[postgis-users] Distance between two furthest points of a group

[Rémi Cura]

Maybe I'm wrong, but your 2 farthest points should be on the boundary of
the maximum bounding circle (feels right but couldn't prove it).
Thus you would compute this circle, then filter points not too far from it,
then take the points with the max distance using an inner join (same as
Nick, but you can save half the computation because dist(A,B)=dist(B,A), so
simply add a condition a.id<b.id)).

Another solution is to use bbox n nearest neighbour, which is indexed.
You wouldn't car too much about using bbox, because for points it only
reduce precision to float instead of double.
This would be like :

SELECT a.id,b.id, st_distance(a.geom,b.geom) AS d
FROM my_points AS a , my_points AS b
ORDER BY a.geom <-> b.geom DESC
LIMIT 1

This is the probably the better easiness/speed ratio.

Cheers,
Rémi-C

2015-06-26 22:06 GMT+02:00 Nick Ves <vesnikos at gmail.com>:

> You can cross join to create the cartesian product of them and use it to
> calculate the distance of each with regards to the other:
>
> select a.id,b.id, st_distance(a.geom,b.geom) d from points a cross join
> points b order by d desc limit 1;
>
> ofc that will take forever because it will have to create an m x n table
> (800 secs and counting...)
>
> As I see it the two points with the furthest distance between them should
> touch the borders of you datasets convexhull. So you can filter out those
> inside the boundaries and do the calculations with  the remaining points
> along the borders :
>
> with f as
> (
> select a.geom,a.id from
> points foo,
> (select ST_ExteriorRing (st_convexhull(st_collect(geom))) geom from
> points) bar
> where st_Dwithin(foo.geom,bar.geom,0.00000001)
> )
> select a.id,b.id, st_distance(a.geom,b.geom) d from f  a cross join f b
> order by d desc limit 1;
>
> ​should give you the id of your targets and the distance between them​
>
>
>
> On Fri, Jun 26, 2015 at 2:54 PM, Jonathan Moules <
> J.Moules at hrwallingford.com> wrote:
>
>>  Hi List,
>>
>> I have sets of points (up to 250,000 in a set) and I want to get the
>> furthest distance between any of them.
>>
>>
>>
>> In theory the simplest way is to use
>> ST_MinimumBoundingCircle(ST_Collect(geography) and then get the diameter of
>> that.
>>
>>
>>
>> The problem is – I don’t seem to be able to get the diameter of that
>> circle (which would give me the distance I want).
>>
>>
>>
>> Does anyone have any thoughts on this? Is there a good way to get the
>> diameter? Or some other way of getting the distance I desire.
>>
>>
>>
>> Thanks,
>>
>> Jonathan
>>
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