Re: [postgis-users] Distance between two furthest points of a group
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.idb.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 vesni...@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,0.0001) ) 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.mou...@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 -- *HR Wallingford and its subsidiaries* uses faxes and emails for confidential and legally privileged business communications. They do not of themselves create legal commitments. Disclosure to parties other than addressees requires our specific consent. We are not liable for unauthorised disclosures nor reliance upon them. If you have received this message in error please advise us immediately and destroy all copies of it. HR Wallingford Limited Howbery Park, Wallingford, Oxfordshire, OX10 8BA, United Kingdom Registered in England No. 02562099 -- ___ postgis-users mailing list postgis-users@lists.osgeo.org http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users ___ postgis-users mailing list postgis-users@lists.osgeo.org http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users ___ postgis-users mailing list postgis-users@lists.osgeo.org http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users
Re: [postgis-users] Distance between two furthest points of a group
Hallo I haven't followed the whole conversation. But one way is to collect alk points and usr ST_maxdistance on the colnections. or st_longestline. longestline will return a line where the end points is the two points furthest from eath other. Those functions is quite fast. /Nicklas Sent from my Cat® phone. Den 27 jun 2015 13:25 skrev Rémi Cura remi.c...@gmail.com: 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.idb.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 vesni...@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,0.0001) ) 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.mou...@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 HR Wallingford and its subsidiaries uses faxes and emails for confidential and legally privileged business communications. They do not of themselves create legal commitments. Disclosure to parties other than addressees requires our specific consent. We are not liable for unauthorised disclosures nor reliance upon them. If you have received this message in error please advise us immediately and destroy all copies of it. HR Wallingford Limited Howbery Park, Wallingford, Oxfordshire, OX10 8BA, United Kingdom Registered in England No. 02562099 ___ postgis-users mailing list postgis-users@lists.osgeo.org http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users ___ postgis-users mailing list postgis-users@lists.osgeo.org http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users ___ postgis-users mailing list postgis-users@lists.osgeo.org http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users
