[postgis-users] Re: st_dwithin in SRID 4269

[Sean Fulton]

On 2009-04-25 09:50:55 -0400, Sachin Srivastava 
<sachin.srivastava1984 at gmail.com> 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