"left" and "right" sides of glacier boundaries

[Deborah Lee Soltesz]

Hi Gang,

Oracle's convention on this subject agrees with the GML specification. I'm 
not 100% certain if Oracle is fully GML compliant, but that's beside the 
point. I believe we should follow Open GIS standards as much as humanly 
possible.

deborah

At 09:21 AM 2/25/2005, Siri Jodha Singh Khalsa wrote:
> From a spatial metadata standpoint:
>
>In ECS a GPolygon "must have at least 3 unique points ordered in a 
>clockwise direction with the interior to the right."
>
>The FGDC is silent on the ordering of the points of a GPolygon.
>
>But Oracle, in it's spatial database extension, uses the convention - 
>outer polygons are counterclockwise and interior polygons are clockwise.
>
>I say we go with Oracle's convention.
>
>Cheers,
>
>Siri Jodha
>Graham Cogley wrote:
>
>>*     One problem with hydrological "handedness" (look downstream and 
>>identify
>>"left" with your left hand) is that it does not work for polygons for which
>>"downstream" has no meaning, such as drainage divides. In computer graphics,
>>and more generally in topology, the decision between "inside on the left" and
>>"inside on the right" becomes arbitrary - it doesn't matter which you choose,
>>*but* it is essential that you make your choice and then stick to it. In the
>>nesting of polygons, for example, it is desirable for nunataks to have the
>>opposite polarity from glaciers and for glaciers inside nunataks to have the
>>same polarity as glaciers. If they do, then all the glacier areas come out
>>positive and all the nunatak areas come out negative. In other words, the
>>polygons carry valuable information implicitly, and for this basic 
>>measurement
>>it is not necessary to rely on explicit coding of the polygon type.
>>
>>      For what little it is worth (because I am not contributing actively to
>>GLIMS), my vote would be for the opposite convention from Bruce's 
>>(because all
>>of the polygons on my computer are that way around). But I would urge the
>>desirability of getting as soon as possible to the state described in Bruce's
>>second paragraph. There are ways of achieving this, but they mean a little
>>work. As an interim measure, would it be feasible to provide a button to *
>>*reverse the polarity of any polygon (or polyline)? Or even a button to 
>>reverse *
>>*the **polarity of all polygons/polylines sharing some chosen attribute?
>>
>>      Graham.
>>      gcogley@
>>           _/                                            _/
>>          _/                                            _/
>>       _/_/_/     _/ _/_/     _/_/_/      _/ _/_/    _/_/_/    _/     _/
>>        _/       _/_/   _/  _/     _/    _/_/   _/    _/      _/     _/
>>       _/       _/         _/_/_/_/_/   _/      _/   _/      _/     _/
>>      _/  _/   _/          _/          _/      _/   _/  _/  _/     _/
>>      _/_/    _/            _/_/_/    _/      _/    _/_/     _/_/_/  .ca
>>
>>              J. Graham Cogley, Ph.D., Professor of Geography,
>>                 Department of Geography, Trent University,
>>         1600 West Bank Drive, Peterborough, Ontario, CANADA K9J 7B8.
>>     Fax 705-742-2131.  Tel 705-748-1011-ext1440.  Email gcogley at trentu.ca
>>              http://www.trentu.ca/geography/glaciology.htm
>>
>>----- Original Message -----
>>From: Bruce Raup
>>To: gcogley1 at cogeco.ca ; GLIMS Mailing List
>>Sent: Thursday, 24 February 2005 17:28
>>Subject: "left" and "right" sides of glacier boundaries
>>
>>
>>Hi all,
>>
>>As you probably know, the GLIMS database provides for storing information
>>about what materials and features are on the left and right sides of
>>glacier boundary segments, and the GLIMSView program makes it fairly easy
>>to assign such attributes to digitized glacier boundaries.  However, the
>>definition of "left" and "right" hasn't really been firmly specified.
>>
>>Ultimately, it would be nice if in GLIMSView, you could just click on one
>>side or the other and not worry about whether it is "left" or "right".
>>But we're not there yet.
>>
>>I propose the following solution.  Since it might negatively impact those
>>who have already assigned such attributes, I want to get your feedback.  I
>>propose ordering all polygons to be in counter-clockwise order (interior
>>on left) at ingest time.  References to left and right would be according
>>to this vertex ordering.
>>
>>GLIMSView currently outputs polygon segments with points ordered in the
>>direction of digitization, and can therefore output segments with
>>inconsistent direction when placed within the polygon.  My ingest software
>>already puts the segments together so that they match up end-to-end in a
>>consistent direction.  Now I'd like to make the handedness consistent too.
>>
>>Comments?
>>
>>Bruce
>>
>>--
>>Bruce Raup                                               Phone:
>>303-492-8814
>>National Snow and Ice Data Center, U. of Colorado, 449 UCB, Boulder, CO 80309
>>http://cires.colorado.edu/~braup/*
>
>
>
>--
>Siri Jodha Singh Khalsa, Ph.D. / Space Sciences Sector, L-3 GSI
>ECS Science Data Coordinator for the National Snow and Ice Data Center 449 
>UCB / 1540 30th St. Boulder, CO 80309 Phone: (303) 492-1445 FAX: (303) 
>492-2468 http://cires.colorado.edu/~khalsa