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

Chapter 4
PostGIS Raster
This section deals with representing raster structure and explaining from how raster is stored to
the mechanisms used to index, retrieve raster in PostgreSQL. It is also important to address the way
that raster and vector layers overlap - a relevant feature that makes PostGIS Raster more robust than
ORACLE GEORaster.
4.1 Implementation
PostGIS Raster chooses to implement a minimal raster data structure. That means that a raster
type is as simple as a single type and is stored in a single table. This data structure is very similar
to the one of PostGIS vector data and very different from the Oracle Spatial SDO_GEORASTER and
SDO_RASTER raster structures. In PostGIS Raster:
• One table with a column of type raster is referred to as a raster coverage.
• One table row with a column of type raster corresponds to one tile.
The choice of such simple data structure will facilitate the achievement of four objectives described
in Chapter 3. With a single raster type, it implies that each raster attribute is a complete and self
sufficient georeferenced raster. So raster attributes composing a table are not necessarily related to
each other to form a significant coverage. This also means that:
• Rasters from the same table may have different size.
• The upper left corner and the pixel size of a raster are not necessarily the same values as those
of another rasters. They may therefore snap to different grids.
• Finally, different rasters may overlap like polygons in a vector layer may overlap. This is a
fundamental feature to implement meaningful vector to raster conversions in which all attributes
of a vector are conserved in the resulting raster table. In this case, if the vector features and the
data values part of the resulting raster features do not necessarily overlap, the nodata values of
the raster features will overlap.
In addition, PostGIS Raster provides also a raster_columns table to help application get a quick
overview of raster tables and information (such as metadata) related to the rasters stored in these
tables.
4.2 Structure
PostGIS Raster implements only one type of raster instead of two like SDO_GEORASTER and
SDO_RASTER in Oracle Spatial. It supports no metadata when a raster contains all data about itself
such as attribute information, georeference information, band information and band data. Figure
?? describes these components in details [6]. As shown in this figure, a raster has no mask and
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