Project Personnel
Jason E. Box
Principal Investigator
Assistant Professor,
Department of Geography
Byrd Polar
The
Henry H. Brecher
Byrd Polar
The
Dean
Merchant
President
Topo Photo Corp.
Professor Emeritus, The
Fellow
of American Society for Photogrammetry & Remote Sensing
James Balog
Photographer
and Aesthetic Production Leader
Introduction and Motivation
Very recently, scientists have been
surprised by how quickly such a large ice mass as the Greenland ice sheet can
respond via its outlet glaciers to inter-annual climate variability. Regions of
enhanced flow on the ice sheet, i.e. ice streams, have demonstrated large
fluctuations in speed, thickness, and end position (Joughin et al. 2004;
Podlech et al. 2004; G. Hamilton), in apparent response to positive and
negative temperature trends. Even the ice sheet flow rates outside regions of
streaming exhibit a significant sensitivity to the duration and intensity of
melting (Zwally et al. 2002). While glacier discharge estimates have been made
for Greenland’s largest outlet glaciers (Rignot et al. 2000; 2001; 2004), these
have not captured what we now understand to be a system characterized by
significant seasonal and interannual flow variability (Joughin et al. 2004;
Luckman and Murray, 2005). The need for higher temporal resolution discharge
data is evident.
This proposal describes autonomous
ground-based mono- and stereo-imaging systems capable of monitoring surface
velocity across a glacier surface at high temporal resolution using standard
terrestrial photogrammetry techniques. Support for the maintenance of existing
systems already ‘watching’ three Greenland outlet glaciers is requested. The
addition of a second camera station at each sites is proposed to achieve
unambiguous velocity determination from using photogrammetry techniques. Two
additional glacier monitoring sites are proposed to represent regional ice
sheet behavior. Graduate student support is also sought under this project.
Major Objectives
1. measure seasonal and interannual
variability in Greenland outlet glacier surface velocities
2. use surface velocity cross section and
existing ice thickness data to estimate variations in glacier ice-volume
discharge near the grounding line
3. determine how much of the ice flow
speed variance can be explained by climate variability
4. evaluate inter-regional correlation
in glacier flow variability
This work is proposed to coincide with and commemorate
the International Polar Year 2007/08.
Existing Measurements
In 2004, investigator Box was awarded
pilot funding (Spatial variability in glacial ablation rates from digital
photography, NASA grant NAG05-GA66G, Oct
01 2004 - Sep 30 2006, $34 k) from the NASA ICESat program to develop and
deploy automatic camera systems in western Greenland. ‘IceCam’ systems were
installed May 2005 featuring different targets. One IceCam captured the
formation and drainage timing of a supraglacial melt lake (Figure 1) near the
JAR 1 continuous GPS and Greenland Climate Network automatic weather station
(Steffen et al. 1996).
10 AM,
Figure 1. IceCam image sequence from Lake JAR showing
different stages of development. Midnight images are shown, as they provide
high brightness contrast. Daytime images provide useful color contrast.
Monitoring Spatial Variations in Ablation
The Smart Stake 3 IceCam monitored
the surface surrounding an automatic weather station 16 May 2005 – 14 August
2005. The position of six fixed targets in the field of view of the camera have
been surveyed. Two of six targets are shown in the Figure 2 image subsets, i.e.
the Smart Stake and the pole to its right. All six targets will be used to
assess spatial variability of ablation rates at this site. Analysis of this
data is planned as an honors undergraduate project beginning Fall 2005.
Figure 2. IceCam image
sequence from Smart Stake 3 showing different stages of the melt season. One of
four metal poles, in addition to the Smart Stake mast are available to
determine spatial variations in surface ablation rate.
Sermeq Avannarleq (Dead Glacier)
An IceCam was activated 12 August
2005 to monitor variations in the Sermeq Avannarleq glacier, near the line of
flight between Ilulissat and Swiss Camp (Figure 3). The first four days of data
were collected from this site on 16 August with helicopter flight time provided
by The Greenpeace Project Thin Ice expedition, July-August 2005 (Figure 3). A
visit to this site is planned in 2005 as part of field work at Swiss Camp (K.
Steffen letter attached). This is one of three outlet glacier sites that host
single IceCam, i.e. mono systems. Mono imaging can provide information on flow
rates with precise camera orientation measurements and flow direction
assumption. However, uncertainties for mono systems are significant. At modest
additional cost, outlet glaciers can be monitored with two cameras, i.e. stereo
systems, which provide unambiguous position detection and higher likelihood of
continuous surveying, in the event that one of the cameras fails. Figure 4
features existing and proposed IceCam sites.
Figure 3. Image sequence showing
IceCam coverage of Sermeq Avannarleq and ice front changes over a 24-hour
period (2000 UTC 15 August (left) and 2000 UTC 16 August (right) 2005) show
calving of an ice bulge near the center of the ice front.
Figure 4. Greenland location map
showing existing and proposed IceCam sites.
A note on Isbraes, Ice streams, and
Glaciers
Isbrae-type outlet glaciers: have
very high driving stresses; flow through a deep bedrock channel significantly
deeper than the surrounding ice; have relatively steep surface slopes; and have
relatively high ice flux, as compared to ‘ice streams’ and especially as
compared to ‘glaciers’ (Truffer and Echelmeyer, 2002). Here the term ‘outlet
glacier’ is used to refer to all such ice flow systems. Imaging both isbrae and
‘glacier’-type’ outlet glaciers is proposed to test the hypothesis: only isbrae-type outlet glaciers are
sensitive to short term climate variations through meltwater interactions.
Jakobshavns Isbrae
After three months of measurements at
the JAR1 melt lake (Figure 1), the Ice Cam equipment was relocated to a site
along side the Jakobshavns Isbrae (Figure 5), at the easternmost land position
with a view of nearly the entire 7 km long ice front. The Jakobshavns Isbrae is
one of the most productive glaciers in the world and is perhaps the best
documented of all Greenland outlet glaciers (e.g. Weidick et al. 1990; 2003;
Weidick 1992; 1994; Abdalati et al. 2001; Thomas et al. 2003; Podlech and
Weidick 2005). Jakobshavns Isbrae has been observed by repeat satellite
measurements to be in a recent state of doubled flow speed (Joughin et al.
2004), thinning (Abdalati et al. 2001; Krabill et al. 2004), break-up and
retreat (e.g. Weidick et al. 2003), in particular since 2002. Understanding its
speed fluctuations (Luckman and Murray, 2005) and continued retreat is of
importance to ice sheet mass balance and global sea level assessments.
Installation of this camera in this ‘World Heritage’ site has been formally
approved by the Greenland Home Rule government.
Figure 5.
IceCam placed near the Jakobshavns Isbrae ice front 11 August 2005.
Figure 6 illustrates the proposed
camera setup at Jakobshavns Isbrae and Sermeq Avannarleq. NASA ice sounding
radar (ISR) and laser altimeter flights from 22 May 2005 are shown (J. Sonntag,
E. Frederick personal communication). An IceCam horizontal field of view (66
degrees) is shown for the Jakobshavns Isbrae situation. Details of stereo
calculations are provided in appendix A.
Figure 6. Existing and proposed IceCam sites at the
Jakobshavn and Sermeq Avannarleq outlet glaciers. 07 August 2005 MODIS image
resolution is 250 m, processed by J. Box.
Sermilik Brae
A new IceCam was installed 04 August
2005 near the Sermilik Isbrae glacier in extreme south Greenland (Figure 7).
Sermilik Isbrae has thinned 120 m since 1985 (Podlech et al. 2004). Work at
this site is in collaboration with Carl Bøggild of the Geologic Survey of
Denmark and Greenland (GEUS), and is complemented by an existing automatic
weather station network, letter of support attached. Collaboration is planned
to study this glacier response using the IceCam, weather station data and an
ice dynamics model. This is one of three existing sites where a mono system is
proposed to be upgraded to a stereoscopic system.
‘trim line’
Figure 7. IceCam at the ice front of Sermilik Isbrae,
installed 4 August 2005 (left). The ice front is visible on the left half of the
image. The glacier ‘trim line’ to the right of the glacier is visible as the
interface of darker vegetated land above bare land, suggesting 245 m (800 ft)
thinning since c. 1850, Half of the thinning appears to have occurred since
1985 (Podlech et al. 2004). MODIS image featuring site positions (right).
Proposed Measurement Sites
Rinks Isbrae
Rinks Isbrae has high fjord walls
advantageous for IceCam viewing of surface roughness features (Figure 8). MODIS
imagery show a relatively small front retreat for this glacier, as compared to
Jakobshavns Isbrae. A motivating question in the site selection is: will glaciers further north begin to
accelerate as southern glaciers already have? This glacier is in the
neighborhood of Uummannaq, serviced by commercial air flights and Air Greenland
Bell 212 helicopter charter. The Bell 212 range is 225 Nautical miles (416 km)
or 2.3 h, with sufficient fuel to divert to alternate airport.
Figure 8. Proposed IceCam sites at the Rinks Isbrae outlet
glacier, overlain on MODIS image.
Helheim Glacier
Representing east Greenland glaciers is important to provide a larger-scale perspective. Recent observations from G. Hamilton and L. Stearns suggest that the Helheim Glacier has begun to flow more rapidly since 2001. Figure 9 illustrates the proposed camera setup.
Figure 9. Proposed IceCam sites at the Helheim
glacier, south east Greenland, overlain on MODIS image.
Equipment
IceCams were developed in
consultation with equipment manufacturers and photographers. Advice from T.
Pfeffer (Department of Civil Engineering, University of Colorado, Boulder) was
most valuable. Commercial-grade cameras costing $400-$800 are triggered at 4-6
hour intervals using an intervalometer ($250). Data are stored on 2-4 Gb memory
cards ($200-400). Stations do not transmit data, although this capability is
considered, but has not been budgeted as it effectively doubles station cost. Camera
enclosures ($114) protect equipment from rain and snow. Chemical desiccant
inside enclosures prevent enclosure window fogging. Sealed gel cell batteries
($150) supply power that is recharged by a 20 W solar panel and charging
regulator ($450). The integrated system was tested in a Byrd Polar Research
Center cold-room March 2005. The cameras self-heat, even when quiescent,
keeping the imaging sensor working even at sub-freezing temperatures
surrounding the camera. A detailed list of equipment and costs is included with
the budget materials. Incorporating micro-computers is proposed to provide
system restart capability and to log health data, i.e. battery voltage,
temperature, humidity. The cost per
single camera system is $5069 with all capabilities proposed, including
automatic system restart.
Surface Velocity Measurements from Stereo Imaging
Terrestrial stereo photogrammetry is
a well-known technique (e.g. Moffit, 1967; Slama, 1980) and has been applied
extensively to studies of glacier motion for many years (e.g. Brecher and
Thompson, 1993). The determination of
positions of points in three dimensions in a local coordinate system is a
‘space intersection’ problem. That is, by
knowing the positions of two camera stations with respect to each other (at the
ends of a ‘baseline’) and the three rotations of the axes of each camera and
measuring the image coordinates of the terrain points on each of the two
images, the positions of these points result from the intersection of the rays
to the points. Appendix A gives details.
The positions and rotations of the
cameras can be derived by ‘orienting’ the images to (at least three) points
whose positions are known in the local coordinate system (‘control points’), by
measuring them directly or by a combination of the two. The latter technique is often employed for
various reasons, such as ease or difficulty of establishing control points,
ability to measure the baseline distance accurately, etc. (e.g. Brecher and
Thompson, 1993).
In this work, it is easy and straightforward
to measure the positions of the two camera stations with sufficient accuracy by
differential GPS (giving the added benefit of a global, rather than local,
coordinate system). It is also proposed
to level the cameras to the horizontal plane, which will allow easy definition
and re-definition (after maintenance visits) of two of the three required
camera axis rotation angles. Given that the camera orientations are to remain
fixed throughout a given measurement period, the camera azimuthal rotation
angles will be valid for all the subsequent photography. In the case of Sermeq
Avannarleq, and possibly some of the other sites, it is proposed to establish
at least one control point on (fixed) rock outcrops in order to allow the
determination of the third rotation angle at subsequent time periods, to
evaluate errors associated with camera motion, i.e. from wind vibrations or
other displacements, e.g. by snow creep.
In the case of Jakobshavns Isbrae, it
appears that fixed terrain can be included in the images, but at some detriment
to the stereo-coverage. Alternatively, it may be also be feasible to measure
the third rotation angle directly, i.e. measuring the viewing azimuth angles of
the cameras using a precision compass. However, establishing at least one
targeted control point on the (moving) ice appears to be the option that
provides the most precise control. Photos from the existing IceCam site and
some from the air suggest that a distinct feature on the ice front may be
selected as a control point and thus can be measured with a helicopter hovering
over and making a GPS measurement. However, the use of one or more artificial
targets (such as a black plastic banner) to unambiguously define this control
point is also considered, although the Greenland Home Rule Government may not
approve landing permission for such a site within the World Heritage site.
Several features across the glacier,
perpendicular to its flow, will be identified in image pairs for instants in
time. The positions of these features can be calculated in ‘terrain
coordinates’ from measurement in a ‘stereo model’. See Appendix A for details.
The same features will be identified for image pairs at some later time, with
time interval on the order of days, for the same time of day, to have surface
illumination roughly the same, so shadows contribute a small amount to feature
re-identification ambiguity. It may be possible and would be desirable to place
at least a few artificial targets on the ice in order to ensure unambiguous
identification at each measurement. The
series of feature-displacements, in the 3D ‘terrain’ coordinate system, will
then provide a velocity measurement. Depending on glacier velocity, daily to
weekly velocity measurements are expected to exceed one standard deviation of
the expected uncertainty.
Ice Volume Flux from Surface Velocities
Ice volume flux can be estimated
using surface velocity cross sections and ice thickness for floating ice just
below the grounding line (Rignot et al. 1997) because the ice vertical velocity
profile and basal melt rates seem to be negligible. The position of the
grounding line can be identified by synthetic aperture radar interferometry
(Rignot et al. 1997). The positions of the proposed IceCams will feature ice
near the grounding line. Ice sounding radar (ISR) (Gogineni et al. 2001)
bedrock depth measurements in the vicinity of proposed sites may be obtained
from NASA Wallops Facility investigators (W. Krabill, J. Sonntag, E. Frederick,
P. Kanagaratnam). Airborne Topographic Mapper (ATM) altimeter data provide
changes in surface elevation (Krabill et al. 2000; 2004) and thus help
constrain the ice thickness. Ice Cloud and Land Elevation Satellite (ICESat)
and CryoSat altimeter data will be of use in the determination of ice thickness
at times not available from ATM surveys. Given ice thickness information from
ISR and altimetry, it is feasible to derive seasonal variations in glacier
volume flux, given IceCam-derived surface velocity measurements, ice thickness
data, and targeting the part of the glacier that is at the grounding line
position. For situations where the surveyed part of the glacier is above the
grounding line, an estimate of the vertical velocity profile would be needed.
To address this potential challenge, the incorporation of a glacier flow model
would be planned. Podlech (2004) details such a model and its use is planned in
collaboration with Carl Bøggild of GEUS, letter attached.
Correlation with Climate Data
Box et al. (2004; 2005) have applied a
mesoscale atmospheric data assimilation model to compute spatial and temporal
patterns of surface mass balance, including rates of meltwater production at
sub-daily time-scales. The use of Automatic Weather Station (AWS) data from the
Greenland Climate Network (GC-Net) (Steffen et al. 1996) has proven vital in
this regard, for model error assessment and calibration. Melt water production
information is currently available at 24 km horizontal resolution, with a 12 km
product planned for 2006. These data will be used to compute the time-variation
of the available meltwater volume for drainage basins contributing to
individual outlet glacier flow rates. The flow rates derived from IceCam data
will be compared with meltwater flux information to test the hypothesis: outlet glacier flow rates correlate with
meltwater flux. Other parameters, such as daily to seasonal temperature,
albedo, and precipitation anomalies, will also be compared with the outlet
glacier flow rates to determine how much of the ice dynamics variance can be
explained by short-term climate variability.
Lens Calibration
Although
the Nikon cameras in use are not sold as ‘metric’ cameras, the careful
determination of interior orientation parameters (focal length, lens
distortion, principal point offset) for each camera, through calibration
procedures, illustrated in Figure 10, allows retrieved metric imagery to be of
more than sufficient accuracy for the purposes of this study.
Figure 10. Dean Merchant
(left) and Henry Brecher (right) maneuver inexpensive Nikon digital camera into
a variety of positions to obtain lens calibration data at a target field at
Topophoto Inc., London, OH, USA.
Error Considerations
We calculate conservative
uncertainties of ±10 to ±40 m in position determinations for features moving
between 60±40 degrees of the line of sight at 2 km to 8 km distances,
respectively. Given daily glacier motions of 10-70 m, 4-day velocity
determinations significantly larger than the uncertainty. Uncertainty calculations that include sensor
dimensions, uncorrectable lens distortions, and a small amount of vibration,
suggest that the Nikon 5400 camera yields measurements at 2 km to 4 km
positions good to ±10 and ±20 m, respectively. However, to resolve a feature 4
pixel in diameter, uncertainties are expected to be ±40 m. In cases when uncertainties are largest, the
time interval for displacement calculations must be increased. Even with 2-week
time resolution, seasonal variability in ice flow can be determined.
Maintaining camera stability is
important, to minimize the need to co-register images using fixed image
features on land (Harrison et al. 1992). Camera stability, as such, is a
problem of the current design that will be minimized by using a more robust
camera enclosure mounting flange. In almost all cases, unmoving features in
image foreground and background help identify error from camera orientation
changes.
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Additional Materials
Appendix: Digital Stereo Photogrammetry Basics
Figure A1 shows proposed IceCam
situation in a two-dimensional horizontal plane. Camera orientation angles a1 and a2, focal
lengths (f1 and f2) are key parameters to set up a stereo
model. What remains are angle measurements derived from image pixels
coordinates relative to the camera lens axis. Pixel units are converted to
distance units given the camera focal length and ‘format size’ of the camera
detector. With the focal lengths of cameras known from the calibration
procedure (featured in Figure 10) and measurements of the image coordinates of
a feature at positions ci from the center of each image, the angles di are calculated from which the angles fi to the feature of rays di are determined.
6 to 10 features across the glacier surface are expected to be tracked in image
pairs with time intervals of 1-7 days. Interaction of the rays yields the
coordinates xi and yi.
Figure A1. Schematic representation
of proposed terrestrial photogrammetry.