PostGIS: How to find N closest sets of points to a given set?

Question

I am using PostGIS/Rails and have sets of points with geolocations.

class DataSet < ActiveRecord::Base  # these are the sets containing the points
  has_many :raw_data
  # attributes: id , name
end

class RawData < ActiveRecord::Base  # these are the data points
  belongs_to :data_set
  # attributes: id, location which is "Point(lon,lat)"
end

For a given set of points I need to find the N closest sets and their distance;

or alternatively: For a given max distance and set of points I need to find the N closest sets.

What is the best way to do this with PostGIS?

My versions are PostgreSQL 9.3.4 with PostGIS 2.1.2

Answer 1

The answer on how to find the N-closest neighbours in PostGIS are given here:

Postgis SQL for nearest neighbors

To summarize the answer there:

You need to create a geometry object for your points. If you are using latitude, longitude, you need to use 4326.

UPDATE season SET geom = ST_PointFromText ('POINT(' || longitude || ' ' || latitude || ')' , 4326 ) ;

Then you create an index on the geom field

CREATE INDEX [indexname] ON [tablename] USING GIST ( [geometryfield] ); 

Then you get the kNN neightbors:

SELECT *,ST_Distance(geom,'SRID=4326;POINT(newLon newLat)'::geometry) 
FROM yourDbTable
ORDER BY
yourDbTable.geom <->'SRID=4326;POINT(newLon newLat)'::geometry
LIMIT 10;

Where newLon newLat are the query points coordinates.

This query will take advantage of kNN functionality of the gist index (http://workshops.boundlessgeo.com/postgis-intro/knn.html).

Still the distance returned will be in degrees, not meters (projection 4326 uses degrees).

To fix this:

SELECT *,ST_Distance(geography(geom),ST_GeographyFromText('POINT(newLon newLat)') 
FROM yourDbTable
ORDER BY
yourDbTable.geom <->'SRID=4326;POINT(newLon newLat)'::geometry
LIMIT 10;

When you calculate the ST_distance use the geography type. There the distance is always in meters:

http://workshops.boundlessgeo.com/postgis-intro/geography.html

All this functionality will probably need a recent Postgis version (2.0+). I am not sure though.

Check this for reference https://gis.stackexchange.com/questions/91765/improve-speed-of-postgis-nearest-neighbor-query/

EDIT. This covers the necessary steps for one point. For set of points:

SELECT n1.*,n2.*, ST_Distance(n1.geom,n2.geom) 
FROM yourDbTable n1, yourDbTable n2
WHERE n1.setId=1 AND n1.setId=2 //your condition here for the separate sets
AND n1.id<>n2.id // in case the same object belong to 2 sets
ORDER BY n1.geom <->n2.geom
LIMIT 20;