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Daniel R. Roman and Vicki A. Childers

45 th Annual Alaska Surveying & Mapping Conference February 21-25, 2011 Hilton Anchorage Hotel Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Part III: Geoid Modeling. Daniel R. Roman and Vicki A. Childers. Outline. Introduction Crossovers

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Daniel R. Roman and Vicki A. Childers

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  1. 45th Annual Alaska Surveying & Mapping ConferenceFebruary 21-25, 2011 Hilton Anchorage Hotel Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Part III: Geoid Modeling Daniel R. Roman and Vicki A. Childers

  2. Outline • Introduction • Crossovers • Gridded Residual Gravity • Equivalent Geopotential • Residual Terrain Modeling • Future Geoid Models • Outlook Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  3. Introduction • Airborne gravity = Aerogravity • Relative gravity observations (i.e., need a base) • Collected on a fast-moving platform (400-500 kmh) • Kinematic GPS fixes positions • Also helps to determine gravity (a/c accelerations) • Very complicated process to “observe” aerogravity • Product is a function of observations and processing • Lastly, it is observed at very high altitudes (20 kft) Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  4. Purpose of GRAV-D • Aerogravity are for a geoid – not a gravity grid • A pattern of observation poor for gravity might be sufficient for geoid determination • The longest wavelengths affect the geoid • Gravity data highlight shorter wavelengths • Random gravity observation errors are “short” • Transformation into geoid effectively removes • Systematic errors are the cause of geoid errors Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  5. Creating a Gravimetric Geoid • Satellites provide global consistency & accuracy • GRACE has been up a long time – likely follow-on • GOCE supplements coverage of GRACE • Combined satellite model would cover 200 km > • Significant errors exist below 200 km • GRAV-D aerogravity will help to resolve these • Terrain modeling captures the smallest features • Melding all these together remains the challenge Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  6. Crossover Plots for AK08 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  7. AK08 Residual Gravity Profiles Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  8. AK08 Residual Geopotential (N=1440) Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  9. AK08 Residual Geopotential (Min=175, Max=1440) Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  10. Crossover Plots for AK09 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  11. AK09 Gridded Residual Gravity

  12. AK09 Residual Geopotential (N=1440) Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  13. AK09 Residual Geopotential (Min=175, Max=1440) Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  14. Statewide Geoid Modeling • Each month-long survey covers 400x500 km • All surveys collected in a consistent manner • All constrained to a GRACE/GOCE model • GRACE/GOCE will account for long wavelengths • Aerogravity will bridge the gap to surface data • Shortest wavelengths will come from DEM’s • Must properly account for all these signals • “Short” wavelength gravity can have big effects Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  15. Residual Terrain Modeling • Terrain creates the smallest gravity signal • Density contrast between rock and air • EGM2008 accounted for signal to 5’ (10 km) • Remaining signal between 5’ and 3” (90 m) • This neglected signal has a systematic effect • Examples are given for CONUS and Alaska Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  16. Point Gravity Minus EGM2008 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  17. RTM Gravity Signal from 3” to 5’ Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  18. Point Gravity Less EGM2008 and RTM Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  19. Filtered Residual Gravity Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  20. Equivalent Residual Geoid Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  21. Alaska RTM • For CONUS, SRTM was available for uniformity • In Alaska, several models were melded into one • SRTM below 60 N Latitude • NED was used for Alaska • CDED was used in Canada • GTOPO30 was used in Russia • Dr. Xiaopeng Li used GPSBM’s to unify them all • Final grid is improved, but probably not consistent • ASTER was looked at but didn’t perform as well Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  22. Combined Terrain Models Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  23. Alaska DEM (USGG2009) Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  24. Point Gravity Minus EGM2008 Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  25. RTM Gravity Signal from 3” to 5’ Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  26. Point Gravity Less EGM2008 and RTM Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  27. RTM Summary • RTM signal 3”-5’ significantly helped • Residual gravity much reduced • DEM for CONUS is from SRTM 3” – uniformity • DEM for Alaska built from many sources • In CONUS, long wavelength residuals • Small signal but yields big geoid residual • No equivalent signal for Alaska – too many gaps Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  28. Future Geoid Models • By the end 2012, plan is to be about 80% done • Comparison to Canadian data in overlap regions • Potentially, in next national gravimetric geoid • (e.g. USGG20xx) • GRACE/GOCE - long wavelengths (200 km >) • Aerogravity – mid-wavelengths (20 - 400 km) • RTM – short wavelengths (0.1 – 20 km) • Terrestrial gravity – all wavelengths – just too few Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  29. Outlook • Aerogravity is improving local gravity field • Significant improvements being implemented • Aim is for 2 mgal crossovers or better • Random errors have minimal impact • Systematic errors create geoid errors • Aerogravity will bridge satellite and terrestrial • DEM’s provide shortest wavelengths • Satellite, airborne, terrestrial, & terrain all merged • Removing errors in each yields an accurate geoid Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

  30. Questions? • Aerogravity Team • Vicki A. Childers, Ph.D. • Theresa Diehl, Ph.D. • Sandra A. Preaux (DST) • Processing Support • William Waickman • Xu Yang (SGT) Websites • http://www.ngs.noaa.gov/GEOID/ • http://www.ngs.noaa.gov/GRAV-D/ GEOID Team Daniel R. Roman, Ph.D. Yan Ming Wang, Ph.D. Xiaopeng Li, Ph.D. (ERT) Simon Holmes, Ph.D. (SGT) Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200 30

  31. Why we didn’t use Aster Impact of Airborne Gravity Surveys on Geoid Modeling in Alaska Thursday, 0800-1200

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