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Monitoring geoid change in CanadaMarc Véronneau, Joe Henton, Jianliang Huang*, Jacques Liard,
Michael Craymer and Pierre Héroux
Geodetic Survey Division , CCRS, NRCan
2009 Workshop on Monitoring North American Geoid Change
Boulder, CO
21-23 October 2009
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Outline
Geodetic Survey Division … 3 Height Modernization … 4 GNSS infrastructure (h and h-dot) … 7 Gravity infrastructure (g and g-dot) … 11 Satellite altimetry (sea level) … 24 Conclusion … 26
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Geodetic Survey Division
The primary role of the Geodetic Survey Division (GSD) is to maintain, continuously improve, and facilitate efficient access to the Canadian Spatial Reference System (CSRS). This includes the responsibility to maintain the Canadian Gravity Standardization Net (CGSN) that provides datum control for gravity observations across Canada.
In order to better contribute to the definition of the vertical component of a highly accurate, multi-purpose, active and integrated Canadian Spatial Reference System (CSRS), GSD is in the process of consolidating the CGSN primary control sites with geometric reference stations (e.g. continuous and episodic GPS) of the CSRS
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Height Modernization - 2013
1. Time consuming 2. Expensive 3. Limited coverage 4. BMs are unstable 5. BMs disappear 6. Local networks
Levelling Networks:
1. Established over the last 100 years 2. 120,000 km of levelling lines 3. Some 80,000 benchmarks
The geoid model: 1. Entire coverage of the Canadian territory (land, lakes and oceans) 2. Compatible with space- based positioning (e.g., GNSS, altimetry)
3. Less expensive for maintenance4. Fairly stable reference surface
H = hGNSS – NModel
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Unifying Vertical and Horizontal Networks
NAD27
CGVD28
NAD83
NAVD88
NAD83(CSRS)NAD83(CSRS)
Geoid model
Horizontal network (, )
Vertical network (H)
3-D network (, , h)
Adopted in the USA,but not in Canada
3-D network(, , H = h – N )
4-D network(, , H = h – N, t)
NAD83(CSRS)Geoid model
h-dot & N-dot
Directtransformation
ITRFSNARF
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Vertical Reference System
Definition of the Vertical Reference system Equipotential surface (W0)
Specific potential value determined by NA agencies or IAG Physical height realization at one or several CACS/CBN
Realization of the Vertical Reference System Mean static solution representing a 1-to-2-year period Remove-restore approach; Degree banded Stokes Kernel GRACE, GOCE, Altimetry, Terrestrial gravity data, DEM, Topo density Error estimates
Maintenance of the Vertical Reference Frame Monitoring height (h and h-dot)
CACS/CBN + collaborative stations (e.g., provincial)
Monitoring gravity (g and g-dot) Improvement at GRACE wavelengths only Absolute gravity stations co-located at CACS/CBN/Tide Gauges No plans for short wavelengths (unless something is observed locally that requires action)
Actions to be taken regarding significant changes in geoid models What is a significant change in the geoid model?
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Federal active and passive stations
CACS:• 50 stations• Concrete pillar anchored to bedrock• GNSS receivers (9 stations)• 3 stations at St-Johns, ARO, Yellowknife, and Penticton (all former VLBI sites)• Real-time (1 sec.)CHAIN (New):• 9 stations (some co-located with CACS)CBN:• 151 stations• 5-year obs. cycleNot shown: active• Provincial active stations
• NB, QC, BC• Private RTK networks Not shown: passive• High Precision Network (HPN)
• Provincial
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National Velocity Grid v1.0
10.14 mm/yr
-1.29 mm/yr
Halifax, NS
Churchill, MB
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GPS on BMs
• GPS surveys: 1986 to 2008
• 2729 stations
• h accuracy: ± 1-2 cm to ± 10-15 cm
• h are not corrected for PGR
• Current levelling data have somewhat reach their limitation in evaluating geoid models
• Deflections of the vertical may be the most suitable data to validate geoid models
Nmodel = hGPS - HLev
Validation of geoid models(monitoring static solutions)
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GPS on Tide Gauges
GPS on BMs (near tide gauge) GPS on Tide gauges
Halifax, NS
Churchill, MB
-11.12 mm/yr
3.22 mm/yr
Tide gauges with more than 100 months of observations
West Coast East Coast
Arctic
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Gravimetry at NRCan
CSRS objectives, including: Datum support for gravity surveys Support time evolution of vertical component of geometric RF Maintenance of a new gravity-based/geoid height reference
system (e.g. direct measurement of g-dot/h-dot ratio to provide simplified connection for corresponding reference standards)
Scientific applications/priorities (with NRCan partners): Subduction/earthquake zone deformation studies Sea-level rise studies Hydrological/ground-water mass monitoring Post-glacial rebound studies
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Instrumentation
Superconducting gravimeter (CAGS) Observing since November 1989 (no activities between 1994 and 1996)
Absolute gravimeters FG5-236 NRCan/GSD (field operation) FG5-106 NRCan/GSC (field operation) FG5-105 NRC Watt Balance JILA-2 NRCan/GSD (CAGS) A-10 NRCan/GSD No absolute gravimeters with universities or private industry in Canada
Relative gravimeters CG5 (4) L&R (8) SL-1 dynamic gravimeter (shipborne and airborne surveys)
Tidal gravimeter 1 instrument in preparation for field operation
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Absolute gravity survey
FG-5
Survey tent
Vertical gradient of gravity
A-10
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Fundamental Absolute Gravity Station in Canada
• Superconducting gravimeter • Canadian Active Control Station• Four absolute gravity stations• Weather station• Two wells• Possible North American Inter-Comparison site (4 inst. max.)
CAGS
TMGO
Canadian Absolute Gravity Site (CAGS)
GWR superconducting gravimeter at CAGS
CAGS was established in 1987.The site is located on bedrock.
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Absolute Array• 63 sites co-located with ACP, CBN and/or TG (1) (15) (37) (10)• 8 proposed new sites• 44 other sitesCGSN Primary stations• 74 stations
Canadian Absolute Gravity Array
VancouverIslandLaurentian Profile
Mid-continent Profile
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Regular GSC Survey Sites in SW-BC
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Preliminary Uplift Rate Comparisons
Algonquin Park
Louise
Gananoque
Charlevoix
Eastmain
Val D’Or
La Grande-1
La Forge
Schefferville
Kuujjuarapik
9.7 ± 2.2
10.3 ± 3.9
10.8 ± 2.8
12.3 ± 2.7
11.0 ± 2.2
17.5 ± 8.8
4.5 ± 1.3
2.9 ± 0.6
2.2 ± 2.4
2.0 ± 4.4
10.1 ± 2.1
(1 Obs.)
15.9 ± 3.3
(1 Obs.)
(2 Obs.)
18.2 ± 5.9
21.5 ± 19.5
1.5 ± 3.7
8.4 ± 2.7
1.7 ± 2.3
11.5
10.6
4.6
-0.2
1.8
8.3
12.1
Station Name/Location
GPS UpliftRate (mm/yr)
Abs-GravityRate (mm/yr)
PGR Model(mm/yr)
9.4
8.6
0.0
La Pocatiere 2.0 ± 2.0 -0.5
-1.3St. John’s -0.9 ± 1.1 (1 Obs.)
-2.8Halifax -2.6 ± 3.0
(2 Obs.)
-0.7 ± 4.7
Eastern Canada GPS Uplift Rates
• Co-located at/near AG• Continuous & Episodic
(CBN & CACS)• Data period equivalent
to JILA AG surveys
[after Henton et al., 2004]
AG to Uplift Conversion• -0.15 μGal/mm• Theoretical relationship
for Laurentide GIA
[Lambert et al., 2001]
PGR Model• ICE-4G (VM1) [Peltier, 1994]
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CGSN (relative gravity network)
Primary gravity ties in Canada
Primary and secondary gravity stations
• 50+ years of relative gravity observations in Canada.• g-dot determination (Pagiatakis and Salib)
• Absolute gravity stations are tied to primary stations of CGSN by relative measurements
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“G-dot” (Relative Gravity Change)
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
μg
al/
yr
▪ CGSN (Relative) Primary Control Points [Pagiatakis & Salib, 2003 – JGR] ▪
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GRACE Analysis of monthly and static solutions
CSR, GFZ, JPL Analysis of combined solutions (e.g., EGM08) Estimation of monthly and secular variations of the geoid
Validate variation models against ground terrestrial gravity measurements
Availability of secular and monthly geoid variations (preliminary stage)
GOCE Will analyze GOCE-contributing global gravity models Will contribute to the static geoid for half-wavelength components
between ~700 km and 100 km (1 cm) in North America Evaluate needs for surface/airborne gravity measurements in Canada
Satellite Gravity Missions
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Secular geoid velocity
The secular geoid change from the monthly GRACE models (2002-2008).
The solution represents the effect due to total mass changes in the Earth interior.
The solution uses a 400-km Gaussian filter.
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Monthly geoid change - 2008
Jan MarFeb
OctSep
AugJulJun
Apr
May
Nov Dec
Color scale: -10 mm to 10 mm, GIA trend removed, and wrt to mean static 2002-2003 model
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GRACE comparison
GRACE gravity vs mean abs. surface gravitySouthern Vancouver Island
GRACE gravity vs SG gravity dataCanadian Absolute Gravity Site
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Sea Surface Topography
SSH: DNSC08 (Denmark)N: PCG08I (Canada)
SST = SSHAlt – N
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MSS variation and currents: East coast
Area: N65/N41/W72/W46
SST Atlantic Ocean
-0.55
-0.5
-0.45
-0.4
-0.35
-0.3
Date
SS
T (m
)
Data: AVISO TOPEX-POSEIDON SSH; GEOID CGG2005 (RIM)
SSH: DNSC08 (DK)N: PCG08I (Canada)
Speed of ocean currents (DNSC08 & PCG08I)
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Conclusion
Canada is moving to a geoid-based vertical reference system by 2013
Canada has a well-established active and passive GNSS network based on high reliability standards
Canada is presently establishing a national absolute gravity array co-located at GNSS stations
Canada relies heavily on satellite gravity missions in realizing an accurate static geoid model and monitoring geoid change
Canada is making use of tide gauge and altimetry data in monitoring the coastal and marine geoid (sea level)