[postgis-users] Measure distance to nearest border

[Andreas Forø Tollefsen]

Thanks.
I have found a way to do this calculation.
However, since i gave 64818 cells and almost 200 countries, the distance
from every cell to every country is calculated before the minimum distance
is selected.
This takes awful lot of time.

Any ideas on how to increase the performance?

DROP TABLE IF EXISTS borddisttest2;
CREATE TABLE borddisttest2 AS
select a.gid, MIN(ST_Distance(a.centroid, b.the_geom)) AS distance FROM
pgfinal2008 a, cshapes b, cshapes c WHERE a.gwcode != b.gwcode AND b.gwsyear
<= 2008 AND b.gweyear >= 2008 GROUP BY a.gid LIMIT 1;

DROP TABLE IF EXISTS borddist2008;
CREATE TABLE borddist2008 AS
SELECT pgfinal2008.cell, borddisttest2.gid, MIN(borddisttest2.distance) FROM
borddisttest2, pgfinal2008 WHERE borddisttest2.gid = pgfinal2008.gid
GROUP BY pgfinal2008.cell, borddisttest2.gid;

2010/10/13 Nicklas Avén <nicklas.aven at jordogskog.no>

>  Hi
>
> You can not cast to geography that way (if I haven't missed something
> essential)
> And if you could you would mix geometry and geograph type in ST_Distance,
> which wouldn't work.
>
> St_Distance for geometry vs geography uses totally differnt algorithms. The
> geography calculations is far more advanced.
>
> ST_Shortestline only works for geometry type.
>
> /Nicklas
>
>
> 2010-10-13 Andreas Forø Tollefsen wrote:
>
> Hi>
>
> >
> >
> Thanks,
> >
> I will give it a try.
> >
> The reason I used the geography types in the distance query, were to get
> the distance in meters instead of degrees.
> >
>
> >
> >
> Andreas
> >
> >>
> 2010/10/13 Nicklas Avén <nicklas.aven at jordogskog.no>
> > >
> >
> Hi
> >
>
> >
> As I understand you you want to get the distance from each cell to the
> closest neighbour country. Then, why don't you just query the distance from
> your cell to closest country with other gwcode?
> >
> Maybe something like:
> >
>
> >
> Create table borddisttest as
> >
> Select ST_Distance(st_collect(b.the_geom), c.centroid)
> >
> from cshapes a, cshapes b, pgfinal2008 c
> >
> where a.gwcode=c.gwcode and b.gwcode != c.gwcode and
> st_intersects(a.the_geom, b.the_geom)
> >
> group by c.gwcode and c.the_geom;
> >
> >
> With some tweaking like ordering the cells by gwcode I don't think the
> intersection calculation have to be done for each cell. Otherwise this is a
> very bad approach if there are many grid-cells.
> >
>
> >
> indexing on gwcode on both tables and spatial index on the country geoms
> for the intersection will also be of importance.
> >
>
> >
> I don't understand:
> >
> geography(pgfinal2008.centroid)
> >
>
> >
> what does geography means here?
> >
>
> >
> I would also go for calculating the centroids on the fly. It should be fast
> and you will not get the trouble of updating the centroid column if
> adjustuing the grid.
> >
>
> >
> About doing calculations comparing geometries inside a dataset you use self
> join.
> >
>
> >
> You can join a table with itself as long as you put an alias so you can
> identify them like I did above with a, b and c
> >
>
> >
> Well, I am not sure I answered the right question but anyway...
> >
> HTH
> >
> Nicklas
> >
> >
>
> >
> >
>
> > 2010-10-13 Andreas Forø Tollefsen wrote:
> >
> > >
> >
> Hi all,
> > >
>
> > >
> > >
> I have two datasets. On is a quadrate grid, and the other is the boundaries
> of all countries.
> > >
> What i want to do is to measure the distance from each centroid of the grid
> cells to the nearest border, but not all borders. Only the international.
> > >
>
> > >
> > >
> First i convert my country polygon dataset into line features:
> > >
> > >
> select gwcode, gwsyear, gweyear, ST_Boundary(the_geom) AS line Into
> cshapes_line from cshapes;
> > >
>
> > >
> > >
> Next i calculate the distance from the centroid of each cell to the nearest
> border where the gwcode (country code) of the cell is the same as the gwcode
> in the line feature.
> > >
> drop table if exists borddisttest;
> > >
> select pgfinal2008.cell, pgfinal2008.gwcode,
> ST_ShortestLine(pgfinal2008.centroid, st_boundary(cshapes.the_geom))
> > >
> AS shortestline, ST_Distance(geography(pgfinal2008.centroid),
> st_boundary(cshapes.the_geom))/1000 AS borddist
> > >
> into borddisttest
> > >
> from cshapes, pgfinal2008
> > >
> where cshapes.gwcode = pgfinal2008.gwcode;
> > >
>
> > >
> > >
> The problem here is that when using ST_Boundary, it converts all boundaries
> into lines, while i only want to get the boundaries where two polygons with
> different gwcodes meet.
> > >
> The lines where two countries meet are overlapping, meaning these borders
> have two gwcodes but in different line entries.
> > >
>
> > >
> > >
> Is there a way to calculate the overlaps within one dataset? If i could do
> this, then i could put a clause so the measurement only measures to lines
> with 2 or more gwcodes.
> > >
>
> > >
> > >
> Thanks.
>
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