[postgis-users] RFQ: WKT Raster SQL API for being able to intersect vector and raster tables

[Pierre Racine]

Hi PostGIS folks,

Now that you have a little bit of time after the release of PostGIS 1.5, I would like to request your comments on the WKT Raster specifications drawn to meet objective 0.1.6 - "Being able to intersect vector and raster to produce vector". In other word: what set of function do we need to be able to intersect a geometry layer with a raster layer? You will find the last version of the written specs here:

http://trac.osgeo.org/postgis/wiki/WKTRaster/SpecificationWorking01#Objective0.1.6e-Beingabletointersectvectorandrastertoproducevector

This kind of operation is a generalisation of the operation needed by many people doing spatial analysis involving raster and vector. Some examples of users expressing their needs for a similar functionality:

http://postgis.refractions.net/pipermail/postgis-users/2009-April/023218.html

http://postgis.refractions.net/pipermail/postgis-users/2009-March/022890.html

Some from the r-sig-geo group:

https://stat.ethz.ch/pipermail/r-sig-geo/2009-December/007177.html

https://stat.ethz.ch/pipermail/r-sig-geo/2009-November/006999.html

We plan on reusing as much as possible existing PostGIS functions. The basic idea is to convert (only the) WKT Raster tiles involved in the intersect to geometries and then to procede to a normal vector intersection using the PostGIS intersection function. It involves functions like: ST_Envelope(raster), ST_ConvexHull(raster), ST_Shape(raster, integer), ST_AsPolygon(raster, integer), ST_AsWKTPolygon(raster, integer), ST_Intersects(raster, integer, geometry), ST_Intersection(raster, integer, geometry). Jorge is in charge of the implementation.

We are now in the process of testing if it is worth using the polygonize function of GDAL, thus linking with GDAL, or implementing our own. A summany of the pros and cons is available at this location:

http://trac.osgeo.org/postgis/wiki/WKTRaster/SpecificationWorking01#TheprosandconsofusingGDALvsimplementingourownrasterservices

We are also discussing the pros and cons of directly returning PostGIS geometry objects, thus having to link with PostGIS at the C level and all it involve in term of release management or to return WKT geometries and thus using PostGIS just at the SQL level, facilitating supports for former/newer PostGIS versions. An argument against linking with PostGIS is available at this location:

http://trac.osgeo.org/postgis/wiki/WKTRaster/SpecificationWorking01#WhyisitimportnattoavoidlinkingwithPostGIS

The most cited tool to do such operation right now is StarSpan (http://starspan.projects.atlas.ca.gov)

There is plan to implement a raster/raster intersection function as well but the exact result of such an operation is still to determine.

Thanks for your thoughts.

Pierre