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

[Nick Ves]

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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