Proposed GLIMS definition of "glacier"

[Bruce Raup]

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/