Comparing GEOID99 to the GSD95 Canadian geoid model

Dru A. SmithDaniel R. RomanNational Geodetic Survey, U.S.A.

Presented at the 2000 Meeting of the CGUBanff, Alberta, Canada

Topics of Discussion

Models: NGS (USA) and GSD (Canada)

Comparison and discussion

Future plans

G99SSS: NGS gravimetric model

2.6 Million gravity points 30 arcsecond DEM 1 arcsecond DEM for PNW USA EGM96 underlying structure Faye anomalies (not Helmert) 1-D FFT Geocentric (ITRF96/GRS-80)

New for G99SSS (vs. G96SSS)

KMS98 altimetry Ellipsoidal solution to the Stokes

Integral (Fei and Sideris) Multi-band 2-D FFT TCs 3 arcsecond TCs for PNW USA 1 arcminute resolution North to 58 degrees

New for G99SSS: Ellipsoidal solution to the Stokes Integral

Fei, Z.L. and M.G. Sideris, 2000: A new method for computing the ellipsoidal correction for Stokes’s formula, Journal of Geodesy, in review.

Significant reduction in N/S tilts

Ellipsoidal Corrections used in G99SSS (cm)

New for G99SSS: Multi-band 2-D FFT TCs Lowers N/S errors in TCs; Not as good as 1-D FFT In PNW (3 arcsecond):

10 areas (5 x 2) 18 x 18 degrees (keep 6 x 6) Overlap: 2 degrees E/W and N/S

In CONUS (30 arcsecond) 3 areas (1 x 3)

164 x 46 deg (keep 76 x 17) Overlap: 8.5 degrees N/S

PNWOverlap

TC DiffRMS

(mGals)

TC DiffMax

(mGals)

E/W 0.054 0.310

N/S 0.508 31.275

Basic Statistics of TC Differences using overlapping 2-D FFT regions

CONUSOverlap

TC DiffRMS

(mGals)

TC DiffMax

(mGals)

N/S 0.837 74.6

GEOID99: NGS hybrid model

Begin with G99SSS

6169 (NAD 83) GPS heights on (NAVD 88) leveled benchmarks inside USA

Collocation and transformations

Non-geocentric: NAD 83 <--> NAVD 88

PICTURE OF 6169 GPSBMS

PICTURE OF GEOID99

G99BM: NGS geocentric hybrid model

Need a geocentric hybrid model to compare to GSD95 (geocentric)

G99BM = geocentric GEOID99

Transformed NAD 83 into ITRF96

G99SSS/G99BM vs GSD95 Re-grid GSD95 from 5’ to 1’

G99BM valid only inside USA

G99SSS - GSD95 Ave = -63 cm +/- 33 cm

Trend: 0.15 ppm (72 cm E/W)

Detrended: Ave = 0 cm +/- 19 cm

PNW, BC, Hudson Bay remain outliers

G99BM - GSD95

Valid in USA only

Ave: -78 cm +/- 49 cm

New issues: WA/OR tilt New England rise

Conclusions E/W tilt remains, but reduced from 150 cm to 72 cm

GPS on BM do not significantly help the NGS/GSD disagreement

Data differences in altimetry, and DEM yield systematics that cause tilts NGSDEM99 in PNW Quebec DEM

FUTURE PLANS Helmert Anomalies, not Faye

GPS on BM through all of North America Re-adjustment in 2003(?)

Density anomalies

1 arcsecond DEM over all of CONUS (USGS) Possibly NA (SRTM)