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