Keynote

Dru Smith NSRS Modernization Manager

Daniel R. Roman

Spatial Reference System Division Chief

NOAA’s National Geodetic Survey

Why are we here?

• Of course: • To learn • To exchange information • To get out of the office for a week

• But why are we here (as in “near NGS headquarters”)? • The new vertical datum and its cornerstone:

• GRAV-D

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The new vertical datum

• 2007 – NGS formally declares an end-of-life date for NAVD 88 – Only sustainable way forward: geoid-based datum – Biggest hurdle: terrestrial gravity data

• Heterogeneous in about every way – Instruments, Dates, Observers, Spacing, Metadata

• Old • Huge voids in littoral regions

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Within-survey Errors

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Across-surveys Errors

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Impact on the geoid

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Ship gravity

Terrestrial gravity

New Orleans

20-100 km gravity gaps along coast

Airborne gravity is the only technique that can adequately connect existing terrestrial data to existing ship and altimetry data in the oceans and fill coverage gaps. Airborne data will not replace existing data, but will be used as a baseline for correcting that data to be consistent across the country.

Terrestrial Data before GRAV-D

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May 23, 2016

Approximate extent of 2022 geoid model used for the “North American” part of the new vertical datum

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Historical geoid accuracy…or not

• From Geodesy (Bomford, 1952, section 6.00): – “For geodetic purposes, an error of 1 mGal matters little…”

• From Physical Geodesy (Heiskanen and Moritz, 1967, section 2.16): – “…an error of probably less than 1 meter in N, which can be neglected

for most practical purposes…”

• These issues are related! If you can’t get accurate gravity, you can’t get an accurate geoid. – Gravimeter accuracy and geoid accuracy march forward together – Geoid theory must also advance (but that is the other IAG summer

school!)

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A very brief history of gravimetry

• 1670s-1680s: Discovery that gravity varied geographically – Came out of clock measurements

• Relation between pendulum periods and gravity

– Accuracy Problem: Ideal pendulums (point mass

attached to a massless swing arm) swing differently than true pendulums

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A very brief history of gravimetry

• 1817: Kater reversible pendulum – Pendulum hung normally, then

upside down

– Exploits results of 1673 paper by Huygens

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A very brief history of gravimetry

• 1960s: Three absolute techniques vying for dominance (see Cook, 1968): – Kater pendulums still in use (~ 150 years!) – Ballistic meters – Freefall meters

• Calibrated rods, corner cubes, slotted rods • Lasers playing an ever-increasing role

• Accuracies at 0.1 mGal and improving

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Shipborne Gravimetry

• 1920s-1950s – Initiated by Vening-Meinesz – Submarines (to avoid weather) with pendulums

• 1930s on: – Multiple techniques for surface ships

• Spring-based dominated • Some pendulum • Vibrating string accelerometer

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Airborne Gravimetry

• We’ll save this for Dr. Jekeli’s lectures…

• Suffice to say, by the 1990s, the practice of airborne gravimetry had still not sufficiently advanced to use the data in most geoid modeling

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Gravimetry and the Geoid at NGS

• NGS holds 2 million terrestrial (including shipborne) gravity measurements – Many thanks to NGA’s efforts

• Geoids have been made at NGS with this data

since 1990 – With ever-increasing accuracy

• But you can only massage bad data into good geoids for so long

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Proving the accuracy of a geoid based on airborne gravity:

Geoid Slope Validation Surveys

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“...the gravimetric geoid used in defining the future vertical datum of the United States should have an absolute accuracy of 1 centimeter at any place and at any time.”

-- The NGS 10 year plan (2008-2018)*

Admirable!...Achievable?

May 23, 2016

Proving Geoid Accuracy

* Latest 10 year plan runs 2013-2023, but does not change this basic goal

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Goal of the surveys

• Observe geoid shape (slope) using multiple independent terrestrial survey methods – GPS + Leveling – Deflections of the Vertical

• Compare observed slopes (from terrestrial surveys) to modeled slopes (from gravimetry or satellites) – With / Without new GRAV-D airborne gravity

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GSVS11

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325 km 218 points 1.5 km spacing

South Texas July-October, 2011 hot…Hot…HOT!

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Surveys Performed

• GPS: 20 identical. units, 10/day leapfrog, 40 hrs ea.

• Leveling: 1st order, class II, digital barcode leveling

• Gravity: FG-5 and A-10 anchors, 4 L/R in 2 teams

• DoV: ETH Zurich DIADEM GPS & camera system

• LIDAR: Riegl Q680i-D, 2 pt/m2 spacing, 0.5 km width

• Imagery: Applanix 439 RGB DualCam, 5000’ AGL

• Other: – RTN, short-session GPS, extra gravity marks around Austin, gravity

gradients May 23, 2016 20

May 23, 2016

GPS DoV

Leveling

Gravity

LIDAR/ Imagery

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Empirical Error Estimates

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• σ∆h (OPUS-S) : 2 - 6 cm – GPSCOM combination: ~ 4 mm – (no significant baseline dependency)

• => 16 mm RMS over GSVS11

• σξ , ση : 0.06 arcseconds

– ~ 0.43 mm / 1.5 km => 6.6 mm RMS over GSVS11

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GSVS11: Proving why we need GRAV-D

May 23, 2016

Geoids without new GRAV-D data: 1-3 cm differential accuracy over distances from 0.4 to 325 km

Geoids with new GRAV-D data: 1 cm differential accuracy over distances from 0.4 to 325 km

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GSVS11 Conclusions

• For GSVS11, adding airborne gravity data improves geoid slope accuracy at all distances <325 km • Low elevation, flat topography

• Gravimetric geoid models and GPS are a viable alternative to long-line leveling

• Paper: Confirming regional 1 cm differential geoid accuracy from airborne gravimetry: the Geoid Slope Validation Survey of 2011

– J Geod DOI 10.1007/s00190-013-0653-0

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More GSVSs

• GSVS14: Took place in Iowa – Results less dramatically improved – Likely due to reasonableness of terrestrial data in

the center of the continent

• GSVS17: Southern Colorado – Marks being installed this year

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Thank you!

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