Jakobshavn isbrae using high resolution digital camera Rignot, Friedt, Moreau Data acquisition and processing Greenland
About the tide oscillations and icebergs motion recorded at Jakobshavn isbrae during summer 2007 using high resolution digital camera, Icefjord, West Greenland.
French alps Conclusion
Discussion about acceleration of glacier flow in Greenland, lubrication at the ice-rock interface, vertical circulation of melt water (moulins) and lubrication recorded in situ under Argentire glacier (Mt-blanc).
´ Rignot, J.-M Friedt, L. Moreau E.
5 mars 2008
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Jakobshavn isbrae using high resolution digital camera
Data acquisition
Rignot, Friedt, Moreau Data acquisition and processing Greenland French alps
Automated digital image capture using a custom circuit for triggering a commercial digital camera
Conclusion
• low power consumption of control circuitry (< 200 µA) for an
autonomy of several years • camera consumption limits the global autonomy to theoretically '1000 frames • date & time stored in EXIF header for quantitative processing
(problem of powering the camera’s internal clock) Two testbeds : a 1 month long continuous sequence shot in Greenland, a 6 month long webcam archive from Chamonix, France.
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Jakobshavn isbrae using high resolution digital camera
Selected analysis area
Rignot, Friedt, Moreau Data acquisition and processing Greenland French alps Conclusion
blue
red
green
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black 3 / 15
Jakobshavn isbrae using high resolution digital camera Rignot, Friedt, Moreau Data acquisition and processing Greenland French alps Conclusion
Measurement horizon (theory) • Fixed window : correlation technique only works as long as reference
and anlyzed frame look similar. • Short term difference of successive frames provides poor accuracy
(motion > 1 pixel between two frames) • Shift reference window and connect drift curves
Reference frame (fixed)
+ illumination artifact : apparent “Y motion” with 24 hour period due to sun motion 4 / 15
Jakobshavn isbrae using high resolution digital camera Rignot, Friedt, Moreau Data acquisition and processing Greenland French alps Conclusion
Measurement horizon (theory) • Fixed window : correlation technique only works as long as reference
and anlyzed frame look similar. • Short term difference of successive frames provides poor accuracy
(motion > 1 pixel between two frames) • Shift reference window and connect drift curves
Frame 1
Reference frame (fixed)
+ illumination artifact : apparent “Y motion” with 24 hour period due to sun motion 5 / 15
Jakobshavn isbrae using high resolution digital camera Rignot, Friedt, Moreau Data acquisition and processing Greenland French alps Conclusion
Measurement horizon (theory) • Fixed window : correlation technique only works as long as reference
and anlyzed frame look similar. • Short term difference of successive frames provides poor accuracy
(motion > 1 pixel between two frames) • Shift reference window and connect drift curves
Frame 1 Frame 2
Reference frame (fixed)
+ illumination artifact : apparent “Y motion” with 24 hour period due to sun motion 6 / 15
Jakobshavn isbrae using high resolution digital camera Rignot, Friedt, Moreau Data acquisition and processing Greenland French alps Conclusion
Measurement horizon (theory) • Fixed window : correlation technique only works as long as reference
and anlyzed frame look similar. • Short term difference of successive frames provides poor accuracy (motion > 1 pixel between two frames) • Shift reference window and connect drift curves Frame 1 Frame 2
Frame 3
Reference frame (fixed)
+ illumination artifact : apparent “Y motion” with 24 hour period due to sun motion 7 / 15
Jakobshavn isbrae using high resolution digital camera Rignot, Friedt, Moreau Data acquisition and processing Greenland French alps Conclusion
Motion detection (application) • Basic but robust technique : intercorrelation looks for translation
vector for best match of a reference picture and a measurement picture (xcorr2() with Matlab) • We will use an Eulerian descritpion of fluid motion (fixed window, monitor the mass entering and leaving this frame) image(t)
image(t+24)
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Jakobshavn isbrae using high resolution digital camera
0.0215 pixel/min
50 0 −50
French alps Conclusion
reference frame 9
displacement (pixel)
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0.0323 pixel/min
X
0.0193 pixel/min
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−100
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displacement (pixel)
displacement (pixel)
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0.0321 pixel/min
displacement (pixel)
Greenland
0.0326 pixel/min
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reference frame 70
50 0 0.023 pixel/min
−50 −100
displacement (pixel)
Data acquisition and processing
Measurement horizon (results) displacement (pixel)
Rignot, Friedt, Moreau
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Jakobshavn isbrae using high resolution digital camera Rignot, Friedt, Moreau
Influence of tide Visually, obvious influence of tide on “vertical” motion of ice 40
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Data acquisition and processing
displacement (pixel)
high tide
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0.0055 pixels/min
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low tide 0.0033 pixel/min
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−10 reference frame 230
−30 1.4
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x 10
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1.6 27/07 frame 70
Y motion X motion
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1.4 13/08 frame 260
21/07 frame 10
1.2
22/08 frame 330
high tide
reference frame (330)
10 displacement (pixel)
Conclusion
displacement (pixel)
French alps
Y motion X motion
30
reference frame (130)
20
tide amplitude (m)
Greenland
Y motion X motion
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1
0.8
5
Y motion X motion
0 −5
0.6 01/08 frame 130
−10 low tide
0.4 29/07 frame 100
0.2 3.4
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−15
3.7 3.8 3.9 4 date (minutes since 01/01/2007)
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0.0024 pixel/min
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x 10
4.8 time (min)
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x 10
• Two tide amplitude maxima over 1 month record : visible signal
synchronous with predicted tide max/min. • No obvious signal synchronous with tide during amplitude minimum
(bot. right) 10 / 15
Jakobshavn isbrae using high resolution digital camera Rignot, Friedt, Moreau Data acquisition and processing
Influence of wind 1
No visible influence of wind : the local influence on a given iceberg is not tracked : would require a Lagrangian description of fluid motion (object tracking)
Greenland French alps
40
magenta
tide amplitude x50, +30
wind
0
Conclusion
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red (ref.) yellow
0 displacement (pixel)
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−500 blue
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green yellow
−40 −60 −80
high tide x10, −100
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−700 −800
magenta
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blue
1 month
displacement (pixels)
−200
low tide x10, −100
−120 0.5
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x 10
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x 10
Long term “vertical motions” appears related to tide2 amplitude (right) 1 Hourly METAR data collected from http://english.wunderground.com/history/airport/BGJN/ 2 Web interface to xtide at http://tbone.biol.sc.edu/tide/sites othernorth.html
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Jakobshavn isbrae using high resolution digital camera Rignot, Friedt, Moreau Data acquisition and processing
Influence of picture quality We have demonstrated the use of high resolution (10 Mpixels), low compression images on a fast moving object. What about poor quality images of a slow glacier ?
Greenland X motion (pixel)
French alps red
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green magenta
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Conclusion
20 0
20 0 40 20 0
time (frame number)
Requires 1 interpolation of the original image to smooth JPEG compression boundaries and improve resolution 2 histogram equalization 12 / 15
Jakobshavn isbrae using high resolution digital camera
Influence of picture quality
Rignot, Friedt, Moreau Data acquisition and processing
Strong JPEG compression ⇒ artifacts
Greenland French alps Conclusion
Matlab’s imresize() bilinear interpolation (weighted average of pixels in the nearest 2-by-2 neighborhood) 13 / 15
Jakobshavn isbrae using high resolution digital camera
Results and improvements
Rignot, Friedt, Moreau Data acquisition and processing
• Some frames3 with poor weather induce high noise level : would
require pre-processing to avoid noise
Greenland French alps
• Strong effect of shadow → histogram equalization
Conclusion
⇒ reduce influence of noise when connecting successive windows by using linear fit of glacier motion 3 http://www.compagniedumontblanc.fr/webcam/CMM1MERDEGLACE.jpg 14 / 15
Jakobshavn isbrae using high resolution digital camera
Results and improvements
Rignot, Friedt, Moreau Data acquisition and processing Greenland French alps Conclusion
• The motion is an average over a given part of the picture :
the larger the picture, the longer the horizon, but the worse the average • Connection of analysis periods is not always accurate • Conversion from pixel to meters requires terrain model + camera
characteristics • Validation using calibrated instruments (GPS) Yet, digital image processing provides a means of continuous monitoring of ice flow in area where instuments cannot be positioned (icebergs, strong slopes)
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