About the tide oscillations and icebergs motion recorded at ...jmfriedt.free.fr/shf2008.pdfJakobshavn isbrae using high resolution digital camera Rignot, Friedt, Moreau Data acquisition

Jakobshavnisbrae using highresolution digital

camera

Rignot, Friedt,Moreau

Data acquisitionand processing

Greenland

French alps

Conclusion

About the tide oscillations and icebergs motion recorded at Jakobshavnisbrae during summer 2007 using high resolution digital camera, Icefjord,

West Greenland.

Discussion about acceleration of glacier flow in Greenland, lubrication atthe ice-rock interface, vertical circulation of melt water (moulins) and

lubrication recorded in situ under Argentire glacier (Mt-blanc).

É. Rignot, J.-M Friedt, L. Moreau

5 mars 2008

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Greenland

French alps

Conclusion

Data acquisition

Automated digital image capture using a custom circuit for triggering acommercial digital camera

• low power consumption of control circuitry (< 200 µA) for anautonomy 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, a6 month long webcam archive from Chamonix, France.

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Greenland

French alps

Conclusion

Selected analysis area

blue red green magenta black

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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 dueto sun motion

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Greenland

French alps

Conclusion






Frame 1


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Greenland

French alps

Conclusion





Frame 2


Frame 1


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Greenland

French alps

Conclusion

Measurement horizon (theory)

• Fixed window : correlation technique only works as long as referenceand anlyzed frame look similar.



Frame 2


Frame 1Frame 3


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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 measurementpicture (xcorr2() with Matlab)

• We will use an Eulerian descritpion of fluid motion (fixed window,monitor the mass entering and leaving this frame)

image(t)

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image(t+24)

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image(t+24)

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xcorr2

100 200 300 400 475

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295

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Greenland

French alps

Conclusion

Measurement horizon (results)

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t (pi

xel)

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disp

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xel)

0.0326 pixel/min

0.0215 pixel/min

24 h

referenceframe 9

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xel)

0.0323 pixel/min

0.0193 pixel/min

referenceframe 40

24 h

X

Y 24 h

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referenceframe 70

0.0321 pixel/min

0.023pixel/min

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Greenland

French alps

Conclusion

Influence of tideVisually, obvious influence of tide on “vertical” motion of ice

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referenceframe (130)

low tide

high tide

0.0033pixel/min

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referenceframe 230

0.0055pixels/min

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x 105

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1

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date (minutes since 01/01/2007)

tide

ampl

itude

(m

)

22/08frame 330

29/07frame 100

01/08frame 130

21/07frame 10

27/07frame 70

13/08frame 260

4.2 4.4 4.6 4.8 5 5.2

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referenceframe (330)

0.0024 pixel/min

high tide

low tide

• Two tide amplitude maxima over 1 month record : visible signalsynchronous with predicted tide max/min.

• No obvious signal synchronous with tide during amplitude minimum(bot. right)

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French alps

Conclusion

Influence of windNo visible influence of wind1 : the local influence on a given iceberg isnot tracked : would require a Lagrangian description of fluid motion(object tracking)

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wind

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onth

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spla

cem

ent (

pixe

l)

tide amplitudex50, +30

high tide x10, −100

low tide x10, −100

blue

red (ref.)

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yellow

magenta

Long term “vertical motions” appears related to tide2 amplitude (right)

1Hourly METAR data collected fromhttp://english.wunderground.com/history/airport/BGJN/

2Web interface to xtide athttp://tbone.biol.sc.edu/tide/sites othernorth.html 11 / 15

http://english.wunderground.com/history/airport/BGJN/http://tbone.biol.sc.edu/tide/sites_othernorth.html


camera



Greenland

French alps

Conclusion

Influence of picture qualityWe have demonstrated the use of high resolution (10 Mpixels), lowcompression images on a fast moving object.What about poor quality images of a slow glacier ?

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n

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enta

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k

Requires

1 interpolation of the original image to smooth JPEG compressionboundaries and improve resolution

2 histogram equalization12 / 15


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Greenland

French alps

Conclusion

Influence of picture quality

Strong JPEG compression ⇒ artifacts

Matlab’s imresize() bilinear interpolation (weighted average of pixelsin the nearest 2-by-2 neighborhood)

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Greenland

French alps

Conclusion

Results and improvements

• Some frames3 with poor weather induce high noise level : wouldrequire pre-processing to avoid noise

• Strong effect of shadow → histogram equalization

⇒ reduce influence of noise when connecting successive windows byusing linear fit of glacier motion

3http://www.compagniedumontblanc.fr/webcam/CMM1MERDEGLACE.jpg14 / 15

http://www.compagniedumontblanc.fr/webcam/CMM1MERDEGLACE.jpg


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Greenland

French alps

Conclusion

Results and improvements

• The motion is an average over a given part of the picture :the larger the picture, the longer the horizon, but the worse theaverage

• 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 monitoringof ice flow in area where instuments cannot be positioned (icebergs,strong slopes)

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Data acquisition and processingGreenlandFrench alpsConclusion

Documents

About the tide oscillations and icebergs motion recorded at ...jmfriedt.free.fr/shf2008.pdfJakobshavn isbrae using high resolution digital camera Rignot, Friedt, Moreau Data acquisition