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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
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Topics of Discussion
Models: NGS (USA) and GSD (Canada)
Comparison and discussion
Future plans
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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)
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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
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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
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Ellipsoidal Corrections used in G99SSS (cm)
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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
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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
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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
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PICTURE OF 6169 GPSBMS
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PICTURE OF GEOID99
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G99BM: NGS geocentric hybrid model
Need a geocentric hybrid model to compare to GSD95 (geocentric)
G99BM = geocentric GEOID99
Transformed NAD 83 into ITRF96
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G99SSS/G99BM vs GSD95 Re-grid GSD95 from 5’ to 1’
G99BM valid only inside USA
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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
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G99BM - GSD95
Valid in USA only
Ave: -78 cm +/- 49 cm
New issues: WA/OR tilt New England rise
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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
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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)