daniel r. roman and vicki a. childers

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

Outline

• Introduction• Crossovers• Gridded Residual Gravity• Equivalent Geopotential• Residual Terrain Modeling• Future Geoid Models• Outlook

45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011


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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)




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 errors45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011



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 challenge45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011



Crossover Plots for AK08




AK08 Residual Gravity Profiles




AK08 Residual Geopotential (N=1440)




AK08 Residual Geopotential (Min=175, Max=1440)




Crossover Plots for AK09



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AK09 Gridded Residual Gravity

AK09 Residual Geopotential (N=1440)



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AK09 Residual Geopotential (Min=175, Max=1440)



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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 effects45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011


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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



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Point Gravity Minus EGM2008



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RTM Gravity Signal from 3” to 5’



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Point Gravity Less EGM2008 and RTM



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Filtered Residual Gravity



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Equivalent Residual Geoid



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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 well45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011


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Combined Terrain Models



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Alaska DEM (USGG2009)



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Point Gravity Minus EGM2008



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RTM Gravity Signal from 3” to 5’



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Point Gravity Less EGM2008 and RTM



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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



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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 few45th Annual Alaska Surveying & Mapping Conference February 21-25, 2011


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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



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Questions?GEOID Team• Daniel R. Roman, Ph.D.• Yan Ming Wang, Ph.D.• Xiaopeng Li, Ph.D. (ERT)• Simon Holmes, Ph.D. (SGT)

Websites• http://www.ngs.noaa.gov/GEOID/• http://www.ngs.noaa.gov/GRAV-D/

Aerogravity Team• Vicki A. Childers, Ph.D.• Theresa Diehl, Ph.D.• Sandra A. Preaux (DST)

Processing Support• William Waickman • Xu Yang (SGT)

http://www.ngs.noaa.gov/GEOID/

http://www.ngs.noaa.gov/GRAV-D/

Why we didn’t use Aster



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daniel r. roman and vicki a. childers

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