Jason E. Box
Assistant Professor, Department of Geography
Henry H. Brecher
President Topo Photo Corp.
Professor Emeritus, The
Fellow of American Society for Photogrammetry & Remote Sensing
Very recently, scientists have been
surprised by how quickly such a large ice mass as the
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
1. measure seasonal and interannual
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.
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).
Figure 1. IceCam image sequence from
The Smart Stake 3 IceCam monitored
the surface surrounding an automatic weather station
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.
An IceCam was activated
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.
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.
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
IceCam placed near the Jakobshavns Isbrae ice front
Figure 6 illustrates the proposed
camera setup at Jakobshavns Isbrae and Sermeq Avannarleq. NASA ice sounding
radar (ISR) and laser altimeter flights from
Figure 6. Existing and proposed IceCam sites at the
Jakobshavn and Sermeq Avannarleq outlet glaciers.
A new IceCam was installed
Figure 7. IceCam at the ice front of Sermilik 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
Figure 8. Proposed IceCam sites at the Rinks Isbrae outlet glacier, overlain on MODIS image.
Figure 9. Proposed IceCam sites at the Helheim
glacier, south east
IceCams were developed in
consultation with equipment manufacturers and photographers. Advice from T.
Pfeffer (Department of Civil Engineering,
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 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 (
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.
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
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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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.