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Efforts to Monitor Geoid Change in Mexico

This article discusses the current geoid solution in Mexico, the state of the gravimetric network, variations in heights, and the re-construction of the geodetic vertical control network. It also examines the magnitude of vertical movements detected and proposes strategies for improving accuracy. Additionally, it highlights weaknesses in the current geoid solution and the need for further developments in the gravimetric and vertical control networks.

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Efforts to Monitor Geoid Change in Mexico

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  1. Efforts to Monitor Geoid Change in Mexico Towards a Significant Improvement for National and Regional Geoid Solution David Avalos-Naranjo. August 2009

  2. Contents • Current geoid solution: GGM06 • Gravimetric network, current state and perspective. • Heights and its Variations • Re-construction of the geodetic vertical control network. • Magnitude of vertical movements detected.

  3. Contents • Current geoid solution: GGM06 • Gravimetric network, current state and perspective. • Heights and its Variations • Re-construction of the geodetic vertical control network. • Magnitude of vertical movements detected.

  4. Geoid Solution • Geoid solutions with free access have been released since August 2004. • Kind of solution from INEGI: • Constraints: pure gravimetric • Geoid fixed in time • Approach: Stokes-Helmert technique (UNB)

  5. Geoid Solution • Currently recommended: GGM06 software available at: www.inegi.org.mx

  6. Geoid Solution • Last geoid solution (GGM06) used three terrain gravity sources: • INEGI • DMA (1993) • PEMEX (1987) • Sea anomalies: derived from altimetry T/P

  7. Geoid Solution • Input coverage for GGM06

  8. Geoid Solution Weaknesses detected • Extended data voids and lack of offshore data buffer prevent an exact determination of geoidal heights. • Regional disagreement among gravity sources can be high (10 and 20 mGal). • Geoid accuracy has been regionally affected in dm level by some biases in gravity anomaly. • Offshore accuracy has not been determined.

  9. Geoid Solution • Perspective: • Strategy for improving accuracy • Analysis of input gravity • Removal of biases medium/long wavelength • Insertion of recent GRACE-derived g anomalies • Implementation of most recent software version (SHGeo, UNB) • For next solution: • Sought < 10 cm absolute accuracy point by point • Assessment by improved GNSS/BM

  10. Contents • Current geoid solution: GGM06 • Gravimetric network, current state and perspective. • Heights and its Variations • Re-construction of the geodetic vertical control network. • Magnitude of vertical movements detected.

  11. Gravimetric Network • Gravity surveys continue to densify homogeneously throughout the country. 62,065 points IGSN71

  12. Gravimetric Network • Current densification criteria: • 1 point per 2.5’x2.5’ cell • all benchmarks observed ongoing

  13. Gravimetric Network • Weaknesses detected • Offshore-buffer gravity values missing! • Regional and general accuracy is unknown! • Absolute g reference missing! • Calibration line for gravimeters missing!

  14. Gravimetric Network • Solutions for near future • Diagnosis of INEGI’s base network: • study robustness of the current base network, • design of reinforcement links, • evaluate a simultaneous adjustment. • Obtain new absolute g observations for: • assessment of network consistency, • detecting regional biases, • establishment of one calibration line, • regular monitoring of time variations. • Possible redefinition of g values for the whole net (if significant distortions were detected).

  15. Gravimetric Network • Not implemented yet: • Airborne gravimetry • Shipborne gravimetry • Absolute g

  16. Contents • Current geoid solution: GGM06 • Gravimetric network, current state and perspective. • Heights and its Variations • Re-construction of the geodetic vertical control network. • Magnitude of vertical movements detected.

  17. Vertical Control Network • Datum adopted: NAVD88 • First order network is built in stages: • Older network: surveys 1949 – 2002 analog devises • Recent observations (re-construction) surveys 2002 – to date. electronic devises

  18. Vertical Control Network Accumulated levelling 1949 – 2002

  19. Vertical Control Network Recent levelling, 2002 to date 30,000 km 25,000 benchmarks

  20. Vertical Control Network Benchmarks re-observed: Re-levelling helps monitoring vertical deformations.

  21. Vertical Control Network Weaknesses detected: • No rigorous modelling of some systematic errors has been applied to the network (field data is insufficient for this), • Most of older benchmarks are destroyed. • Reference NAVD88 heights are uncertain due to time variations.

  22. Vertical Control Network EXPECTATIONS: • To complete the construction of a recent network within next few years, • To organize re-observed level differences, • To estimate time-variations in level differences along the country.

  23. Contents • Current geoid solution: GGM06 • Gravimetric network, current state and perspective. • Heights and its Variations • Re-construction of the geodetic vertical control network. • Magnitude of vertical movements detected.

  24. Recent Vertical Movements • Mexico is mostly built on deformation areas. • Current vertical displacements are studied. • Existing estimates come from: • MSL tracking, • GPS time-series, • Re-levelling.

  25. Recent Vertical Movements • Point velocity from MSL records: After D. Avalos, 2009.

  26. Recent Vertical Movements • Point velocity from GPS time-series: Network: INEGI-RGNA After D. Avalos, 2009.

  27. Recent Vertical Movements • Point velocity from GPS time-series: Network: SOPAC After D. Avalos, 2009.

  28. Recent Vertical Movements • Point velocity from GPS time-series: Network: UNAM-SSN After D. Avalos, 2009.

  29. Recent Vertical Movements • Relative velocity from re-levelling Detection of General Trends per Profile And unstable monitoring Sites MAZATLAN AGS

  30. Recent Vertical Movements • Vertical Crustal Movements, Preliminary map: • (excluding variations of local extent) mm/y

  31. Recent Vertical Movements IMPLICATIONS: • Current displacements are significant and should be accounted for in height determination, • Geoidal heights from GNSS/leveling could be significantly improved considering local vertical motions to achieve epoch consistency, • Actual VCM (medium wavelength features) seems to have potential to change the geoid at 1cm level only after accumulating 20 years of motion.

  32. Recent Vertical Movements IMPLICATIONS: • Point velocities can be highly deceiving for detecting VCM in Mexico, • Re-leveling is potentially best (mitigation of systematic errors needs attention).

  33. Remarks • Ongoing studies aim to improve the input for geoid computation. • An effort in monitoring time-variations for geodetic quantities is giving preliminary results. • Rates of Geoid change might be obtained in the future as an implication of changes in heights and gravity field.

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