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

[David Fawcett]

Brent

Great, informative answer.

On Sun, Mar 22, 2015 at 3:14 PM, Brent Wood <pcreso at pcreso.com> wrote:

> Hi Aaron,
>
> Hopefully this (simplistic) description helps.
>
> Setting a SRID value for a feature tells Postgis what coordinate reference
> system (CRS) the coordinates are in. This includes the unit, which can be
> any linear unit of measurement, such as degrees (measured at the surface of
> the earth), meters, feet, etc. Obviously getting this right is important to
> calculate distances correctly.
>
> Different CRS's generally apply to different parts of the globe, and
> include the projection parameters. Note that every projection applies some
> distortion - trying to map (project) a part of a spherical surface to a
> flat plane has this problem. There are three main types of distortion -
> angular (changes shapes), area and distance. Normally to measure distance
> using a projected CRS, you'd use an equidistant projection (which minimises
> distance distortions) centered near the location you are trying to measure.
>
> An alternative approach is to measure it on a spheroid, in 3D space,
> instead of on a projected 2D space. This is basically what a Postgis
> geography allows you to do. But the coordinate units in a geography are
> degree coordinates, so you need to specify that your coordinates are
> unprojected lon/lat values when you use them in a geography. The SRID
> (Spatial Reference ID) for such a CRS is 4326.
>
> In your case, try:
> SELECT * FROM users
> WHERE ST_DWithin (users.location::geography, st_setsrid(st_makepoint
> (146.0,
> 138.19), 4326)::geography, 100);
>
> This assumes your location is a geometry with a SRID of 4326 (ie: the
> coordinate values are unprojected lon/lat degrees). It then converts this
> geometry to a geography datatype, which it tests against a point geometry
> in SRID 4326, which is also converted to a geography for the test, to see
> if it is within 100m. So this SQL tests geography against geography
> datatype.
>
> If your location feature is not SRID 4326, you'll need to reproject
> (transform) it to 4326 to for this to work:
>
> SELECT * FROM users
> WHERE ST_DWithin (ST_Transform(users.location,4326)::geography,
> st_setsrid(st_makepoint (146.0,
> 138.19), 4326)::geography, 100);
>
> (I haven't tested it, but I think this should work)
>
> Cheers
>
> Bent Wood
>
>   ------------------------------
>  *From:* Aaron Lewis <the.warl0ck.1989 at gmail.com>
> *To:* postgis-users at lists.osgeo.org
> *Sent:* Monday, March 23, 2015 12:09 AM
> *Subject:* [postgis-users] Need help on basic concepts, I just need
> really simple calculations
>
> 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
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