Plan of the course

1
From tide gauge to CGPS How to perform the
local geodetic link ?
Guy Wöppelmann, CLDG - Université La
Rochelle gwoppelm_at_univ-lr.fr

• Plan of the course
• I. Preliminary remarks on CGPS_at_TG concept
• II. General site overview
• III. Techniques Methods
• IV. Budget error
• V. Application - Case studies

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Preliminary remarks on CGPS_at_TG concept

• CGPS_at_TG - What does it mean ?
• Continuous GPS positioning of tide gauges in a
  well defined global reference frame (ITRF, see
  IAG Recommendation).
• CGPS_at_TG - What is it for ?
• Separate sea level rise from vertical crustal
  motion
• Determine the vertical velocity of the land
  underlying the tide gauge few tens of mm/yr over
  few decades
• Calibration of satellite altimeters
• Determine the exact position of the sea-level
  measured by the tide gauge in the same frame as
  the space altimeter 1-2 cm
• Other applications Hydrography...
• Provide permanent and/or real-time access to
  tidal datum lt 5 cm

Section I Preliminary remarks...
3
IAG Resolution, Vienna 1991

•  The International Association of Geodesy
  recommends
• that groups making highly accurate geodetic,
  geodynamic or oceanographic analysis should
  either use the ITRS directly or carefully tie
  their own systems to it,
• that for mapping, navigation or digital databases
  where sub-metre accuracy is not required, WGS 84
  may be used in the place of ITRS.

Section I Preliminary remarks...
4
Section I Preliminary remarks...
5
CGPS_at_TG and Collocation Concepts

• Why these considerations ?
• CGPS and TG are a priori not physically linked
• The distance results from a large variety of
  parameters
• (see M.Bevis et al, 2002)
• Which criteria to adopt ?
• Geographical (distance)
• Technical (precision)
• Logistical (practical)
• An attempt to answer...
• Technical Geodetic tie with an accuracy of
  about 1 mm ? Distance (precision is often
  function of distance)
• Logistical ? Distance (common sense...)

?10 km, a more or less arbitrary upper limit...
Section I Preliminary remarks...
6
Application Bathymetric Survey
1. Seafloor Topography
2. Sounding reduction
Section I Preliminary remarks...
7
Scientific Objectives Technical Constraints

• Sky view
• Multipath
• Power supply
• Communication
• Security
• Site stability
• Local geology and topography
• Monumentation

• GLOSS-LTT(Long Term Trend)
• 0.3 - 0.5 mm/yr
• GLOSS-ALT(Altimetry calibration)
• 1-2 cm

M.Bevis et al, 2002 Technical issues and
recommendations related to the installation of
continuous GPS stations at tide
gauges http//imina.soest.hawaii.edu/cgps_tg/intr
oduction/index.html
Section I Preliminary remarks...
8
General Site Overview
Section II General site overview
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Techniques and Methods

• Differential Leveling
• Trigonometric Leveling
• Differential GPS ( Levelling)

Section III Techniques Methods
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Principle of differential leveling (1)
Backsight
Foresight
AV
AR
HB
HA
DHHB-HAAR-AV
Section III Techniques Methods
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Principle of differential leveling (2)

• Single reading accuracy
• 0.02 to 0.03 mm with Precise Level (sight
  lengths under 50 m)
• 0.2 to 0.3 mm with Digital Level (sight up to
  100 m)
• 2-3 mm with Builders/Engineers Level
• Note First order standard precision between
  national leveling benchmarks about 2?D, with D
  in km

Section III Techniques Methods
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Differential Leveling - Error Sources

• Sources
• Non verticalité des mires
• Erreurs de talon de mire
• Systematic errors due to operator (reading...)
• Collimation Error
• Error due to Earth Curvature
• Error due to Refraction

• Methodology - Rigorous procedures before and
  during observations
• Level and staff calibration
• fore - and backsights of equal length (less than
  30 m)

Section III Techniques Methods
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Error Detection and Reduction

• Rigorous observation and computation procedures

YES
YES
NO
NO

• Simple rules based on the level of tolerance over
  closures and forth-and-back lines
• DNA-gtB DNB-gtA
• DNA-A0

For national leveling survey about 2 mm per
kmFor site stability monitoring, about 0,5mm per
km
Section III Techniques Methods
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Principle of trigonometric leveling
DH HB-HA ha Dp . sin(i) - hv

• Advantages
• Faster over long distances
• Sight lengths up to 600 m and higher height
  differences
• Drawbacks
• Less precise
• Logistic

Section III Techniques Methods
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Error Budget
Section IV Error Budget
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Application - Case studies

• Brief Guide
• Application aimed at ?
• Precision
• Time span
• Local Geology and Site stability
• How often
• Local Topography
• What technique and method
• Funding
• What can I actually do
• and How long should I wait to get scientific
  results
• Lets go for the case studies...

Section V Application - Case studies
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Marseille Sea Level Station (IGN)
Acoustic tide gauge since 27/10/1998
16/07/1998
Trimble 4000 SSI
Since 03/02/1885...
No PC on site... ? Modem and daily remote data
retrieval
18
La Rochelle Sea Level Station (La Pallice)

• Observations
• 1863 - 1874
• 1956 - 1963
• 1963 - 1980
• 1997 - ...

Since 23/11/2001
Less than 100 m

• Radar Tide Gauge (SHOM)
• GPS Ashtech ?Z (CLDG)
• Monumentation and local connections (IGN, 2001)
• Logistical Support Stilling well, Power
  supply... (DDE)

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Brest Sea level Station

• Observations
• 1711 - 1716
• 1778 - 1792
• 1806 - 1835
• 1846 - 1856
• 1961 - 1943
• 1952 - ...

• Acoustic tide gauge (SHOM)
• GPS Trimble 4000 SSI (IGN)
• Monumentation (IGN)
• Local ties(IGN 1999, SHOM 2002)
• Logistical Support (SHOM)

Since 31/10/1998
Somewhere here (350 m)
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Ajaccio Sea Level Station

• Acoustic tide gauge (SHOM)
• GPS ashtech ZXII (IGN)
• GPS monumentation and local connection (IGN,
  2000)
• Stilling well (OCA)
• Logistical Support (Naval Base)

Since 20/01/2000
Distance separating GPS and TG 500 m

• Sea level Observations
• 1981 - 1982
• 1996 - 1997
• 2000 - ...

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Kerguelen Sea Level Station

• Pressure tide gauge (LEGOS)
• GPS Rogue SNR 8 (CNES)
• Monumentation (IFRTP)
• Local ties(IGN 1994, AUSLIG 1995)

16/11/1994
Distance is about 3 km from TG