You are right Mark, of course :-)
I'm sorry to mislead.
Thanks
Nicklas
2009-04-27 MarkW wrote:
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:
>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:
>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
>>
>>
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>>
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>>
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>>
>>
>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 allcases.
>>
>>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
>>
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>
>
>
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