[postgis-users] Simple question: How to get projected distance?

[Gustavo Henrique Sberze Ribas]

Hi,


> Here's what I've tried and the problems I've encountered:
> 1. Tagging the points with an SRID of 4326 (WGS 84) and 
> directly querying for the distance.  Since it's not a projected coordinate 
> system, it just gives the Cartesian distance.  Pretty useless.

  Don't apply 2D planar formulas to an ellipsoidal coordinate system.
It makes no sense.


> 2. Use distance_sphere() to calculate the distance.  Great, I 
> get distances, but can't create shapes, etc.

  Why not? If you don't need much precision you can say
that 1 degree ~ 100km. 

> 3. Transform the points into a UTM projection and create 
> polygons and query for distance on the transformed points. 
> This works, however  the system must work across the US and 
> I'd have to have a table for each UTM zone and add a
> lot of logic to accommodate not to mention separate indexes, 
> etc.  Very ugly option.

  Agreed.

> 4. Transform into UTM, perform the operations, and transform the UTM
> coordinates back into WGS 84 lat/long, storing the resulting 
> points in one table with the appropriate indexes attached for efficient querying.
> Ideally, I'd like to end up with a set of WGS 84 defined  geometries that I
> can query.  Unfortunately, this doesn't work.  Transforming  the points back
> into WGS 84 yields nonsense results.

  That's strange, I do exactly that and it works just fine. Here's an example that
creates a circle of radius 100m around a lat/long point:

select 
 transform(
   setSRID(
      buffer(
         transform(
           'SRID=4326;POINT(-47.2407989501953 -22.7394008636475)',32723
	 ),
	 100
      ),
      32723
   ),
   4326
 )


  I project the point to UTM using the same WGS84 ellipsoid, then I buffer it
to create a "circle" (32 sided polygon). Finally I translate the circle coordinates
back to lat/lon WGS84. 


> I thought this would be very simple.  At this point, I'm 
> quite frustrated. What am I doing wrong?? 

 Remember: Point(Longitude Latitude).
 

--
Gustavo

> Some of my results:
> -- Create WGS84 point and transform into UTM zone 18N
> ASText(Transform(PointFromText('POINT(40.8117 -74.218)', 
> 4326), 32618));
> Result: "POINT(2192975.52043048 -10779305.4033352)"
> 
> -- Create WGS84 point, transform into UTM zone 18N, and transform back
> ASText(Transform(Transform(PointFromText('POINT(40.8117 
> -74.218)', 4326),
> 32618), 4326));
> Result: "POINT(-75 -90)"
> 
> 
> Thanks,
> Lance
> 
> Lance Arlaus wrote:
> > I know this is a simple question, but I'm just getting started with 
> > PostGIS and I can't seem to perform a simple calculation.
> > 
> > I just need to calculate the distance between two points.  
> What am I 
> > doing wrong?
> > 
> >  
> > 
> > Query:
> > 
> >     SELECT distance(PointFromText('POINT(40.8117 -74.218)', 32617), 
> > PointFromText('POINT(40.8272 -74.1242)', 32617))
> > 
> > Returns:
> > 
> >     0.095.
> 
> This is the distance in the input units. Since the values you 
> are using 
> are decimal degrees the result is 0.095... decimal degrees 
> and probably 
> assumes that you are computing this in a flat X-Y plane.
> 
> > Of course if I do the following, I get a reasonable answer 
> (about 10km), 
> > but it's not exactly what I want
> 
> OK, what is it that you want?
> 
> >     SELECT distance_sphere(PointFromText('POINT(40.8117 -74.218)', 
> > 32617), PointFromText('POINT(40.8272 -74.1242)', 32617))
> 
> This is better than you first try because this is computing the arc 
> distance along the great circles between the two points.
> 
> > Even constructing a line and taking the length doesn't work 
> and I've 
> > tried all SRIDs.
> > 
> 
> SRIDs are an attribute associated with the geometry, like, 
> red, green, 
> blue. Changing them will not change the answer unless you 
> pass them to a 
> utility that is actively aware of them. They are there to you 
> don't try 
> to compare or manipulate two points that were stored in 
> different SRIDs.
> 
> The standard use of them is to use them to perform 
> projections from the 
> objects stored SRID to some common workspace that would be good for 
> manipulation.
> 
> The SRID for WGS84 is I believe 4326
> 
> To do things right you need to understand the projection 
> space that you 
> data is in and how it effects calculations and the postGIS functions.
> 
> > 
> > Sorry to ask such a simple question.
> > 
> > Also, is there a document that describes the well known 
> SRIDs?  I'm not 
> > sure which one to use.  For example, the data I've got 
> references WGS 
> > 84, but there's a ton of SRIDs defined.  I've found docs 
> describing the 
> > format of the specification, but how are the SRIDs created 
> (who serves 
> > as the clearinghouse for this info)?
> 
> HTH,
>    -Steve
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