GLIMS definition issues

[Bruce Raup]

Hello all.

I am readying a manuscript for submission to Eos about the GLIMS database
and the scientific considerations that went (are going) into its design.
Below are a few excerpts which need discussion in the wider GLIMS
community.  The issues are:

1.  Definition of a "glacier"
2.  How to treat ice shelves
3.  Coordinate systems

I'm asking for your feedback on the way I've addressed this issues, and
whether you think there are any problems with this.  Thanks in advance for
your quick and insightful comments.

Bruce

P.S.  Sorry if you get this message more than once.

Bruce Raup
National Snow and Ice Data Center                     Phone:  303-492-8814
University of Colorado, 449 UCB                       Fax:    303-492-2468
Boulder, CO  80309-0449                                    braup at nsidc.org


\section{Scientific Considerations}

\subsection{Glacier as Basic Unit}

The fundamental unit in the GLIMS database is the \emph{glacier}.
The definition of ``glacier,'' however, can be a bit problematic.  Glacier
networks, following the underlying drainage basins, are topologically
tree-like.  At one extreme, one could treat a contiguous, but possibly
highly complex and dendritic, ice mass as one unit, and track changes in
its area over time, in a way similar to how sea ice or seasonal snow cover
are tracked using remote sensing.  At the other extreme, one could decide
to treat each branch and ``leaf'' of the tree as a separate unit, putting
arbitrary boundaries between different branches of the glacier complex at
confluence points.  While more complicated, this approach would have the
advantage of treating as separate units masses of ice that historically have been
given separate names, despite possibly being connected to other ice bodies.
Introducing arbitrary boundaries at confluence points allows the
distinctions between ``glaciers'' to be made that have always been made by
field-based scientists.

A problem with mandating that all GLIMS analyses dice up ice masses to the
ultimate level is that such detailed distinctions are seen as burdensome
and unnecessary in areas where there is no such historical view.  In
regions studied for the first time through remote sensing methods, Regional
Center analysts should be free to include tributaries as part of a larger
trunk glacier.

Thus, the GLIMS group has decided on an intermediate approach:  the
database is designed to store information about any body of ice contained
in a single (simple or compound) drainage basin.  A ``glacier'' can thus be
an ice mass in a simple valley, or a large glacier system with many
tributaries.  Ice flow divides are necessarily considered glacier
boundaries.  In the database, glacier records can be related, so that,
should a compound glacier system be analyzed at a greater level of detail
in later years (e.g., a glacier system is analyzed as three glaciers
instead of one), the database records for the ``children'' glaciers can be
associated with that of their ``parent.''


Within the GLIMS framework, ice shelves are not considered to be part of
glacier systems.  Thus, the ``terminus'' of an ice stream that feeds an
ice shelf is the grounding zone.  If the location of the grounding zone is
uncertain, this will be reflected in the error estimates in the position
recorded in the database.

\subsection{Coordinate Systems and Errors}

Three fundamental coordinate systems are used in the GLIMS processing stream:
\begin{enumerate}
\item Local image coordinate system (abbreviated L/S).  Units are
      line/sample, and are referenced to a corner of a particular band of
      a particular image.
\item Local ground coordinate system (abbreviated N/E).  Units are
      meters north and meters east of a \emph{reference point} on the ground
      that is typically, but not necessarily, contained in a given image being
      analyzed.
      Typically, there is one reference point per glacier, although
      a single reference point can be shared by many glaciers.
\item Global coordinate system (abbreviated L/L).  Units are geodetic
      east longitude and latitude (decimal degrees).
\end{enumerate}

The relationships between the coordinate systems are illustrated in
Figure \ref{fig:coords}.

At the image processing stage, the L/S coordinate system is used.  Once
glacier outlines, centerlines, and other point and vector data have been
produced from the analysis, these coordinates are transformed to either N/E
coordinates, a system of northings and eastings referenced to an easily
identifiable point in the image, or L/L coordinates.  The database has been
designed to store coordinates in either system.

In most cases, the location of objects within an image will be known
relative to each other with higher accuracy (~15 m or less) than the
knowledge of their absolute positions (on the order of 100 m from
spacecraft position and attitude (orientation) knowledge).
When glacier outlines are stored in N/E coordinates, relative and absolute
positions are inherently stored separately, with the N/E coordinates
carrying the relative information, and the L/L position of the reference
point carrying the absolute information.  When glacier outlines are stored
in L/L coordinates, uncertainties in relative and absolute positions will
be explicitly stored separately in the database.

The alignment between the image coordinate system (L/S) and the true L/L
grid is determined by the angle the orbit track makes with the meridian
lines (a known function) and the yaw of the spacecraft, typically known to
better than 10 arc-seconds (48 urad).  This amounts to 3 m over the
60 km width of an ASTER image.  The maximum error in calculated northings and
eastings should therefore be on the order of 3 m, after terrain correction
has been applied.  The angular relation between the N/E system and the L/L
grid varies slightly over a spacecraft scene and depends upon the
cartographic projection used.  Any well-defined cartographic projection may
be used; GLIMS uses the Universal Transverse Mercator (UTM) projection.

For the inclusion of older data from other sources, outlines in L/L
coordinates will also be storable in the database.  If a L/L system is
used, the ellipsoid is also recorded; if a N/E system is used, information
about the reference point is stored.