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/