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1)This was a joint
NOAA-NRL-NASA effort designed to satisfy a number of institutional needs.
2) I’d like to thank my co-authors and their
support staffs for all their hard work on this.
3) What I’m covering today represents the
results from the first of three studies to be covered in the next few years.
This is still a work in progress and will serve as an update on our
4) It has been funded by the National Ocean
Service’s Partnership Proposal program in an effort to refine the gravimetric
geoid model in littoral regions and to better understand the nature of the
errors associated with the North American Vertical Datum of 1988.
5) NAVD 88 is the official U.S. vertical datum
from which all heights on topographic charts are measured.
6) Meter-level biases and trends are thought to
exist within NAVD 88 – bringing into question its utility for emergency and
1) In the Overview, I’ll cover what we hope to
gain from this study and possible impacts.
2) Next, I’ll cover the mission collection –
specifically regarding the gravity data.
3) With a better established gravity field,
analysis of the NAVD 88 datum is possible.
4) Next, I’ll cover where we intend to go in the
near term and beyond.
5) Last, I’ll summarize this oral
The EGM96 surface was enhanced using
local gravity and terrain data to create a regional geoid model (USGG2003).
The difference between the enhanced EGM96 surface and that implied by the
14185 GPSBM’s serving as control data are significant. Not all problems are
related to a bias difference, significant trends occur along the coasts.
question is, which is more correct? Another line of evidence would be need to
ascertain this – namely a comparison with the actual sea surface and a modeled
mean dynamic topography: GEOID + MDT + MSSH
Hence comparison of the gravimetric geoid and MDT models at coastal stations (TBM’s)
might resolve the datum question in an absolute sense.
This seems to offer some hope of deriving a seamless and consistent set of gravity
across the region. These gravimetric geoid values may be directly compared to
the NAVD 88 datum at tidal bench marks to estimate the magnitude of error in
NAVD 88 – ASSUMING the derived gravimetric geoid agrees well with the MDT and
tide models as well as the observed lidar sea surface heights.
Numerous TBM’s exist where the NAVD 88
value and ocean surface (MDT+geoid) are both known. These sites could suffice,
providing that suitable gravimetric geoid and MDT models are generated.
Models for the Gulf of Mexico already
exist, however, data gaps in the littoral regions raise questions about the
continuity of the data at the shoreline. Additionally, possible biases may
exist between offshore and terrestrial data. What is desired is a seamless set
of gravity values from the offshore to the onshore to best evaluate the geoid
at the shoreline.
A mission (GLS05) was planned to estimate
a consistent grid with sufficient crossovers to determine the regional gravity
PLANNED PROFILES: GRAVITY AND LIDAR
Off of Southwestern Coast of Florida & North of Cuba
Profiles from 50 km inshore to 200 km offshore
Track spacing of 10 km
Cross tracks at 50-100 km spacing
Profiles were flown at about 28,000 ft (8.5 km)
Florida data will bridge gap in near shore regions
Cuba data will provide constraints at edge of solution
What we hoped to get …
… and what we actually got. Note that
this is only the gravity field recovered along the profiles. Lidar coverage is
even poorer due to extensive cloud cover in the region. Bad weather during
Hurricane season and other factors negatively affected the collection process
and limited the scope of the resulting models to about 40 km full wavelength.
Gravity data at 28,000 feet collected
during GLS05. Block-meaned at 1/8 degree, then minimum curvature spline.
Difference between upward continued
surface gravity (using GRIDFFT from GMT) and aerogravity grid (min.
curvature). In general, there is a -18 mgal bias present but, otherwise good
agreement. Many of the remaining features are likely artifacts from the rough
data and require further processing.