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

[Mike Leahy]

Correction...that url was misspelled - it's supposed to be
http://www.epsg.org - sorry.

Mike Leahy wrote:

> 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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>>
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>>  
>>
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