"left" and "right" sides of glacier boundaries

[Graham Cogley]

     When I suggested a GLIMS button for reversing polylines and polygons,
I was thinking of Didger (vn3), in which directions are listed and the 
operation itself takes two mouse clicks. Programming the operation is almost trivial, about three lines of code, although determining the polarity is
a bit (but not much) more complicated. If anyone could make use of Fortran
subroutines for these jobs they are welcome to mine. (There are probably
lots of such routines available, however.)

     Graham.
     gcogley@
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             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: Fiona Cawkwell 
To: gcogley1 at cogeco.ca ; glims 
Sent: Tuesday, 01 March 2005 15:51
Subject: RE: "left" and "right" sides of glacier boundaries


Bruce, Frank et al

in response to the very valid practical issues Frank raised, there are tools 
in  Arc that are capable of indicating and reversing the direction of polyline 
and polygon themes e.g. from the ESRI Arcscripts download site, for those 
people using ArcView, line direction tool 'line_dir.zip' to identify current 
polygon direction, and flip lines tool 'AS11421.zip' to change the direction 
of lines, with comparable tools for those using ArcGIS; thus it is possible 
with this software to delineate all exterior polygons in an anticlockwise 
direction and interior polygons in a clockwise direction

best wishes

Fiona


>Hi Bruce and Co.
>
>I full agree with Frank opinion (below)
>
>Cheers
>JPDedieu
>
>
>
>xxxxxxxxxxxx
>Hi Bruce & Co.
>
>I agree with using the hydrological convention for assigning left and
>right, using the opposite direction for internal polygons and
>follow the conventions given by the software. However, for practical
>reasons I want to add two points:
>
>(1) If we obtain glacier outlines automatically from satellite data
>(what I thought would be the 'normal' way of getting them ?) we will
>have no circulation direction of the outline. Using the 'gridpoly'
>command in Arc/Info line segments with 500 vertices each are created
>and the respective polygons have the value (label) from the original
>Geo-tiff (which is 0 for 'glaciers' and 1 [or 255] for 'other'). In
>order to allow calculation of glacier areas the value 1 is set to
>NODATA and in the attribute table the item 'grid-code' is used for
>glacier identification (-9999 for other and 0 for glaciers).
>
>(2) The digitized Swiss glacier inventory from 1973 also did not use
>any specific circulation. The discrimination of glacier polygons from
>internal polygons (rock outcrops) is made by manual assignment of
>a label to each glacier under investigation. As such, also 'problematic'
>glaciers can be excluded from the statistical analysis.
>
>Best regards, Frank

--------------------------------------------
Dr Fiona Cawkwell
Post Doctoral Research Fellow
Department of Earth and Atmospheric Sciences
University of Alberta
Edmonton
Alberta, T6G 2E3

tel. 780 492 7821
fax. 780 492 2030
fiona.cawkwell at ualberta.ca
http://www.ualberta.ca/~cawkwell/fiona_cawkwell.html
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