[postgis-users] Contours of rasters without or with tiles

Mathieu Basille basille at ase-research.org
Tue Nov 27 13:33:06 PST 2012

Dear Hugues,

I see now what you mean. ST_Envelope gives the behaviour you described 
(i.e. one polygon per tile, which is 9 lines). If I try to ST_Union them, I 
get the exact same problem I had before with ST_Polygon: not all 
polygon-tiles are unioned, as can be seen with a ST_Dump:

SELECT ST_Dump(ST_Union(ST_Envelope(rast))) FROM test.raster_tile;
(2 lines)

With this line of code on the tile version of the raster, one would expect 
a single line as output (as is the case on the non-tiled raster).

So my questions are really now:

1) Is this a desirable feature that vector functions such as ST_Envelope or 
ST_Polygon behave differently on tiled and non-tiled rasters? From my point 
of view, it shouldn't be the case, as the decision to tile a raster should 
be transparent from a user point of view (it's just a trick to get better 
performance for most GIS procedures). The same code should run fine on both 
types of rasters in my opinion.

2) How can we circumvent this "feature"? Using ST_Union should do it, but 
it's obviously not working as expected (at least by me).

I feel like I'm getting quite far from my initial request, so maybe it 
should be better to start a new thread... Any opinion?


Le 27/11/2012 01:43, Francois Hugues a écrit :
> Hello,
> There's no mess ! You used my firstname correctly and I thank you for that.
> To (try to) answer your questions :
> 1. I would have use st_envelope to see what was the result and if the problem would comes from no data values. St_envelope will also give you one polygon by tile and the result will be totally different if you use it with a tiled or non tiled raster.
> 2. As st_envelope, I think the correct behaviour of st_polygon is to proceed tile by tile if you tiled your raster (a tile by row = a polygon by row).
> Hugues.


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