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

4.19 Intersection
The fact that PostGIS Raster achieves efficient conversion between raster and vector, is the base
to implement certain operations over data, whether it is in raster or vector format. These operations
are essential in PostGIS because they contribute to the accomplishment of its second goal described
in Chapter 3. This goal includes offering a single set of overlay SQL function operating seamlessly
on vector and raster data. Furthermore, in comparison to ORACLE GeoRaster (also in Chapter 3),
they play an important role that makes PostGIS Raster more performing than ORACLE GEORaster
and more adapted to various groups of users (experienced and inexperienced ones). Such operations
include ST_Intersection() that makes intersection between raster and vector data. Several possible
scenarios are available for this operation:
1. A vector and vector intersection with a vector layer as a result.
2. A vector and raster intersection with a raster layer as a result.
3. A vector and raster intersection with a vector layer as a result.
4. A raster and raster intersection with a raster layer as a result.
The result can be a raster or a set of geometry-pixel value pairs (as in the case of ST_DumpAsPolygons())
representing the shared portion of two geometries, two rasters or between a vectorization of a raster
and a geometry. These cases will be examined one by one in the following examples.
Example 1.1: A vector and vector intersection with a vector layer as a result is illustrated in Figure
4.47.
Figure 4.47: Vector and vector intersection with a resulting vector layer [22].
This example examines the first case of intersection between a geometric circle a with a geometric
vegetation cover (type 1 in blue and type 2 in green). The result is a half circle in vector format with
two attributes: a circle name and a cover type.
Example 1.2: A vector and raster intersection with a raster layer as a result.
In general, the intersection begins to select which raster tiles have to be loaded in order to construct
an intersected raster extent (using ST_intersects()) containing all intersected raster tiles. In the
first view, this paradigm makes the operation more efficient in the sense that the number of intersected
raster tiles is often a small number in compare to whole number of existing raster tiles (see Figure
4.48). In second view, the intersection operation works only with these intersected tiles. The raster
extent construction is just the first phase because true intersection will take pixel values (raster format)
or geometries (vector format) into account (using ST_intersection()).
If the output is a raster, then the operation will attach pixel values in the final result as one in
Figure 4.49. So the vegetation cover type represented by a raster is well known. In addition, the
vector attribute is also conserved and become an attribute of the resulting raster.
Example 1.3: A vector and raster intersection with a vector layer as a result.
If the final result is expressed in vector form, the intersecting raster tiles are first vectorized (using
ST_DumpAsPolygon()) into a set of geometry-pixel value and this set is then intersected with the
geometry using the ST_Intersection(vector, vector) function. At this stage, one more time, as
vectorization is a complex task (it is related to each pixel value) and time consume, the paradigm
56