Effect of High Resolution Altimetric Gravity Anomalies on the North America Geoid Computations

Yan M. Wang and D. RomanNational Geodetic Survey

NOAA

Montreal, Canada, May 17-21, 2004

Recent geoid computations at NGS

1. Milbert, D. G., 1991b: GEOID90: A high-resolution geoid for the United States. Eos, 72(49), pp. 545-554.

2. Smith, D.A. and D.G. Milbert, 1999: The GEOID96 high resolution geoid height model for the United States, Journal of Geodesy, V. 73, N. 5, pp. 219-236.

3. Roman, D.R. and D.A. Smith, 2000, Recent investigations toward achieving a one centimeter geoid, presented at GGG2000 session 9 of the IAG Symposium in Banff, Alberta, Canada from July 31 - August 4, 2000.

4. Roman, et al., Assessment of the New National Geoid Height Model, GEOID03 , ACSM Meeting, 4/19, 2004.

Gravimetric geoid computation

Stokes integral /1DFFT

• Topographic reduction: terrain correction at 1/3/30 arc seconds

• EGM96 remove/restore: gravity anomaly of the reference model is removed, reference geoid is added back to residual geoid

• Indirect effect

Data Used for USGG2003

• NGS gravity database: 1,264,372

• NIMA gravity data: 2,064,898

• Altimetric gravity anomaly: 688,876

• GTOPO30 and NGSDEM99

• EGM96 global gravity model up to degree and order 360

• Ellipsoidal correction from Sideris et al. (2003)

Altimetry Gravity Models

Model Resolution Method

GSFC00.1 2’x2’ 2D Fourier Series/Inv.

Stokes Integral

KMS99 2’x2’ LSC/FFT

Sandwell/

Smith10.1

1’x1’ SSH slope/FFT

RMS Values of Gravity Differences(mgal)

GSFC00.1 KMS99 Sandwell/

Smith10.1

GSFC00.1 — 7.35 3.77

KMS99 — 7.61

Sandwell/Smith10.1

—

Geoid Differences (Mean/RMS values in cm)

GSFC00.1 KMS99 Sandwell/

Smith10.1

GSFC00.1 — -0.6/5.3 -0.7/5.2

KMS99 — -0.1/3.4

Sandwell/Smith10.1

—

GPS/leveling Differences (cm)

GSFC00.1 KMS99 Sandwell/Smith10.1

No. of Pts 6169 6169 6169

Slope/

Azimuth

0.12/333 0.15/331 0.16/333

Mean 51.1 51.2 51.0

STD 20.0 21.8 22.0

GPS/leveling Differencesin Florida (cm)

GSFC00.1 KMS99 Sandwell/

Smith10.1

No. of Pts. 372 372 372

Mean -8.7 -17.1 -18.7

RMS 13.6 21.0 22.4

GPS/leveling DifferencesBy Region (Mean/ RMS values in cm)

Longitude GSFC00.1 KMS99 Sandwell/

Smith10.1

West(230-250)

-1.7/19.7 0.0/19.1 0.7/19.2

Central(250-275)

7.0/16.7 9.0/17.9 8.6/18.4

East(275-300)

-13.3/19.8 -16.7/22.8 -16.7/22.6

Conclusions

• Altimetry gravity models differ at ±3 mgal RMS values (excluding few outliers in KMS99)

• Geoid differences can reach 10 cm in coastal regions (land area), few cm in areas thousand miles away from the ocean

• Geoid changes caused by GSFC00.1 improve the US gravimetric geoid

Conclusions (cont.)

• Sea surface topography implied by GSFC00.1 MSS and USGG03 closely emulate the POCM4b

• Ocean/land interface problems (diff. reference systems, lack gravity data)

Web Information

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