGIS database:
1. ONE RASTER PER ROW
2. ONE RASTER PER TABLE
PostGIS uses the GDAL driver for exporting rasters via a connection string. The syntax of this
connection string is as follows:
PG":host=’’ port:’’ dbname=’’ user=’’ password=’’
[schema=’’] [table=’’] [where=’’]
[mode=’’]"
Certain fields can be omitted like password in some case. The table option specifies name of a
raster table to connect. The where option is used to filter results from the raster table. The mode
option determines which connection mode is evoked:
• Mode = 1 or ONE RASTER PER ROW mode (the default mode): in this mode, a raster table is
considered as a set of different raster images. Then where parameter is used to select which
raster images will be exported.
• Mode = 2 or ONE RASTER PER TABLE mode: in this case, a raster table is considered as a unique
raster image composed by a set of raster tiles (one raster tile per row). This mode is intended
for reading tiled raster.
When the connection is established, the GDAL driver uses gdal_translate command to create
an image file from the connected raster table:
gdal_translate -outsize 50% 50% "PG:host=’localhost’ dbname=’’
user=’’ password=’’ table=’table_name’ mode=’2’"
table_name.tif
The additional option -outsize will create an image file like the original image but at a different
size (a half size in the above example).
4.17.2 Out-database Raster Retrieving
PostGIS Raster enables user to simply register basic metadata of images stored in the filesystem
without having to actually load their data values into the database. This is out-database storage as
opposed to the more natural in-database storage. In this way, web or desktop applications:
• do not have to retrieve images from the database but instead can access them directly from the
file system (such as JPEG files).
• can use almost PostGIS Raster operators and functions on those rasters using GDAL driver
(out-database raster support is under development).
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