[postgis-users] Contours of rasters without or with tiles
basille at ase-research.org
Tue Nov 27 13:33:06 PST 2012
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;
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 :
> 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).
Mathieu Basille, PhD
~$ locate --details
University of Florida \\
Fort Lauderdale Research and Education Center
« Le tout est de tout dire, et je manque de mots
Et je manque de temps, et je manque d'audace. »
-- Paul Éluard
More information about the postgis-users