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

2. OUTLINE G99SSS/GEOID99 processing stream Refinements of hybrid methodology Development of a direct Helmert approach Other future developments

3. G99SSS/GEOID99 Processing Stream Use of Faye anomalies in G99SSS is same as approach taken for G96SSS (GEOID96) 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 GEOID96

7. Refinements of Hybrid Methodology G99SSS & 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

11. Development of a Direct Helmert Approach Terrain Effects broken into inner ( 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

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

15. 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

16. 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 high- resolution geoid at the cm-level May eliminate the need for hybrid geoids and permit analysis of leveling networks

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

18. 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 the theoretical improvements Development of a joint regional gravimetric geoid will enhance international studies