[postgis-users] Using PostGIS for latitude & longitude

[Duncan Golicher]

You can get accurate distances in meters simply by casting to geography.

http://www.postgis.org/docs/ST_Distance.html

This works with ST_DWithin as well in the where clause. Just makes queries
a bit slower. To cast geometry to geography use the :: operator.

Duncan





On Wed, Dec 19, 2012 at 11:08 AM, <pcreso at pcreso.com> wrote:

> Hi Neville,
>
> Obviously you start by installing Postgis in your database.
>
> Follow the instructions for your Postgis version & operating system.
>
> To create your geometry:
>
> 1. add a geometry column to your table. This will be a point type, with a
> SRID of 4326 (ie: lat long degrees - see
> http://www.spatialreference.org/ref/epsg/4326/
> See here for the command to use:
> http://postgis.refractions.net/docs/AddGeometryColumn.html
>
> 2. Populate this column (called geom, for example), from your lat, lon
> columns with:
> update table set geom=ST_SetSRID(ST_MakePoint(lon,lat),4326);
> as described at http://www.postgis.org/docs/ST_MakePoint.html
>
>
> You then need to make your query to retrieve the points with 7 miles of a
> specified location. This can get complicated depending on how accurate you
> want to be. The distance of 1 degree for your latitudes is 60 nautical
> miles. For longitude, it is 60 nm at the equator & decreases proportional
> to the cosine of the latitude for non-equatorial values. Postgis allows you
> to reproject your coordinates, or calculate against a sphere, or a selected
> spheroid (given the Earth is not spherical). I'm assuming here that a
> spherical calculation will work in your case, using 1/60th of a degree = 1
> mile.
>
> given a user location of $x,$y, you need to construct a point geometry &
> check the distance (measured on a sphere in units degrees) against each of
> your points, eg:
> select * from table where ST_Distance_Sphere(ST_MakePoint($x,$y),geom) <=
> 7/60;
>
> see:
> http://postgis.refractions.net/documentation/manual-1.4/ST_Distance_Sphere.html
>
> HTH,
>
>   Brent Wood
>
>
>
> --- On *Wed, 12/19/12, nevillekb <nevillekb at gmail.com>* wrote:
>
>
> From: nevillekb <nevillekb at gmail.com>
> Subject: [postgis-users] Using PostGIS for latitude & longitude
> To: postgis-users at postgis.refractions.net
> Date: Wednesday, December 19, 2012, 10:51 PM
>
>
>
> I have a table in postgresql 9.2 that stores the latitude and longitude of
> locations as integer values.
>
> I intend to do something like when a user searches for a location, he also
> gets information on other locations that are within a 7 mile radius of that
> searched location.
>
> How do i use postGIS for this since i am new to it. Any idea.?
>
>
>
>
> --
> View this message in context:
> http://postgis.17.n6.nabble.com/Using-PostGIS-for-latitude-longitude-tp5002097.html
> Sent from the PostGIS - User mailing list archive at Nabble.com.
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-- 
Dr Duncan Golicher
Investigador Titular,
El Colegio de la Frontera Sur, Chiapas,Mexico
Mexico tel +52 1 967 137 94 20
Skype name duncangolicher

Publications: http://www.mendeley.com/profiles/duncan-golicher

Senior lecturer, Bournemouth University, UK
Centre for Conservation Ecology & Environmental Change
School of Applied Sciences
Christchurch House rm C218a
Bournemouth University
Fern Barrow
Poole (Dorset) BH12 5BB UK
Tel. +44 (0)1202 961682

For list of publications see Researcher ID:
http://www.researcherid.com/rid/B-4240-2009

dgolicher at bournemouth.ac.uk
dgoliche at ecosur.mx

Researcher ID:
http://www.researcherid.com/rid/B-4240-2009
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