[postgis-users] Problems with st_distance

[Nicklas Avén]

The geography type distance calculations gives answers in meters from
unprojected data (4326).

I see no erason to belive that the postgis answer is wrong, especially
not when Stuart says he knows the distance is 1800 meters and he gets
1179.61 meters from PostGIS.

I don't know anything about that draft-logic site but I would definitly
rely more on the PostGIS answer. 

The measurments on Bing, Google maps and so on will be wrong since the
webmercator projection 913900 is a flattened globe. How could it be
correct?

/Nicklas


On Tue, 2012-03-13 at 07:37 -0400, Charles Galpin wrote:
> The distance should be in the projection units, which for 4326 is
> decimal degrees, and if you want meters you'll need to convert the
> results to meters.
> 
> 
> But first, I suspect your data is not in 4326 then if you get 1179.
>  Make sure you are using the SRID when inserting them
> 
> 
> >         update address set geog=ST_GeographyFromText('POINT('||
> >         lon||' '||lat||')',4326) where id=?;
> Also see
> http://postgis.refractions.net/docs/ST_Distance.html
> hth
> charles
> 
> On Mar 13, 2012, at 2:58 AM, Stuart Guthrie wrote:
> 
> > 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.
> >         
> 
> 
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