[postgis-users] Spatial linking or selection

Regina Obe lr at pcorp.us
Fri May 15 05:44:17 PDT 2020

Please don’t count of autocasting of geometry -> geography.  This makes your code brittle as it may change in the future.


You should explicitly cast your geometry to geography like


ST_DWithin(a.geom::geography, b.geom::geography, somedistance)  




ST_DWithin(geography(a.geom), geography(b.geom), somedistance)  



Also keep in mind if you have spatial indexes on geometry – the above function can’t use them .  You’d need to add functional indexes for geography


Something like


CREATE INDEX ix_yourtable_geom_geog_gist USING gist(geography(geom)); 


From: postgis-users [mailto:postgis-users-bounces at lists.osgeo.org] On Behalf Of Shaozhong SHI
Sent: Friday, May 15, 2020 8:35 AM
To: PostGIS Users Discussion <postgis-users at lists.osgeo.org>
Subject: Re: [postgis-users] Spatial linking or selection


Hi, Friends,


I am a little bit confused.


If I use geometry as data type, do I put something like 5 metres and use_spheroid=false?


Is that correct?






On Fri, 15 May 2020 at 13:00, Giuseppe Broccolo <g.broccolo.7 at gmail.com <mailto:g.broccolo.7 at gmail.com> > wrote:

Hi Shao,


Il giorno ven 15 mag 2020 alle ore 11:56 Shaozhong SHI <shishaozhong at gmail.com <mailto:shishaozhong at gmail.com> > ha scritto:

Hi, Nicolas,




That is very interesting.  I will test it.


I could not find information on how distance is defined.  0.01 stands on what unit?




Have a look to this statement in the doc you linked:

For geometry: The distance is specified in units defined by the spatial reference system of the geometries. For this function to make sense, the source geometries must both be of the same coordinate projection, having the same SRID.


For geography units are in meters and measurement is defaulted to use_spheroid=true, for faster check, use_spheroid=false to measure along sphere.





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