"left" and "right" sides of glacier boundaries

[Siri Jodha Singh Khalsa]

 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