[postgis-users] PostGIS - Simple Raster Point Query

[Tim-Hinnerk Heuer]

Good point and apologies I haven't been more clear.

We want to achieve this: http://npm.landcareresearch.co.nz/elevation/
with Open Source technology and our own data. Also, instead of doing a
single line, we want to implement a LineString which can have multiple
control points in it.

So far we've got it working by querying for each point on the line, say n
segments would give us n+1 points on the line and therefore n+1 queries
which could potentially be a lot. At the moment one query is run for each
point on the line, which is fine, but I believe there must be a much better
way with PostGIS to get all the points' st_values at once, preferably, like
in the example above, also the distance of each point from the first point
on the line.

Thought an example application, similar to the one we are developing would
be better than a thousand words.

Thanks for all your help.

Regards,
Tim

Tim-Hinnerk Heuer

Twitter: @geekdenz
Blog: http://www.thheuer.com



On 15 December 2012 13:18, Pierre Racine <Pierre.Racine at sbf.ulaval.ca>wrote:

> Maybe it would be better to describe more generally what you are trying to
> achieve. Most complex processing can normally be achieved in one complex
> query.
>
> PostgreSQL should cache what was loaded in previous queries so that it
> does not necessarily reload tiles at each query.
>
> Pierre
>
> > We query the points on the line with one query for each point. This
> could be
> > optimised like this:
> > SELECT ST_AsText(
> > ST_line_interpolate_point(
> > ST_GeometryFromText('LINESTRING(0 0,0 1,1 1)'), CAST(p AS numeric)/10
> > )
> > )
> > FROM generate_series(1,10) p;
> > to get all points. But this is still requiring one query for each point
> on the line. I
> > suspect, if we were to query all points at once, there will have to be
> less reading
> > of the raster, because the points are likely to be on the same tile.
> However, I do
> > not know how to achieve that. Can I somehow intersect the line with the
> raster
> > and query all the points above?
> >
> > Any hints would be greatly appreciated.
> >
> > Thanks,
> > Tim
> >
> > Tim-Hinnerk Heuer
> >
> > Twitter: @geekdenz
> >
> > Blog: http://www.thheuer.com <http://www.thheuer.com/>
> >
> >
> >
> > On 9 December 2012 20:36, Tim-Hinnerk Heuer <th.heuer at gmail.com> wrote:
> >
> >
> >       Hi Pierre,
> >
> >       Great! Thank you so much, that worked!
> >
> >       You're a genius!
> >
> >       In case you are wondering. That point is Mount Ruhapehu and it gave
> > me the answer: 2611.7314453125 (m) which is indeed correct.
> >
> >       Kind regards,
> >       Tim
> >
> >       Tim-Hinnerk Heuer
> >
> >       Twitter: @geekdenz
> >
> >       Blog: http://www.thheuer.com <http://www.thheuer.com/>
> >
> >
> >
> >       On 9 December 2012 20:24, Pierre Racine
> > <Pierre.Racine at sbf.ulaval.ca> wrote:
> >
> >
> >               SELECT ST_Value(rast,ST_SetSRID(ST_Point(1821416, 5649720),
> > 2193))
> >               FROM nzdem3
> >               WHERE  ST_Intersects(rast,ST_SetSRID(ST_Point(1821416,
> > 5649720), 2193))
> >
> >
>
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