GGG 2000 Session 9 August 4 2000 The

GGG 2000, Session 9 August 4, 2000 The Challenge of the cm-geoid 12: 10 -12: 30 Recent Investigations Towards Achieving a One Centimeter Geoid Daniel R. Roman & Dru A. Smith U. S. National Geodetic Survey

OUTLINE • • G 99 SSS/GEOID 99 processing stream Refinements of hybrid methodology Development of a direct Helmert approach Other future developments

G 99 SSS/GEOID 99 Processing Stream • Use of Faye anomalies in G 99 SSS is same as approach taken for G 96 SSS (GEOID 96) • Higher resolution DEM in PNW increased calculated terrain correction (TC) signal • TC’s calculated in much narrower bands • More GPS on BM data improved regions previously under-represented (Midwest & Plains States) in GEOID 96

Refinements of Hybrid Methodology • G 99 SSS & GPS on BM residuals were modeled both by minimum curvature with a 300 -km low-pass filter and by iterative LSC at 550 & 33 km correlation lengths • Forthcoming models will be based on single -pass LSC using a 33 km correlation length and use of non-uniform sigma values based on GPS network accuracies

Development of a Direct Helmert Approach • Terrain Effects broken into inner ( <4 degrees) and outer ( >4 degrees) zones • Outer zone is being modeled using FFT’s • Inner zone will be modeled in spherical coordinates in the space domain • Merging will be with an equivalent taper applied in both space & frequency domains • Downward continuation consistent with terrain effects is being explored

Other Future Developments • Development of a North American geoid • Impending satellite gravity missions • Anticipated improvements to observations and DEM’s

Development of North American Geoid • Joint (U. S. , Canadian, Mexico, etc. ) effort • Anticipated scope is from the North Pole to the Equator including Greenland & Alaska • Next NAG subcomm. meeting possibly in Mexico, Central America or Caribbean • Largest obstacle remaining: Data gaps in Central America and Cuba

Impending Satellite Gravity Missions • GRACE, CHAMP, & GOCE • Expect cm-level commission errors for the resulting degree 120 -250 harmonic models • Combining these models with surface gravity observations should produce highresolution geoid at the cm-level • May eliminate the need for hybrid geoids and permit analysis of leveling networks

Anticipated Improvements to Observations and DEM’s • National Adjustment of CORS Network in 2004 will create more cohesive GPSderived ellipsoidal heights at bench marks • Increased DEM resolution (1 arc-second for all of CONUS) will improve determination of 3 D masses beneath gravity observations

Conclusion • Ongoing research will improve current approach (near term) and develop avenues for more rigorous solutions (far term) • Expected quality and quantity of future DEM and satellite gravity data sets should complement theoretical improvements • Development of a joint regional gravimetric geoid will enhance international studies