[postgis-users] Newbie questions: SRIDs, function return values

[Burgholzer,Robert]

Paul,
Thanks, I think I did not look at your function carefully enough.  I was
still thinking in terms of my original proposition, which would be
specific to the area or distance function, and choose a coordinate
system for the geometries passed in to it.  I see now that your function
simply was a converter to the closest overlapping UTM zone, operating on
one at a time.  The function that I was envisioning would perhaps use
your st_utm function under the hood, but in it's internal it would do
something as follows:

FUNCTION st_distanceUnits (geom1, geom2, units) 
St_utm(extent( union(geom1, geom2) )) 

So that it would ask the st_utm function to return the best zone for
BOTH geometries passed to it.

r.b.

Robert W. Burgholzer
Surface Water Modeler
Office of Water Supply and Planning
Virginia Department of Environmental Quality
rwburgholzer at deq.virginia.gov
804-698-4405
Open Source Modeling Tools:
http://sourceforge.net/projects/npsource/

-----Original Message-----
From: postgis-users-bounces at postgis.refractions.net
[mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of Paul
Ramsey
Sent: Thursday, May 15, 2008 11:35 AM
To: PostGIS Users Discussion
Subject: Re: [postgis-users] Newbie questions: SRIDs, function return
values

Robert,

You misunderstand my core concern, which is that once you start using
pairs of geometries wrapped in this kind of "magic projection"
function, you have no guarantee that the individual functions are
applying the same magic to their geometries. In the case of data lying
on/near a zone boundary you could end up running a cartesian distance
op on one geometry that is projected into UTM10 and another that is in
UTM9.

P.

On Thu, May 15, 2008 at 8:12 AM, Burgholzer,Robert
<rwburgholzer at deq.virginia.gov> wrote:
> Paul,
> Agreed that there is very little way to prevent people from making
> dangerous inferences about in appropriate topics, so consider the
> following:
>
> Maybe part of the advanced options, or a corollary of such a function
> could be a report of just how far the envelope has been pushed, i.e.,
a
> percentage report describing the portion of the object that resides
> totally within the bounds of the UTM zone, 0.0 - 1.0, and perhaps a
> report of the comparison of area of shape to area of UTM zone, also
0.0
> - 1.0.
>
> Another approach, and now I am totally spitballing since I know little
> about geodetics, is some type of rotating UTM zone.  i.e, can one
simply
> define new center points for a "virtual" UTM zone, and adjust any
other
> related parameters that describe the spheroid and such?
>
>
> Robert W. Burgholzer
> Surface Water Modeler
> Office of Water Supply and Planning
> Virginia Department of Environmental Quality
> rwburgholzer at deq.virginia.gov
> 804-698-4405
> Open Source Modeling Tools:
> http://sourceforge.net/projects/npsource/
>
> -----Original Message-----
> From: postgis-users-bounces at postgis.refractions.net
> [mailto:postgis-users-bounces at postgis.refractions.net] On Behalf Of
Paul
> Ramsey
> Sent: Thursday, May 15, 2008 10:57 AM
> To: PostGIS Users Discussion
> Subject: Re: [postgis-users] Newbie questions: SRIDs, function return
> values
>
> The danger of this approach is that it falls apart as people start
> pushing the limits...
>
> select blah from a, b where
> st_dwithin(st_utm(a.geom),st_utm(b.goem),100)
>
> It's a pandora's box... as are all things geodetic
>
> p
>
> On Thu, May 15, 2008 at 7:45 AM, Markus Schaber <schabi at logix-tt.com>
> wrote:
>> Hi, Paul,
>>
>> "Paul Ramsey" <pramsey at cleverelephant.ca> wrote:
>>
>>> There's no universal answer to this, but I have long thought that a
>>> simple answer suitable for may people would be a ST_UTM(geom)
wrapper
>>> on transform that picks the appropriate UTM zone for a given
> geometry.
>>> It would work perfectly well for any collection of small objects. It
>>> would fall apart for larger things like states and countries that
are
>>> of similar size to a UTM zone.
>>
>> The function could simply use the centroid of the bounding box, that
>> shoul be fast enough.
>>
>>
>> Regards,
>> Markus
>>
>> --
>> Markus Schaber | Logical Tracking&Tracing International AG
>> Dipl. Inf.     | Software Development GIS
>>
>> Fight against software patents in Europe! www.ffii.org
>> www.nosoftwarepatents.org
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