On Sun, Dec 4, 2011 at 6:55 AM, Stefan Keller <sfkel...@gmail.com> wrote: > 2011/12/4 Bborie Park <bkp...@ucdavis.edu> wrote... >>> 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. > > You mean aka 'snap-to-grid', i.e. returning the closest grid point? > Which grid point exactly? >
Correct. It converts your geospatial value into a grid pixel x, y (row, column) and then gets the value at the x, y. > I now found a related thread here where elliott-22 asks "what if the > yourLongitude, yourLatitude values provided in the query are not grid > points in that they fall within a bounding pixel grid. Only values > for the intersection points are known so how does a non-intersecting > point get assigned a value from the query?" (see > http://old.nabble.com/Retrieving-Data-from-PostGIS-Raster-using-ST_Value-td32638248.html > at bottom of thread, Oct 17, 2011; 09:27pm) > > There Pierre answers: >> Point outside the area covered by the grid are assigned NULL. There is not >> inter- or extra- polation done. > > But the question is about a query point (of float type) lying inside > the perimeter but outside the grid nodes (probably integer based! > These query points are inside the convex hull of the whole raster but > they still don't "fall" on a regular grid point (see the figure of > eliott-22 above). > You can't think that the value is only significant for the upper-left corner of a pixel (regular grid point) as each pixel represents an area (hence the geotransform values of scale, upperleft corner and skew). Since each pixel of a raster represents an area bounded by the pixel's four corners with a singular value for that area (per band), any point that falls within that pixel would have the same singular value. > What is exactly happening here in ST_Value()? > My response above. ST_Value has the exact same behavior you'd expect if you used the "Identify" tool in ArcGIS, ENVI, ERDAS, qgis and clicked anywhere on a pixel. It gets the value(s) (if more than one band) of the pixel your cursor is over. > Pierre also answered in the thread mentioned above my interpolation question: >> Interpolating the elevation of a point is another story. >> You could build a grid of 8, 15 or 24 points around your lat/long, >> query the elevation at those points and use your algorithm to compute the >> interpolation. > > How do I really control which grid points are chosen - given an > arbitrary (floating number) coordinate? > For what you need, you may want to convert an area of the raster around your coordinates to geometries. -- 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
