MODIS image composite of the Ross Ice Shelf

A composite MODIS image map of the Ross Ice Shelf at 250 m pixel spacing was
produced as part of an NSF-OPP project studying the history of flow variations
from the ice streams feeding the Ross Ice Shelf, Antarctica. This material is
based upon work supported by the National Science Foundation under Grant No.
0242465.

The composite MODIS image is a multiple-image composite from 250 m pixel size
visible-band (band 1) images from the MODIS sensor on NASA's Terra platform.
The MODIS imagery was obtained from the NASA Goddard Space Flight Center Earth
Sciences Distributed Active Archive Center
("http://daac.gsfc.nasa.gov/DAAC_DOCS/gdaac_home.html"). At any location, the
composite includes a few to ten images spanning a few-week period in November
and December 2001, using multiple local solar times from 1000 GMT to 2200 GMT.
This range of image acquisition times provides a range of sun azimuths in the
images used. The images were acquired from the DAAC as individual MODIS
granules, which were individually remapped into a polar stereographic grid at a
pixel size of 250 m, hand masked for clouds using polygonal areas (the many
straight line segments cutting through the composite image are the edges of the
polygons used in this masking), and high-pass filtered with a 25 km radius to
remove the effects of larger scale slope variations and allow contrast
enhancement for smaller spatial scale slope variations (subtle topographic
ridges, with a short dimension on the order of a kilometer, are visible).

Simple summing of the resultant images produced a composite with a wide (180
degree) range of solar azimuths that allows curving features and features of
any orientation to be well represented. The range of solar azimuths
(illumination directions) requires the viewer to exercise some caution in
interpreting the slope directions of the features that are seen in the
composite image. A large fraction of the included images are illuminated from
the upper left corner of the image, resulting in slopes that are downward
toward the bottom and/or right edge of the image to be dark ? one good example
of this is the dark line produced by the slope down on to the ice shelf at the
grounding line of Kamb Ice Stream.

The improved resolution, geolocation, and image availability of MODIS relative
to the older AVHRR data allow us to show the full extent of features that were
only partially visible before. The possible confusion introduced by multiple
solar azimuths used in this process is made up for by the greatly enhanced
ability to track features as coherent structures for hundreds of kilometers
across the ice shelf. Note that the effect of high-pass filtering is to remove
the shading produced by slopes across broad features such as Siple Dome and
Roosevelt Island (and even Earth curvature); this means that the shading
variations in the composite image are dominantly controlled by local slope
variations that would be overwhelmed in a simple shaded-relief map of the area.

The image is in polar stereographic projection. The projection parameters are:
pixel size 250 m

The USGS GCTP coordinate transformation package header for the image is:
GEOMETRIC DATA MAP PROJECTION =PS ELLIPSOID =WGS84 DATUM =WGS84
USGS PROJECTION PARAMETERS = 6378137.000000000000000 6356752.314000000200000
0.000000000000000 0.000000000000000 0.000000000000000
-71000000.000000000000000 0.000000000000000 0.000000000000000
0.000000000000000 0.000000000000000 0.000000000000000
0.000000000000000 0.000000000000000 0.000000000000000
0.000000000000000 USGS MAP ZONE =0