[Ottawa_users] Lat Long Calculations

[Frank Warmerdam]

Sampson, David wrote:
> Thanks Frank,
> 
> I was looking into maybe doing a projection to UTM, but I fear problems
> when a distance is calculated across two different UTM zones... Does
> this matter?

Dave,

It is important that both points can be converted to a projection
that is well behaved (low error) in the area containing the points.
Cross UTM zone edges isn't a problem, but the further you go "out of
the zone" the greater the distortion.  It would likely be better
to use a custom projection centered between your two points.  I'm
not exactly sure what projection would be best though since my
projection "theory" is pretty weak.

> I looked at GEOS and found that there are measurements between geometry
> objects.
> http://geos.refractions.net/ro/doxygen_docs/html/namespacegeos_1_1operat
> ion_1_1distance.html
> But getting into the GEOS library from python is proving a challenge.
> Some posts even note that the measure function is not exposed to the C
> API of Geos (FRANK?)
> http://lists.maptools.org/pipermail/fwtools/2006-March/000318.html.

GEOS is just doing a cartesian, not great circle, calculation.  So
you might as well do it yourself in python.

> So what about the measuremnt function from PCL?  Well I am having to
> battle PCL install on windows using FWTools. If this works I hope this
> is even what I want. Furthermore I hope I can compile and package up an
> application without an FWTOOLS dependency.

I can't speak for what PCL has built in.  I think mixing PCL binaries
and FWTools is likely to be a nightmare though it might be possible
to graft pure python parts of PCL into FWTools.  Generally though if
PCL isn't doing anything special for you, then it likely isn't worth
while.

> I would have thought someone would have a compas.py program that would
> do all this, but I guess it's so simple that everyone reinvents their
> own wheel when they need it. My chissel is not that precise to carve a
> new wheel. (SMILE)

I imagine such a thing exists somewhere.  Finding and integrating may
prove a challenge.  If you are willing to use cartesian distances in
a projection between points then there is really no reason not to do
the distance yourself, using osr.py to reproject things.

Best regards,
-- 
---------------------------------------+--------------------------------------
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | President OSGeo, http://osgeo.org