242
A. Rango, A.E. Walker and B.E. Goodison
general public. The NOHRSC web site (http://www.nohrsc.nws.gov) provides a
description of the NWS airborne ganuna survey program, locations of flight lines, and
access to data products. Historical data are also available from the u.s. National
Snow and Ice Data Center in Boulder, Colorado. Low-flying aircraft are generally the
platforms used to acquire airborne ganuna measurements. The NWS airborne gamma
survey program uses Aero Commander and Turbo Commander twin engine aircraft
which fly at 150m above the ground during the ganuna surveys (Carroll, 1990), which
prevents airborne gamma measurement in rugged topographic areas.
11.2.2 Visible Imagery
Snow and Ice Cover Extent Methodology. Snow cover can be detected and monitored with a variety of remote sensing devices. The greatest application has been
found in the visible and near infrared region of the electromagnetic spectrum. The red
band (0.6-0.7Ilm) of the multispectral scanner subsystem (MSS) on the Landsat
satellite was used extensively for snow cover mapping because of its strong contrast
with snow-free areas. Originally, snow extent mapping was performed manually using
photointerpretive devices and MSS photographs (Bowley et aI., 1981). More recently,
digital mapping of the snow cover has been the preferred approach (Baumgartner et
aI., 1986; 1987; Dozier and Marks, 1987; Baumgartner and Rango, 1995),
The greatest problem hindering Landsat (and SPOT) snow mapping in the past was
a poor observational frequency, Depending on the Landsat satellite being used, each
study area or drainage basin was only revisited every 16-18 days. In areas with
minimal cloud cover during the snowmelt season, this was a sufficient frequency of
observation. In most mountain snow areas, however, this observational frequency is
inadequate because cloud cover will often hide the underlying snow from the satellite
sensors.
A spectral channel (1.55-1.75/lffi) currently only available regularly on the TM
instrument can be used to assist in mapping snow cover when clouds partially cover
a drainage basin. In this band, clouds are usually more reflective than snow (Dozier,
1989). As a result, automatic discrimination between snow and clouds is possible.
Although useful, this capability does not overcome the problem of a complete cloud
cover or an inadequate frequency of observation.
When a basin is partially snow covered, a method has been developed to estimate
the snow cover in the cloud-obscured parts of the basin (Lichtenegger et aI., 1981;
Baumgartner et aI., 1986; Ehrler et aI., 1997). The method uses digital topographic
data and assumes that pixels with equal elevation, aspect and slope have the same
relative snow coverage over all the basin. With this extrapolation method, information
from the cloud-free portion of the basin can be used to estimate the snow cover in the
cloud-covered parts of the basin.
As a result of the Landsat (and SPOT) frequency of observation problem, many
users have turned to the NOAA polar orbiting satellite with the A VHRR, which has
a resolution of about 1 km in the 0.58-0.68/lffi red band. The frequency of coverage
is twice every 24 hours (one daytime pass and one nighttime pass). The major problem with the NOAA-A VHRR data is that the resolution of 1 km may be insufficient
A. Rango, A.E. Walker and B.E. Goodison
general public. The NOHRSC web site (http://www.nohrsc.nws.gov) provides a
description of the NWS airborne ganuna survey program, locations of flight lines, and
access to data products. Historical data are also available from the u.s. National
Snow and Ice Data Center in Boulder, Colorado. Low-flying aircraft are generally the
platforms used to acquire airborne ganuna measurements. The NWS airborne gamma
survey program uses Aero Commander and Turbo Commander twin engine aircraft
which fly at 150m above the ground during the ganuna surveys (Carroll, 1990), which
prevents airborne gamma measurement in rugged topographic areas.
11.2.2 Visible Imagery
Snow and Ice Cover Extent Methodology. Snow cover can be detected and monitored with a variety of remote sensing devices. The greatest application has been
found in the visible and near infrared region of the electromagnetic spectrum. The red
band (0.6-0.7Ilm) of the multispectral scanner subsystem (MSS) on the Landsat
satellite was used extensively for snow cover mapping because of its strong contrast
with snow-free areas. Originally, snow extent mapping was performed manually using
photointerpretive devices and MSS photographs (Bowley et aI., 1981). More recently,
digital mapping of the snow cover has been the preferred approach (Baumgartner et
aI., 1986; 1987; Dozier and Marks, 1987; Baumgartner and Rango, 1995),
The greatest problem hindering Landsat (and SPOT) snow mapping in the past was
a poor observational frequency, Depending on the Landsat satellite being used, each
study area or drainage basin was only revisited every 16-18 days. In areas with
minimal cloud cover during the snowmelt season, this was a sufficient frequency of
observation. In most mountain snow areas, however, this observational frequency is
inadequate because cloud cover will often hide the underlying snow from the satellite
sensors.
A spectral channel (1.55-1.75/lffi) currently only available regularly on the TM
instrument can be used to assist in mapping snow cover when clouds partially cover
a drainage basin. In this band, clouds are usually more reflective than snow (Dozier,
1989). As a result, automatic discrimination between snow and clouds is possible.
Although useful, this capability does not overcome the problem of a complete cloud
cover or an inadequate frequency of observation.
When a basin is partially snow covered, a method has been developed to estimate
the snow cover in the cloud-obscured parts of the basin (Lichtenegger et aI., 1981;
Baumgartner et aI., 1986; Ehrler et aI., 1997). The method uses digital topographic
data and assumes that pixels with equal elevation, aspect and slope have the same
relative snow coverage over all the basin. With this extrapolation method, information
from the cloud-free portion of the basin can be used to estimate the snow cover in the
cloud-covered parts of the basin.
As a result of the Landsat (and SPOT) frequency of observation problem, many
users have turned to the NOAA polar orbiting satellite with the A VHRR, which has
a resolution of about 1 km in the 0.58-0.68/lffi red band. The frequency of coverage
is twice every 24 hours (one daytime pass and one nighttime pass). The major problem with the NOAA-A VHRR data is that the resolution of 1 km may be insufficient