[postgis-users] Basic postgis raster questions

[Bryce L Nordgren]

I started drooling over ST_Histogram() in postgis trunk. Thus far, I think 
my use case prevents me from using it, but I'd like to confirm with the 
list.

I have a raster (derived image from a Landsat scene) which is coarsely 
tiled: 833x833. This gives me 110 tiles for the scene. Separately, I have 
a polygon coverage of a fire perimeter.

Basically, what I want to do is this: 

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.
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". 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.

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. :) 

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.

Bryce
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