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

[Brent Wood]

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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