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

[Mike Leahy]

Hey there,

I usually use transform() to convert my data to a UTM projection, so
that I can calculate distances in metres with relative ease - maybe
there's a more accurate way, but this works well enough for me.

As for looking up SRID numbers, someone (either in this list or another)
directed me to http://www.espg.org, where you can download a database of
EPSG projection information, including a version in Microsoft Access
format with forms for browsing.  I believe the EPSG identifier is
usually the same as the SRID in the spatial_ref_sys table included with
PostGIS (but you'll want to compare the records for whatever projection
you need to make sure I'm right about that).

Regards,
Mike

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…
>
> Of course if I do the following, I get a reasonable answer (about
> 10km), but it’s not exactly what I want
>
>     SELECT distance_sphere(PointFromText('POINT(40.8117 -74.218)',
> 32617), PointFromText('POINT(40.8272 -74.1242)', 32617))
>
> Even constructing a line and taking the length doesn’t work and I’ve
> tried all SRIDs.
>
>  
>
> 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)?
>
>  
>
> Thanks.
>
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