[postgis-users] PostGIS Raster efficient enough to get height given coords from a large DEM?

[Volkmar Herbst]

Yes, we also do use plr for kriging of soil data, which works very fast, but
there are many different variants of it. It helps to know about geo
statistics and get the right type and formula for the kriging. It is also
possible to do the following steps:
1. create your grid (in plr)
2. calculate kriging (in plr)
3. create contours (in plr) 
4. create contour polygons (I have done it in a postgis function but it
should be also possible in plr)
I abandoned the raster data type as kriging output to postgis as I could not
create the postgis raster from the plr raster in an acceptable time. The
ratser postgis function st_setvalue which works on each pixel takes too long
and there seem to be no other way to set raster values programmatically,
that’s why I switched to contours. We do need array to pixel value or so-
see
http://trac.osgeo.org/postgis/ticket/1228
Hope that helps
Volkmar



-----Ursprüngliche Nachricht-----
Von: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] Im Auftrag von Bborie
Park
Gesendet: Sonntag, 4. Dezember 2011 13:46
An: PostGIS Users Discussion
Betreff: Re: [postgis-users] PostGIS Raster efficient enough to get height
given coords from a large DEM?

On Sat, Dec 3, 2011 at 7:16 PM, Stefan Keller <sfkeller at gmail.com> wrote:
> Hi Bborie
>
> Many thanks for your response.
> To summarize: My goal is to get the height (in meters) given an
> arbitrary lat/lon coordinate from the latest free digital elevation
> model (like mentioned here
>
http://slashgeo.org/2011/12/01/New-Free-Elevation-Dataset-named-Global-Multi
-resolution-Terrain-Elevation-Data-2010-GMTE
> ).
>
> The approach using PostGIS to access raster data is
> 1st. to convert an ASCII GRID text file (an ESRI format) to GeoTIFF
> (not tiled nor "pyramide"-ized).
> Then (2nd) to load it into PostGIS, like described here
> http://trac.osgeo.org/postgis/wiki/WKTRasterTutorial01 .
>

I don't remember but I think GDAL supports ASCII GRID so you don't
need an intermediary file.  The loader uses GDAL to convert them into
the PostGIS Raster format.

> I still have some basic questions about this interesting approach:
>
> Regarding my question of "rules of thumb" in order to set the tile
> size you answered:
>> Generally speaking, the smaller the tile size, the faster the operation.
 In
>> my testing of my rasters, 200x200 is slower than 100x100 which is slower
>> than 50x50.  The smaller tile size results in more rows in the table BUT
the
>> performance dramatically increases.  I can't remember of the top of my
head
>> but the performance gain was linear or better.
>
> Thats a good hint.
> Q1: But are there no calculcation rules on how to estimate an ideal
> absolute size (in pixels)?
>

Nope.  Testing is the only way as there are other possible factors
involved.  And even my estimates above are only guidelines.  I know
I'm going to have to test for my production database servers before I
know what works best for them.  In a pinch, I'd say you'd be happy
between 30 - 70.

> Regarding a query given lat/lon you proposed:
>> Assuming you test to see what the ideal (or "good-enough") tile size
>> is for the elevation rasterset, you should see good performance using
>>
>> ST_Value(rast, ST_SetSRID(ST_MakePoint(long, lat), srid))
>>
>> You could eliminate the ST_SetSRID and ST_MakePoint by passing a
>> correct EWKT instead
>>
>> ST_Value(rast, 'YOUREWKT'::geometry)
>
> 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.

> Q3: If yes, I assume its linear. How could I proceed to get a result
> using non-linear interpolation (like Kriging)?
>

I don't know if you can using only PostGIS raster.  If you're able to
use PL/R as well, you should be able to tap into their kriging tools.

-bborie

-- 
Bborie Park
Programmer
Center for Vectorborne Diseases
UC Davis
530-752-8380
bkpark at ucdavis.edu
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