[postgis-users] Need help on basic concepts, I just need really simple calculations

[Rémi Cura]

Hey,
I would sugger to read a little bit about geographical information
<https://en.wikipedia.org/wiki/Spatial_reference_system> before using
PostGIS.
The unit depend on the way you measure, which depends on the Spatial
Reference System.

If you want it is a bit like measuring the temperature in Celsius degrees
and Kelvin.
You measure the same things but you represent the measure differently.

Cheers,
Rémi-C

2015-03-22 15:42 GMT+01:00 Aaron Lewis <the.warl0ck.1989 at gmail.com>:

> Thanks Nicolas,
>
> I take a few tries, but it looks like ST_DWithin & ST_Distance all
> deals in degrees, not meters ..:
>
> gis=# select nick, ST_AsText(location), ST_Distance(location,
> st_setsrid(st_makepoint (46.3, 36.10), 2600)) from users where
> ST_DWithin(users.location, st_setsrid(st_makepoint (46.3, 36.10),
> 2600), 10);
>  nick  |    st_astext     |   st_distance
> -------+------------------+------------------
>  user1 | POINT(45.2 35.1) | 1.48660687473185
>  user2 | POINT(45.2 35.2) | 1.42126704035518
>  user3 | POINT(45.5 35.3) | 1.13137084989848
>
> I tried to cast it to geography type but:
>
> ST_Distance(location::geography, st_setsrid(st_makepoint (46.3,
> 36.10), 2600)::geography)
>
> Resulted in an error:
> ERROR:  Only lon/lat coordinate systems are supported in geography.
>
> So how should I get this distance in meters?
>
>
> On Sun, Mar 22, 2015 at 8:44 PM, Nicolas Ribot <nicolas.ribot at gmail.com>
> wrote:
> > Hi Aaron,
> >
> > SRID is used to set a consistent coordinate system to the data. It
> refers to
> > an EPSG Coordinate Reference System (CRS). For instance, EPSG:2600:
> > (http://spatialreference.org/ref/epsg/2600/) is a metric CRS.
> >
> > Coordinates (146.0, 138.19) are not valid in SRID 2600 (nor in lat/long
> > coordinates).
> >
> > You would use st_setSRID when the coordinates are already in the correct
> > CRS, and st_transform() to reproject coordinates from one CRS to another:
> >
> > -- Search inside a 100m circle for a point already in SRID=2600:
> > SELECT * FROM users
> > WHERE ST_DWithin (users.location, st_setsrid(st_makepoint (508560,
> 6155116),
> > 2600), 100);
> >
> > -- Search inside a 100m circle for a point in longitude/latitude,
> projected
> > into SRID=2600:
> > SELECT * FROM users
> > WHERE ST_DWithin (users.location, st_transform(st_setsrid(st_makepoint
> (24.1
> > 55.5), 4326), 2600), 100);
> >
> > (users.location SRID should be 2600 in these examples)
> >
> > Nicolas
> >
> >
> >
> > On 22 March 2015 at 12:09, Aaron Lewis <the.warl0ck.1989 at gmail.com>
> wrote:
> >>
> >> Hi,
> >>
> >> I've been searching online for days. Trying to understand why SRID is
> >> required. So I picked some random value.
> >>
> >> Now I'm need to retrieve POINTs within a circle range, e.g a circle at
> >> (146.0, 138.19) with radius of 100 meters:
> >>
> >> SELECT * FROM users
> >> WHERE ST_DWithin (users.location, st_setsrid(st_makepoint (146.0,
> >> 138.19), 2600), 100);
> >>
> >> It's very simple, but the result seems wrong. I have a record contains
> >> a POINT(55 43) that matches this query.
> >>
> >> Anyone know what's wrong with it?
> >>
> >> --
> >> Best Regards,
> >> Aaron Lewis - PGP: 0x13714D33 - http://pgp.mit.edu/
> >> Finger Print:   9F67 391B B770 8FF6 99DC  D92D 87F6 2602 1371 4D33
> >> _______________________________________________
> >> postgis-users mailing list
> >> postgis-users at lists.osgeo.org
> >> http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users
> >
> >
> >
> > _______________________________________________
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>
>
>
> --
> Best Regards,
> Aaron Lewis - PGP: 0x13714D33 - http://pgp.mit.edu/
> Finger Print:   9F67 391B B770 8FF6 99DC  D92D 87F6 2602 1371 4D33
> _______________________________________________
> postgis-users mailing list
> postgis-users at lists.osgeo.org
> http://lists.osgeo.org/cgi-bin/mailman/listinfo/postgis-users
>
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