1. Plan of the course I. Preliminary remarks on CGPS@TG concept II. General site overview III. Techniques & Methods IV. Budget error V. Application - Case studies From tide gauge to CGPS : How to perform the local geodetic link ? Guy Wöppelmann, CLDG - Université La Rochelle gwoppelm@univ-lr.fr

2. zCGPS@TG - What does it mean ? yContinuous GPS positioning of tide gauges in a well defined global reference frame (ITRF, see IAG Recommendation). zCGPS@TG - What is it for ? ySeparate sea level rise from vertical crustal motion xDetermine the vertical velocity of the land underlying the tide gauge [few tens of mm/yr over few decades] yCalibration of satellite altimeters xDetermine the exact position of the sea-level measured by the tide gauge in the same frame as the space altimeter [1-2 cm] yOther applications : Hydrography...  Provide permanent and/or real-time access to tidal datum [ < 5 cm] Preliminary remarks on CGPS@TG concept Section I : Preliminary remarks...

3. z« The International Association of Geodesy recommends ythat groups making highly accurate geodetic, geodynamic or oceanographic analysis should either use the ITRS directly or carefully tie their own systems to it, ythat for mapping, navigation or digital databases where sub- metre accuracy is not required, WGS 84 may be used in the place of ITRS. » IAG Resolution, Vienna 1991 Section I : Preliminary remarks...

5. zWhy these considerations ? yCGPS and TG are a priori not physically linked yThe distance results from a large variety of parameters (see M.Bevis et al, 2002) zWhich criteria to adopt ? yGeographical (distance) yTechnical (precision) yLogistical (practical) zAn attempt to answer... yTechnical : Geodetic tie with an accuracy of about 1 mm  Distance (precision is often function of distance) yLogistical  Distance (common sense...) CGPS@TG and Collocation Concepts 10 km, a more or less arbitrary upper limit... Section I : Preliminary remarks...

6. 1. Seafloor Topography 2. Sounding reduction Application : Bathymetric Survey Section I : Preliminary remarks...

7. zGLOSS-LTT (Long Term Trend…) ³0.3 - 0.5 mm/yr zGLOSS-ALT (Altimetry calibration) ³1-2 cm zSky view zMultipath zPower supply zCommunication zSecurity zSite stability zLocal geology and topography zMonumentation Scientific Objectives & Technical Constraints 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/introduction/index.html Section I : Preliminary remarks...

8. General Site Overview Section II : General site overview

9. Techniques and Methods Section III : Techniques & Methods zDifferential Leveling zTrigonometric Leveling zDifferential GPS (+ Levelling)

10.  H=H B -H A =AR-AV Principle of differential leveling (1) Section III : Techniques & Methods BacksightForesight AR AV HAHA HBHB

11. Principle of differential leveling (2) Section III : Techniques & Methods zSingle reading accuracy y0.02 to 0.03 mm with “Precise Level” (sight lengths under 50 m) y0.2 to 0.3 mm with “Digital Level” (sight up to 100 m) y2-3 mm with “Builder’s/Engineer’s Level” zNote : First order standard precision between national leveling benchmarks : about 2*  D, with D in km

12. Differential Leveling - Error Sources Section III : Techniques & Methods zSources yNon verticalité des mires yErreurs de talon de mire ySystematic errors due to operator (reading...) yCollimation Error yError due to Earth Curvature yError due to Refraction zMethodology - Rigorous procedures before and during observations yLevel and staff calibration yfore - and backsights of equal length (less than 30 m)

13. YES NO Error Detection and Reduction Section III : Techniques & Methods For national leveling survey : about 2 mm per km For site stability monitoring, about 0,5mm per km zRigorous observation and computation procedures zSimple rules based on the level of tolerance over closures and forth-and-back lines DN A->B = DN B->A DN A-A =0

14. Principle of trigonometric leveling Section III : Techniques & Methods zAdvantages yFaster over long distances ySight lengths up to 600 m and higher height differences zDrawbacks yLess precise yLogistic  H = H B -H A = h a + D p. sin(i) - h v

15. Error Budget Section IV : Error Budget

16. zBrief Guide yApplication aimed at ? xPrecision xTime span yLocal Geology and Site stability xHow often yLocal Topography xWhat technique and method yFunding xWhat can I actually do xand How long should I wait to get scientific results zLets go for the case studies... Application - Case studies Section V : Application - Case studies

17. 16/07/1998 Marseille Sea Level Station (IGN) Since 03/02/1885... Acoustic tide gauge since 27/10/1998 No PC on site...  Modem and daily remote data retrieval Trimble 4000 SSI

18. La Rochelle Sea Level Station (La Pallice) yRadar Tide Gauge (SHOM) yGPS Ashtech  Z (CLDG) yMonumentation and local connections (IGN, 2001) yLogistical Support : Stilling well, Power supply... (DDE) Since 23/11/2001 Observations : 1863 - 1874 1956 - 1963 1963 - 1980 1997 -... Less than 100 m

19. Brest Sea level Station Since 31/10/1998 yAcoustic tide gauge (SHOM) yGPS Trimble 4000 SSI (IGN) yMonumentation (IGN) yLocal ties (IGN 1999, SHOM 2002) yLogistical Support (SHOM) Observations : 1711 - 1716 1778 - 1792 1806 - 1835 1846 - 1856 1961 - 1943 1952 -... Somewhere here (~350 m)

20. Ajaccio Sea Level Station yAcoustic tide gauge (SHOM) yGPS ashtech ZXII (IGN) yGPS monumentation and local connection (IGN, 2000) yStilling well (OCA) yLogistical Support (Naval Base) Since 20/01/2000 Sea level Observations : 1981 - 1982 1996 - 1997 2000 -... Distance separating GPS and TG : 500 m

21. Kerguelen Sea Level Station yPressure tide gauge (LEGOS) yGPS Rogue SNR 8 (CNES) yMonumentation (IFRTP) yLocal ties (IGN 1994, AUSLIG 1995) 16/11/1994 Distance is about 3 km from TG