[GIS] Find points that fall within a radius using Hibernate Spatial

hibernate-spatialpostgis

I am using hibernate spatial and postgis and i want to find the points that fall within a certain radius. Using this SQL:

SELECT name FROM institution WHERE ST_DWithin(geom,ST_GeographyFromText('SRID=4326;POINT(36.9881 0)'), 5000)

I get the points within 5KM.

Using Hibernate Spatial i am doing

Criteria criteria = getCurrentSession().createCriteria(Institution.class, "i");
Geometry geometry = ....

criteria.add(SpatialRestrictions.distanceWithin("i.geom", geometry, distance));

But it does not return distance in Metres. What do I need to do?

Best Answer

You have to provide geographic data to evaluate distance but you are passing geometry.

see https://stackoverflow.com/a/20577961/6490042

SRID 4326 geometry is in degrees not meters.

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SELECT name,ref,type,ST_Distance(ST_Buffer(r.geom,20),ST_SetSRID(ST_MakePoint(lon, lat),4326)) 
FROM roads r 
ORDER BY 4 ASC 
LIMIT 1;

For point in lon/lat coordinates:

SELECT name,ST_Distance(r.geom,ST_SetSRID(ST_MakePoint(lon, lat),4326)) FROM roads r ORDER BY 2 ASC LIMIT 1;

[GIS] PostGIS – Geometry or Geography find points within distance in meters

The definition of SRID 4326 is in fact IN DEGREES. ALso as it says in the ST_DWithin page:

Returns true if the geometries are within the specified distance of one another. For geometry units are in those of spatial reference and For geography units are in meters and measurement is defaulted to use_spheroid=true (measure around spheroid),

Since you're are using the 4326 SRID, you can cast from geometry to geography and vice versa like this :

select st_distance(<geom_column>::geography) from myTable;

Most of the functions in postgis are overloaded, meaning, they return values and run a different procedure based on the input data type.

eg:

boolean ST_DWithin(geometry g1, geometry g2, double precision distance_of_srid);

boolean ST_DWithin(geography gg1, geography gg2, double precision distance_meters);

they have the same name, but the first function returns T/F based on units of the srid, and the second returns T/F based on meters.

If you plug into the function geog types, for its calculation, will take into account the curve of the earth so the algorithm 'costs' a bit more.

If you want to check how much more it costs, you can run for each query an EXPLAIN-ANALYSE

Best Answer

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[GIS] PostGIS – Geometry or Geography find points within distance in meters

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