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<DIV><STRONG><FONT face=Fixedsys size=2> Bruce - Your
definition of a glacier sounds excellent to me, both as a<BR>working tool and
more generally.<BR><BR> 1. The only problem I can see is
with observation F, which seems to need<BR>some fine-tuning. E.g. what does
"separate part" mean?<BR><BR> I developed this
definition for an article in the Encyclopaedia of<BR>Hydrological Sciences
(2005, Mass and energy balances of glaciers and ice<BR>sheets, volume 4,
2555-2574, Wiley):<BR> "A glacier is a collection of contiguous complete
flowlines through snow and<BR>ice which persists on the Earth's surface for more
than one year."<BR> Also<BR> "A flowline is a sequence of ice columns
of infinitesimal cross-section<BR>arranged so that each column gains mass by
flow from an up-ice neighbour and<BR>loses mass to a down-ice neighbour. To
a good approximation, flowlines may<BR>be identified by beginning at any point
where either the slope changes sign -<BR>at a flow divide - or the ice thickness
drops to zero, and following the<BR>direction of steepest ascent or descent to
another such point. The first<BR>column in the sequence has zero flow through
one boundary."<BR><BR>This definition - obviously for a purpose different from
yours - allows the<BR>arbitrary subdivision or non-subdivision of ice bodies as
long as dividing<BR>lines are either flowlines or inflections of the surface
slope. Ice caps with<BR>radial drainage can be recognized as one or arbitrarily
many units according<BR>to the judgement of the investigator. (Note that
two flowlines are</FONT></STRONG></DIV>
<DIV><STRONG><FONT face=Fixedsys size=2>contiguous if they have adjacent first
columns, i.e. at a divide.)<BR><BR> 2. A question not
considered in your text is "Is an ice shelf a<BR>glacier?" My answer is "That's
a good question ... ." I think the answer is<BR>actually "No, because the
grounding line is neither a divide nor a flowline",<BR>although that doesn't
feel very convenient. To complement the definitions<BR>above, I
concocted<BR><BR>"An ice shelf consists of the floating parts of two or more
glaciers."<BR><BR>which implies that a floating tongue (e.g. Petermann Glacier
in NW Greenland)<BR>is not an ice shelf. But the ice cover of Lake Vostok is
part of a glacier<BR>(unless somebody wants to take a flowline passing through
it as a division<BR>between two parts of the ice
sheet).<BR><BR> Perhaps this discussion is too pedantic.
We all know what we are talking<BR>about, don't we? But your statement of the
rationale shows that there are<BR>good practical reasons for getting it
right.<BR><BR> 3. Any attempt to define things must end
up using a number of terms<BR>which are *not* defined. I think it is reasonable
to leave "body of ice" and<BR>"ice mass" undefined, but what about "snowfield"?
It may not be necessary,<BR>but I think your definition of "glacier" would be
improved by a definition of<BR>"snowfield", if anyone feels like suggesting
one.<BR><BR> Best
wishes,<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/glaciology.htm<BR><BR>----- Original
Message ----- <BR>From: Bruce Raup <BR>To: gcogley1@cogeco.ca ; GLIMS Mailing
List <BR>Sent: Wednesday, 22 February 2006 17:10<BR>Subject: Proposed GLIMS
definition of "glacier"<BR><BR><BR>Hi all,<BR><BR>Please send comments about the
below, preferrably to the list, or directly <BR>to me.<BR><BR>Best
regards,<BR>Bruce<BR><BR><BR>Through experience with data submitted to the GLIMS
Glacier Database, as<BR>well as the GLACE experiments, it has become apparent
that a more precise<BR>practical definition of "glacier" is needed within the
GLIMS project. Such<BR>a definition will help ensure consistency of
analysis results in the<BR>database, facilitating more meaningful multi-temporal
comparisons and<BR>change-detection results in the future.<BR><BR>The following
candidate definition is the result of discussion at the GLIMS<BR>Workshop in
Twizel, New Zealand. Trevor Chinn began the discussion by<BR>reciting
several definitions of "glacier", and the ensuing discussion<BR>touched on
various practicalities of observing glaciers from space. The<BR>resulting
definition is heavily influenced by the original glacier mapping<BR>instructions
for the World Glacier Inventory (WGI).<BR><BR>Candidate definition of "glacier"
for the GLIMS Project:<BR><BR> A glacier consists of a body of ice that
persists for a number of years<BR> and is greater in area than
approximately 0.01 km2 (approximately 40<BR> ASTER VNIR pixels), measured
at end of summer, or, in the case of<BR> tropical glaciers, after
transient snow melts. This includes all<BR> tributaries, connected
feeders, disconnected or icefall-interrupted<BR> feeders that
contribute ice to the main glacier, plus all debris-covered<BR> ice.
Excluded is all exposed ground, including nunataks, together with<BR> any
adjacent snow and ice areas that do not contribute ice, even though<BR>
they may be the source of snow avalanches and wind redistributed snow,<BR>
both of which are considered to be part of seasonal accumulation.<BR>
Joined glaciers should generally be separated at their topographic<BR>
divides, though in some cases, an analyst may treat several glaciers
that<BR> are connected but that have divergent flow (i.e., they don't
contribute<BR> mass to each other) as a single unit.<BR><BR>Consequences
and observations:<BR><BR>A. Snowfields above the accumulation zone of a glacier
shall be considered<BR> part of the glacier, because they contribute
snow (through avalanches)<BR> and ice (through creep flow) to the
glacier.<BR><BR>B. A tributary in a glacier system that has historically been
treated (and<BR> named) as a separate glacier should, within the
GLIMS framework, be<BR> included as part of the glacier into which
it flows.<BR><BR>C. Any steep rock walls that avalanche snow on to a glacier but
do not<BR> retain snow themselves are NOT included as part of the
glacier.<BR><BR>D. Separate snowfields above the accumulation zone of a glacier
shall NOT<BR> be considered part of the glacier, because they
contribute only snow<BR> (through avalanches) in the same manner as
avalanches off rock.<BR><BR>E. A stagnant ice mass still in contact with a
glacier is part of the<BR> glacier, even if it supports an
old-growth forest.<BR><BR>F. If no flow takes place between separate parts of a
continuous ice mass<BR> they should, in general, be treated as
separate units. However, for<BR> practical purposes, such an
ice mass may be analyzed as a unit at the<BR> analyst's
discretion. See question 1 below.<BR><BR>G. Marginal and terminal moraines
should be included if they contain ice.<BR><BR>Questions:<BR><BR>1 Does this
mandate the delineation of flow divides?<BR><BR> ANSWER: No, an ice mass
containing flow divides can still be analyzed as<BR> a unit, if
delineation of the flow divides is impossible or impractical.<BR> If the
same system is analyzed in the same way later, it will have the<BR> same
glacier ID, and can therefore be compared. If the system is<BR>
analyzed in more detail later by breaking it into its component
glaciers,<BR> those pieces will get new IDs (ID of system will be "parent
icemass" ID),<BR> and future analyses of those pieces, if done in the same
way, will be<BR> comparable.<BR><BR>2 What about the lower parts of
lateral snowfields, whose extent varies<BR> from year to
year?<BR><BR> ANSWER: Map only at the end of summer to exclude
seasonal snow. Then<BR> map everything that is connected to the
glacier.<BR><BR>For details on how to produce outlines consistent with this
definition,<BR>see the GLIMS Glacier Analysis Tutorial
at<BR>http://www.glims.org/MapsAndDocs/assets/GLIMS_Analysis_Tutorial.pdf<BR>[to
be updated soon].<BR><BR>-- <BR>Bruce Raup<BR>National Snow and Ice Data
Center<BR>University of Colorado<BR>449 UCB, Boulder, CO
80309<BR>Phone: 303-492-8814<BR></FONT></STRONG><A
href="http://cires.colorado.edu/~braup/"><STRONG><FONT face=Fixedsys
size=2>http://cires.colorado.edu/~braup/</FONT></STRONG></A></DIV></BODY></HTML>