differential GPS measurements.

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Austre Lov´en and its bedrock topography using

Ground Penetrating

Radar and differential GPS

measurements.

Friedt &al.

Introductions DEM source comparison Error analysis Comparison with classical methods Conclusion

Evaluating the Austre Lov´ en and its bedrock topography using Ground Penetrating Radar and

differential GPS measurements.

J.-M. Friedt¹, D. Laffly², A. Saintenoy³, E. Bernard⁴, M. Griselin⁴, C. Marlin³

1Univ. de Franche-Comt´e, FEMTO-ST, UMR 6174 CNRS, Besan¸con, France

2Universit´e de Toulouse, GEODE, UMR 5602 CNRS. Toulouse, France

3Universit´e de Paris-sud Orsay, IDES, UMR 8148 CNRS, Orsay, France

4Université de Franche-Comté, THÉMA, UMR 6049 CNRS, Besan¸con, France

jmfriedt@femto-st.fr

slides available athttp://jmfriedt.free.fr

September 21, 2010

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Intoduction

Objective ¹: estimate the mass balance and area of a polar glacier exhibiting typical average thickness variations of a few tens of centimeters/year

1 classically, a few stakes measurements/year at best, with coarse spatial accuracy

2 complement with fine spatial accuracy using historical Digital Elevation Models (DEM) ?

⇒what is the resolution of DEMs when used for estimating mass balance ?

“... an absolute requirement of using glacier maps is that they are of a very high accuracy. Otherwise it is a doubtful task to try to determine volume changes over periods of 10-12 years.”,

N. Haakensen, Annals of Glaciology8(1986)

1sequel to the presentation given by Griselin& alat 10th International Circumpolar Remote Sensing Symposium, Whitehorse, Canada (2008)

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Relationship between area and volume

1 Air temperature might appear as the most obvious (most commonly available) climate index.

2 The delay between area and volume changes has been estimated to 31 years for the neighbour Middre Lov´enbreen (S. Hansen, Master Thesis, 1999)

• How to relate these measurements with actual glacier mass balance ?

• compare historical temperature records with historical DEMs (1965-2010)

• All discussion will focus on thickness (altitude) measurements rather than water equivalent thicknesses.

• Application to Austre Lovenbreen, 79^oN, Svalbard (selected for its hydrological properties)

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Available data: DEMs from 1965 to 2010

DEMs are generated from awide variety of sources

1 1/25000map traced by the German scientists between 1962 and 1965 in the framework of the “Deutschen Spitzbergen-Expeditionen 1962-1965 des Nationalkomitees f¨ur Geod¨asie und Geophysik der DDR”.

2 1995 from the Norsk Polarinstitutt was derived from six stereo-overlappingaerial photographstaken in August 1995 (Rippin et al., 2003).

3 airborne LiDARdata from the Scott Polar Institute Cambridge (Rees, 2003, 2005)² working on the Middre Lov´enbreen (15 cm vertical accuracy).

4 2007 SPIRIT (CNES) dataset was obtained from stereography of couples of September 2007SPOT satelliteHRS images (high stereoscopic resolution)

5 skidoo tracked(D)GPS in 2007 and 2010 over glacier surface only

2

•Arnold, N.S., Rees, W.G., Devereaux, B.J. and Amable, G. 2006. Evaluating the potential of high-resolution airborne LiDAR in glaciology. International Journal of Remote Sensing27(5-6), 1233-1251.

•Rees, W.G. and Arnold, N.S. (2007) Mass balance and dynamics of a valley glacier measured by high-resolution LiDAR. Polar Record

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Introductions DEM source comparison

Error analysis Comparison with classical methods Conclusion

DEM source comparison

X-Y: 5 m/pixel interpolation of all DEMs

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DEM source comparison

Comparison of a few static refence points on historical DEMs:

DEM moraine1 moraine2 moraine3 moraine4 bird cliff

map 1964 67 111 35 111 21

LiDAR 2005 61 121 30 87 24

NPI 1995 62 116 33 86 21

SPIRIT 2007 54 121 34 129 29

max-min 13 10 5 43 8

→Offset is not constant

We complement these data with skidoo-tracked GPS units (C/A in 2007, phase corrected in 2010 – Trimble Geo XH)

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DEM source comparison (2)

Within a year: C/A GPS v.s SPIRIT (stereography of satellite images)

• >15 m differences in the flattest part of the glacier, consistent with

C/A GPS elevation errors

• + missing parts due to shadows in the SPIRIT DEM (steep slopes)

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DEM source comparison (3)

2010 DGPS - 2005 LiDAR (partial coverage)

• Most consistent dataset: elevation errors<1 m, remaining error due to environment (snow) or measurement procedure (antenna height)

• We are interested in elevationdifferences: offset might be removed using a constant correction over the whole image based on a few reference points

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DEM source comparison

2007-2010 (D)GPS

1 2010 dataset was RINEX-corrected using the reference station in Ny

˚Alesund (6 km away)

2 Most significant errors on the borders of the glacier, where the slopes are steepest (cannot be reached by skidoo)

3 depending on the assumption in the circuses and ice-rock junction, volume loss between 2007 and 2010 would be between 8.5 and 7.2 Mm³(over a 4.8 km²basin: 1.8 to 1.5 m on average)

4 significant altitude loss (-5 to -7 m/3 years) in the glacier front

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DEM source comparison

2007-2010 (D)GPS

1 2010 dataset was RINEX-corrected using the reference station in Ny

˚Alesund (6 km away)

2 Most significant errors on the borders of the glacier, where the slopes are steepest (cannot be reached by skidoo)

3 depending on the assumption in the circuses and ice-rock junction, volume loss between 2007 and 2010 would be between 8.5 and 7.2 Mm³(over a 4.8 km²basin: 1.8 to 1.5 m on average)

4 significant altitude loss (-5 to -7 m/3 years) in the glacier front

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DGPS 2010 analysis

Search all neighbours within 5 m of each point: what is the elevation difference ?

78.86 78.87 78.88 78.89 78.9 78.91

12.08 12.1 12.12 12.14 12.16 12.18 12.2 12.22 12.24

latitude (deg, WGS84)

longitude (deg, WGS84)

0 1 2 3 4 5

0 2000 4000 6000 8000 10000 12000 14000

distance (m)

point index (integer) -1

0 1 2 3 4 5

0 2000 4000 6000 8000 10000 12000 14000

time interval (days)

point index (integer) -2

-1.5 -1 -0.5 0 0.5 1 1.5 2

0 2000 4000 6000 8000 10000 12000 14000

altitude difference (m)

point index (integer)

Elevation difference standard deviation: ±50 cm describes>66% data

⇒if DGPS altitude error is no longer significant, what other sources of error ?

• bias (altitude of receiving antenna)

• snow height/weight of skidoo

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DGPS 2010 analysis

Search all neighbours within 5 m of each point: what is the elevation difference ?

78.86 78.87 78.88 78.89 78.9 78.91

12.08 12.1 12.12 12.14 12.16 12.18 12.2 12.22 12.24

latitude (deg, WGS84)

longitude (deg, WGS84) 0

2 4 6 8 10 12 14

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

probability (\%)

altitude difference (m) (1:2000) ; (6500:10000) ; (15000:end)

Elevation difference standard deviation: ±50 cm describes>66% data

⇒if DGPS altitude error is no longer significant, what other sources of error ?

• bias (altitude of receiving antenna)

• snow height/weight of skidoo

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Snow cover influence

Snow cover might be significant, yielding a bias on the height measurements

Interpolated snow (drill) thickness from 2008 to 2010

Hardly an issue for airborne measurements (july-august), but april is most favorable for skidoo tracked GPS

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2010-1964

• Focus on the dataset extremes, where measurement errors might be small with respect to ablation

• most significant error still visible on the ice-rock boundaries

• in the flattest part of the glacier, ablation maximum would be -100 m : is this result reasonable ?

-200 -100 0 100 200 300 400 2010-1964 500

0 200 400 600 800 1000 1200

0 100 200 300 400 500 600 700 800

0 100 200 300 400 500 600 700 1964 800

0 200 400 600 800 1000 1200

0 100 200 300 400 500 600 700 800

0 100 200 300 400 500 600 700 2010 800

0 200 400 600 800 1000 1200

0 100 200 300 400 500 600 700 800 -100

-80 -60 -40 -20 zoom 200:676, 240:450 0

200 250 300 350 400 450 500 550 600 650

250 300 350 400 450

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Ablation stake results

Comparison of the DEM values with field measurements using ablation stakes + interpollation (∼cm uncertainty on thickness measurements)

100 m/45 years=2.2 m/year average, consistent with ablation stake measurements at the glacier snout + litterature³.

3N.E. Barrand& al, J. Glaciology (2010)56(199), 771-780

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GPR bedrock mapping

• Purpose: estimate the glacier volume, and relate the ablation to total volume (52000 points collected using 100 MHz antennas)

• Deepest parts of the glacier: 160 m

• Thickness error (100 MHz antenna): 1 wavelength=1.7 m⇒1%

• Borders: the glacier is still 15-20 m deep on the steepest slopes⇒ 13% error

-160 -140 -120 -100 -80 -60 -40 -20 0 substrate map (GPS2010-GPR2010)

0

200

400

600

800

1000

1200

0 100 200 300 400 500 600 700 800

The largest ablation value would be at the glacier snout, where the current thickness is -20..-60 m

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GPR bedrock mapping

• Purpose: estimate the glacier volume, and relate the ablation to total volume (52000 points collected using 100 MHz antennas)

• Deepest parts of the glacier: 160 m

• Thickness error (100 MHz antenna): 1 wavelength=1.7 m⇒1%

• Borders: the glacier is still 15-20 m deep on the steepest slopes⇒ 13% error

-120 -100 -80 -60 -40 -20 0 substrate map (GPS2010-GPR2010)

200 250 300 350 400 450 500 550 600 650

250 300 350 400 450

-160 -140 -120 -100 -80 -60 -40 -20 0 substrate map (GPS2010-GPR2010)

0

200

400

600

800

1000

1200

0 100 200 300 400 500 600 700 800

The largest ablation value would be at the glacier snout, where the current thickness is -20..-60 m

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Glacier evolution

• Strong ablation on most of the glacier, accelerating in the latest years

• Rich information from DEM: which area is accumulating ?

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Glacier evolution

100 200 300 400 500

0 1000 2000 3000 4000 5000

height (m)

distance (m)

19641996 2010

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Glacier evolution

2010-1964 1996-1964 2010-1996

∆h₆₄₋₁₀=−43 cm/a ∆h₆₄₋₉₆=−35 cm/a ∆h₉₆₋₁₀=-63 cm/a surf₆₄=5.73 km² surf₆₄=4.84 km² surf₆₄=4.57 km_{20 / 21}²

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Conclusion

• Based on more recent measurement technologies ((D)GPS), snow balance over 3-years periods is accessible through DEM,

• at an average ablation rate of 63 cm/year, 3σDEM elevation error is reached every3 years+ highspatial resolution

• beyond intrinsic height measurement errors (instrument),

experimental procedures (antenna height, snow thickness) remain a source of bias,

• from this analysis, up to 100 m ablation in the glacier snout since 1964, with only 20 to 60 m-thick ice left.

Perspectives

ASTER beyond GDEM + RADAR Acknowledgements:

• Hydro-Sensors-FlOWS French National Research Agency (ANR) funding,

• IPY #16 program,

• IPEV

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