[postgis-users] Problems with st_distance

[Nicklas Avén]

Sorry, wrong from me

Something is wrong of course if the answer is 1179 compared to 1800. My
brain was on vaccation

/Nicklas


On Tue, 2012-03-13 at 12:48 +0100, Nicklas Avén wrote:
> 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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> 
> 
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