[postgis-users] Problems with st_distance

[Nicklas Avén]

Hallo again

Stuart, what makes you say that you know the distance is 1800 meters?
I just measured by hand in google earth and I get the same answer as
PostGIS there about 1180 meters.

I think you have been tricked by some application using web mercator
projection in the first.

Google earth, like PostGIS geography knows how to calculate distances on
a sphere or spheroid which for instance google maps doesn't do since it
is a flattened globe.

HTH

Nicklas






On Tue, 2012-03-13 at 12:50 +0100, Nicklas Avén wrote:
> 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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