Okay, you said to correct you! A minute of latitude is the unit that is a nautical mile. If each second where a mile, there would be 3600 nautical miles in a degree - rather than the actual 60.
Mark On Sat, Apr 25, 2009 at 2:05 PM, <[email protected]> wrote: > Hallo > > Someone have to correct me if I'm wrong but as I remamber it from my > nautical experience from quite along ago: > A nautical mile is 1852 meters yes. But not a degree. the distance of a > degree is depending on if you are talking about a longitude or latitude > degree. A latitude degree can be converted to meters. I think one degree is > divided in 60 minuites. And one miniuite is divided in 60 seconds and if I > remember right on second is the same as one nautical mile; 1852 meter. But > if we are talking about longitude degrees it depends on how close you are to > the poles. If you are just one meter from the pole you just have to take a > walk of some meters to pass all 360 degrees but if you are walking at the > equator you will have to walk and swim about 40000 km. > > This is why you have to transform your geometry to a meter-based srid > before calculating distances right. > > What you first have to do is to tell the system what srid your data > originally has if that isn't already done when loaded. That you do with > st_setsrid: > http://www.postgis.org/documentation/manual-svn/ST_SetSRID.html > > then when the system knows what you have (that is stored together with your > geometry), then you can transform it to desired meterbased srid with > st_transform: > http://www.postgis.org/documentation/manual-svn/ST_Transform.html > > Hope things get clearer (and hope I'm right) > > /Nicklas > > 2009-04-25 Sachin Srivastava wrote: > > > > > >> > 2009/4/25 Sean Fulton <[email protected]> > > > On 2009-04-25 09:50:55 -0400, Sachin Srivastava < > [email protected]> said: > > > > I am a newbie, could somebody explain what distance transformation > should i > > use to eliminate discrepancy in results for the following 2 queries > > > > 1) SELECT * from table1 where st_dwithin(geom1, geom2, distance); > > *(Note, geom1 and geom2 are in SRID 4269) > > * > > 2) SELECT * from table1 where st_dwithin(transform(geom1, 2163), > > transform(geom2,2163), distance); > > * > > Note:Here distance is in meters > > > > The things that i understood so far are, distance has to be in the same > SRID > > as the two geoms, I knew distance in meters, so the second query will > give > > me correct results however what transformation should i do in the first > > query distance value to get the same result as of query 2. > > > > > Sachin, > > > > I believe the answer is that you use query 2 that you provided. Query 1 > will always give you distance in degrees which is going to be meaningless. > You will never get meters out of Query 1 without transforming the > coordinates as you did in Query 2. You could try to convert the result of > Query 1 from degrees to meters but that's really doing it backwards and I > wouldn't trust the results. Query 2 is the correct approach. > > > > Sean > > > > > > _______________________________________________ > > postgis-users mailing list > >[email protected] > >postgis.refractions.net/mailman/listinfo/postgis-users > > > > >Sean, > > > > Thanks a lot for your answer. > > > > 1 degree is 1852 meters, so if i do distance/1852, in the Query1 i should > get the correct results, but that is not happening. > > I guess that conversion has to do a lot with what SRID i am using. > > > > So what you suggest is the best way to find whether two geoms are at > 'dist' distance apart. Whether the following query will do in all cases. > > > > SELECT * FROM table where st_dwithin(transform(geom1, 2163), > transform(geom2, 2163), dist); > > > > Or It will again depend on which SRID i am using? > > > > > > > > > >-- > >An Expert is the one who has made all the mistakes that are possible > within a narrow field. > > ---------- Sachin > > > > _______________________________________________ > postgis-users mailing list > [email protected] > http://postgis.refractions.net/mailman/listinfo/postgis-users > >
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