Proposed GLIMS definition of "glacier"
Graham Cogley
gcogley1 at cogeco.ca
Wed Feb 22 20:39:16 MST 2006
Bruce - Your definition of a glacier sounds excellent to me, both as a
working tool and more generally.
1. The only problem I can see is with observation F, which seems to need
some fine-tuning. E.g. what does "separate part" mean?
I developed this definition for an article in the Encyclopaedia of
Hydrological Sciences (2005, Mass and energy balances of glaciers and ice
sheets, volume 4, 2555-2574, Wiley):
"A glacier is a collection of contiguous complete flowlines through snow and
ice which persists on the Earth's surface for more than one year."
Also
"A flowline is a sequence of ice columns of infinitesimal cross-section
arranged so that each column gains mass by flow from an up-ice neighbour and
loses mass to a down-ice neighbour. To a good approximation, flowlines may
be identified by beginning at any point where either the slope changes sign -
at a flow divide - or the ice thickness drops to zero, and following the
direction of steepest ascent or descent to another such point. The first
column in the sequence has zero flow through one boundary."
This definition - obviously for a purpose different from yours - allows the
arbitrary subdivision or non-subdivision of ice bodies as long as dividing
lines are either flowlines or inflections of the surface slope. Ice caps with
radial drainage can be recognized as one or arbitrarily many units according
to the judgement of the investigator. (Note that two flowlines are
contiguous if they have adjacent first columns, i.e. at a divide.)
2. A question not considered in your text is "Is an ice shelf a
glacier?" My answer is "That's a good question ... ." I think the answer is
actually "No, because the grounding line is neither a divide nor a flowline",
although that doesn't feel very convenient. To complement the definitions
above, I concocted
"An ice shelf consists of the floating parts of two or more glaciers."
which implies that a floating tongue (e.g. Petermann Glacier in NW Greenland)
is not an ice shelf. But the ice cover of Lake Vostok is part of a glacier
(unless somebody wants to take a flowline passing through it as a division
between two parts of the ice sheet).
Perhaps this discussion is too pedantic. We all know what we are talking
about, don't we? But your statement of the rationale shows that there are
good practical reasons for getting it right.
3. Any attempt to define things must end up using a number of terms
which are *not* defined. I think it is reasonable to leave "body of ice" and
"ice mass" undefined, but what about "snowfield"? It may not be necessary,
but I think your definition of "glacier" would be improved by a definition of
"snowfield", if anyone feels like suggesting one.
Best wishes,
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/glaciology.htm
----- Original Message -----
From: Bruce Raup
To: gcogley1 at cogeco.ca ; GLIMS Mailing List
Sent: Wednesday, 22 February 2006 17:10
Subject: Proposed GLIMS definition of "glacier"
Hi all,
Please send comments about the below, preferrably to the list, or directly
to me.
Best regards,
Bruce
Through experience with data submitted to the GLIMS Glacier Database, as
well as the GLACE experiments, it has become apparent that a more precise
practical definition of "glacier" is needed within the GLIMS project. Such
a definition will help ensure consistency of analysis results in the
database, facilitating more meaningful multi-temporal comparisons and
change-detection results in the future.
The following candidate definition is the result of discussion at the GLIMS
Workshop in Twizel, New Zealand. Trevor Chinn began the discussion by
reciting several definitions of "glacier", and the ensuing discussion
touched on various practicalities of observing glaciers from space. The
resulting definition is heavily influenced by the original glacier mapping
instructions for the World Glacier Inventory (WGI).
Candidate definition of "glacier" for the GLIMS Project:
A glacier consists of a body of ice that persists for a number of years
and is greater in area than approximately 0.01 km2 (approximately 40
ASTER VNIR pixels), measured at end of summer, or, in the case of
tropical glaciers, after transient snow melts. This includes all
tributaries, connected feeders, disconnected or icefall-interrupted
feeders that contribute ice to the main glacier, plus all debris-covered
ice. Excluded is all exposed ground, including nunataks, together with
any adjacent snow and ice areas that do not contribute ice, even though
they may be the source of snow avalanches and wind redistributed snow,
both of which are considered to be part of seasonal accumulation.
Joined glaciers should generally be separated at their topographic
divides, though in some cases, an analyst may treat several glaciers that
are connected but that have divergent flow (i.e., they don't contribute
mass to each other) as a single unit.
Consequences and observations:
A. Snowfields above the accumulation zone of a glacier shall be considered
part of the glacier, because they contribute snow (through avalanches)
and ice (through creep flow) to the glacier.
B. A tributary in a glacier system that has historically been treated (and
named) as a separate glacier should, within the GLIMS framework, be
included as part of the glacier into which it flows.
C. Any steep rock walls that avalanche snow on to a glacier but do not
retain snow themselves are NOT included as part of the glacier.
D. Separate snowfields above the accumulation zone of a glacier shall NOT
be considered part of the glacier, because they contribute only snow
(through avalanches) in the same manner as avalanches off rock.
E. A stagnant ice mass still in contact with a glacier is part of the
glacier, even if it supports an old-growth forest.
F. If no flow takes place between separate parts of a continuous ice mass
they should, in general, be treated as separate units. However, for
practical purposes, such an ice mass may be analyzed as a unit at the
analyst's discretion. See question 1 below.
G. Marginal and terminal moraines should be included if they contain ice.
Questions:
1 Does this mandate the delineation of flow divides?
ANSWER: No, an ice mass containing flow divides can still be analyzed as
a unit, if delineation of the flow divides is impossible or impractical.
If the same system is analyzed in the same way later, it will have the
same glacier ID, and can therefore be compared. If the system is
analyzed in more detail later by breaking it into its component glaciers,
those pieces will get new IDs (ID of system will be "parent icemass" ID),
and future analyses of those pieces, if done in the same way, will be
comparable.
2 What about the lower parts of lateral snowfields, whose extent varies
from year to year?
ANSWER: Map only at the end of summer to exclude seasonal snow. Then
map everything that is connected to the glacier.
For details on how to produce outlines consistent with this definition,
see the GLIMS Glacier Analysis Tutorial at
http://www.glims.org/MapsAndDocs/assets/GLIMS_Analysis_Tutorial.pdf
[to be updated soon].
--
Bruce Raup
National Snow and Ice Data Center
University of Colorado
449 UCB, Boulder, CO 80309
Phone: 303-492-8814
http://cires.colorado.edu/~braup/
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