[postgis-users] Measure distance to nearest border

[Andreas Forø Tollefsen]

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.
>
> _______________________________________________
> postgis-users mailing list
> postgis-users at postgis.refractions.net
> http://postgis.refractions.net/mailman/listinfo/postgis-users
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/postgis-users/attachments/20101013/f886f8d2/attachment.html>