GEOID03 in Louisiana and Alaska Dr. Yan M Wang and Dr. Daniel R Roman Geodesist, NGS/NOAA ACSM Annual Conference and Technology Exhibition Orlando, FL April 21-26, 2006

Outline GEOID03 update in Louisiana GEOID03 in Alaska

Geoid03 update in Louisiana Subsidence corrections to the leveling network (85 pts) provided by LSRC Subsidence corrections are computed from a model, and further improvement is expected The leveling network will be readjusted in the National Spatial Reference System in February, 2007.

Few Remarks Updated Vertical Time-dependent Positioning bench marks must be used for all applications New survey along one long level line of 40 km, running from the coast inland as close to North/South is under discussion

The Geoid in Alaska NGS has computed a gravimetric geoid for Alaska as a separate computational area since GEOID96 Gravimetric only; No hybrid possible –Lack of wide-spread GPS on leveled benchmarks –Refers to ITRF origin, not NAD 83

~2.24 m NAD 83 origin ITRFxx origin Earth’s Surface h NAD83 h ITRF Simplified Concept of NAD 83 vs. ITRF h NAD83 – h ITRF varies smoothly by latitude and longitude

Gravity Holdings Zoom near Nome - Note lack of good near-shore data - Some sparseness in terrestrial network - Airborne gravity could solve both of these

GEOID99 vs GEOID03 in Alaska An error in how OPUS estimates orthometric heights was found during FAA airport surveys –OPUS/GPS yields NAD 83 ellispoid height –GEOID99 is an ITRF/GRS80 geoid height –Incompatible! GEOID03 was created in Alaska to fix this problem

Alaskan GEOID99 vs GEOID03 Erroneously, OPUS was doing: H≈h 83 -N 99 Now, correctly, OPUS does: H≈h 83 -N 03 ~2.24 m NAD 83 origin ITRF96 origin Earth’s Surface h NAD83 h ITRF00 GEOID99 (refers to ITRF96 centered ellipsoid) N 99 N 03 GEOID03 (refers to NAD 83 centered ellipsoid) Note: Physical location of geoid In space has not changed from 99 to 03! Geoid

The Geoid in Alaska (2) Ongoing research –GPS on leveled benchmarks vs GEOID03 –Accuracy assessments –Better theory –Better data Airborne gravity –Most logical way to collect accurate data in areas previously unvisited

Final snag: Heights change over time! All heights needs monitoring: –Crustal motion (affects H, h) –Gravity changes (affects H, N) –h=H+N Monitoring can be done efficiently: –GPS for crustal monitoring –Spot gravity re-visits for gravity monitoring –Re-leveling is not efficient Lest we forget, Alaska has some very active geology…

Questions/Comments? Dr. Yan M Wang and Dr. Daniel R Roman Geodesist, National Geodetic Survey x x 161