On Fri, Jun 3, 2011 at 5:00 AM, Pierre Racine <pierre.rac...@sbf.ulaval.ca> wrote: >>1] Globally (i.e., not for an individual tile, but for the entire scene) I >>want one histogram of raster values for pixels which are inside the fire >>perimeter, and one histogram of raster values for pixels which are outside. > > Stats functions like ST_Histogram() do not allow yet to compute stats > restricted to the area of a polygon directly from raster objects. We are > working on that. But if your raster dataset is small enough you can convert > everything to vector and compute those stats in vector mode. >
Yes, this is on my todo list. >>2] For each individual tile, I want separately computed histograms for pixels >>inside and outside the fire perimeter. > >>My understanding is that I cannot do #2 because ST_Intersection() always >>returns a setof geomval, and never returns a "subset raster". > > Right. But you can compute stats on the result of ST_Intersection() using > ST_Area(geom)/areaOfOnePixel. Since the results are vector, the ST_histogram > function is useless here. > >>I believe I cannot do #1 because in addition to this ST_Intersection >>behavior, there is no way to re-assemble the tiles into one giant raster >>prior to feeding it to ST_Histogram. It appears I can run ST_Histogram only >>on individual, whole tiles. > > It is true that PostGIS raster do not allow yet to reassemble raster tiles > into bigger rasters but you don't have to do that in order to compute > histogram on a tiled coverage. Just compute the histogram for every tiles and > aggregate the results using a GROUP BY query. (Bborie: shouldn't we have a > ST_Histogram variant working on a whole table like we have for the other stat > functions?) > Have you considered looking at ST_ValueCount? That function will count the number of times a user-specified value (or no value to get all unique values) appears in the raster. It is not a true histogram in the statistical sense but does give you the distribution of data in your raster. It also works on a whole table unlike ST_Histogram. http://postgis.refractions.net/documentation/manual-svn/RT_ST_ValueCount.html >>I am currently pursuing a vector solution using ST_DumpAsPolygons() but this >>is very slow. Hence I have time to send questions to the list. :) > > If the polygon area is small comparing with the raster area you should retile > your raster coverage into smaller tiles so the query has to vectorize less > useless raster area. It is important to index the coverage and to add "WHERE > ST_Intersects()" in this case. Make sure also that both your raster coverage > and your vector coverage have indexes. This makes a HUGE difference. I see > that you don't do that in your tutorial. You can do it while loading your > data with the "-I" option or afterward with a "CREATE INDEX" query. > >>I have outlined my vector plan of attack here: >>https://collab.firelab.org/software/projects/modisfire/wiki/Calculating_histograms_with_PostGIS_Raster >> If you believe this to be a worthy tutorial-in-progress, you may copy or >>link to it as you see fit. I expect it will change as I pursue this activity >>further. > > As I said we are very near to have a geometry parameter to the stats > functions restricting the computation to defined area. Once we have this all > your tutorial should be much shorter... Converting raster to vector solutions > are easy when raster coverages are small but impracticable on large raster > datasets (this is why we developed PostGIS raster!). > -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
