GEOID03 in Louisiana and Alaska

Dr. Yan M Wang and Dr. Daniel R RomanGeodesist, NGS/NOAA

ACSM Annual Conference and Technology ExhibitionOrlando, FLApril 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

hNAD83 hITRF

Simplified Concept of NAD 83 vs. ITRF

hNAD83 – hITRF 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≈h83-N99

Now, correctly, OPUS does:H≈h83-N03

~2.24 m

NAD 83 origin

ITRF96 origin

Earth’s Surface

hNAD83

hITRF00GEOID99 (refers toITRF96 centered

ellipsoid)

N99

N03

GEOID03 (refers toNAD 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

• Yan.Wang@noaa.gov• 301-713-3202 x 127

• Dan.Roman@noss.gov• 301-713-3202 x 161