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

Figure 4.37: Tiled images (2 tables of 54 tiles) [6].
6. A raster table may be a raster coverage resulting from any analysis operations (such as ST_AsPolygon(),
ST_Intersection()) that imply rasterization of a vector coverage. From the rasterization, each
vector feature becomes a small raster with the same extent as the original vector feature. This
type of coverage is not necessarily complete nor rectangular. Tiles should be of different sizes
and might overlap. It all depends on the characteristics of the vector layer being rasterized. For
example, a continuous layer of mutual exclusive geometries (without gaps or holes like a forest
cover) would result in a raster coverage in which significant pixels (with data values) would not
overlap, but non-significant pixels (nodata values) would. On the other end, a discontinuous
layer of mutual exclusive geometries (like a lake or building layer) would result in a coverage of
mostly disjoint raster objects. In practice, this arrangement is identical to a vector layer in the
sense that all pixels of each raster have the same value.
Figure 4.38: Raster object coverage (9 raster objects corresponding 9 rows in a raster table) [6].
All these arrangements are possible and PostGIS Raster does not impose one over the other even
though types 3. and 4. and 6. are the most practical for a GIS analyst. This is due to the fact
that users might add or delete rows (or tiles) of a raster table and PostGIS Raster must support
variable-sized tiles for vector to raster conversion. So it is very difficult to enforce a certain type
of configuration.
The raster arrangements play a fundamental role in implement meaningful vector to raster
conversion (see Section 4.18), where all attributes of a vector are conserved in the resulting
raster table. Example, 10 lake vector features with attributes like name, type, area and etc. are
converted to 10 lake raster features with the same attributes. 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 (see Figure 4.39). These characteristics enable an easy
integration of raster with vector.
In PostGIS, the conversion from raster to vector can be done using ST_DumpAsPolygons() function
as shown in Figure 4.40. So it is not matter whether the layers are in raster or vector format
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