[postgis-users] Problems with st_distance

[Stuart Guthrie]

Could it be returning a distance in decimal miles? I'm out by around 1.6 -
1.6km = 1mile?

On Tue, Mar 13, 2012 at 5:43 PM, Stuart Guthrie <stuartguthrie at gmail.com>wrote:

> OK, I'm a newbie to postgis but an old-timer with open source in general.
>
> Sorry to bother everyone but this problem is driving me a little nuts.
>
> I've bought the 'postgis in action' book and now I think I still know
> nothing.
>
> I'm trying to store long lats and do queries on them to establish distance
> and use that for reporting.
>
> I'm using this to update the table 'address'
>
> update address set geog=ST_GeographyFromText('POINT('||lon||' '||lat||')')
> where id=?;
>
> the address table field 'geog' has this definition:
>
>  geog            geography(Point,4326)
>
> When I compare two addresses that I know are around 1800m apart I get this:
>
> select a.addressline2, a.suburb, ST_Y(a.geog::geometry),
> ST_X(a.geog::geometry),  b.addressline2,b.suburb,ST_Y(b.geog::geometry),
> ST_X(b.geog::geometry), ST_distance(a.geog,b.geog ) from address a join
> address b on b.id=41491 where a.id=1238 ;
>
> addressline2  | suburb |    st_y     |    st_x     |  addressline2  |
> suburb    |    st_y     |    st_x     |   st_distance
>
> ---------------+--------+-------------+-------------+----------------+-------------+-------------+-------------+------------------
>  140 Sussex St | Sydney | -33.8692429 | 151.2036312 | 1 Riley Street |
> Surry Hills | -33.8726426 | 151.2157115 | 1179.61881540556
>
> Postgis says they are 1179.61m apart.
>
> When I use google maps/yahoo maps/bing maps scale with a good-old
> fashioned ruler, I get closer to 1800-2000m.
>
> This site says 2230m:
>
> http://www.daftlogic.com/projects-google-maps-distance-calculator.htm
>
> Is there somewhere I'm missing a magic incantation? I can't find it. Tried
> geometry instead of geography no solution for me there.
>
> I'm using Postgis 1.5.3-1 ubuntu amd64 11.10
>
> Thanks if you can help.
>
> --
> Best regards
>
> Stuart Guthrie
>
>
>
>


-- 
Best regards

Stuart Guthrie
(m) 0403 470 123
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