On Sat, Dec 3, 2011 at 7:16 PM, Stefan Keller <sfkel...@gmail.com> wrote: > Hi Bborie > > Many thanks for your response. > To summarize: My goal is to get the height (in meters) given an > arbitrary lat/lon coordinate from the latest free digital elevation > model (like mentioned here > http://slashgeo.org/2011/12/01/New-Free-Elevation-Dataset-named-Global-Multi-resolution-Terrain-Elevation-Data-2010-GMTE > ). > > The approach using PostGIS to access raster data is > 1st. to convert an ASCII GRID text file (an ESRI format) to GeoTIFF > (not tiled nor "pyramide"-ized). > Then (2nd) to load it into PostGIS, like described here > http://trac.osgeo.org/postgis/wiki/WKTRasterTutorial01 . >
I don't remember but I think GDAL supports ASCII GRID so you don't need an intermediary file. The loader uses GDAL to convert them into the PostGIS Raster format. > I still have some basic questions about this interesting approach: > > Regarding my question of "rules of thumb" in order to set the tile > size you answered: >> Generally speaking, the smaller the tile size, the faster the operation. In >> my testing of my rasters, 200x200 is slower than 100x100 which is slower >> than 50x50. The smaller tile size results in more rows in the table BUT the >> performance dramatically increases. I can't remember of the top of my head >> but the performance gain was linear or better. > > Thats a good hint. > Q1: But are there no calculcation rules on how to estimate an ideal > absolute size (in pixels)? > Nope. Testing is the only way as there are other possible factors involved. And even my estimates above are only guidelines. I know I'm going to have to test for my production database servers before I know what works best for them. In a pinch, I'd say you'd be happy between 30 - 70. > Regarding a query given lat/lon you proposed: >> Assuming you test to see what the ideal (or "good-enough") tile size >> is for the elevation rasterset, you should see good performance using >> >> ST_Value(rast, ST_SetSRID(ST_MakePoint(long, lat), srid)) >> >> You could eliminate the ST_SetSRID and ST_MakePoint by passing a >> correct EWKT instead >> >> ST_Value(rast, 'YOUREWKT'::geometry) > > Q2: Given the raster has a resolution (grid) of - say at best - 30m, > does this function really interpolate, given the input is an arbitrary > coordinate (I can't find a hint to that in the docs > (http://postgis.refractions.net/documentation/manual-svn/RT_ST_Value.html))? > There is no interpolation. It literally is a pin stuck through the raster. > Q3: If yes, I assume its linear. How could I proceed to get a result > using non-linear interpolation (like Kriging)? > I don't know if you can using only PostGIS raster. If you're able to use PL/R as well, you should be able to tap into their kriging tools. -bborie -- Bborie Park Programmer Center for Vectorborne Diseases UC Davis 530-752-8380 bkp...@ucdavis.edu _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
