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-Still in progress
-however, note the desire for 2-5 cm accuracy
Residual signal between 2003 GPSBM’s and GEOID03
-after modeling with MMLSC, very little systematic signal is left
-several spike values are observed and represent points that are similarly
suspect to those that are already removed
-some of these points were
subsequently removed (they are counted in the 123 total removed points) but
the overall model did not change by their removal
Definition (1) is preferred.
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
A geoid model should predict the surface
between two known points (blue) and the unknown point (green)
GEOID03 permits that at 1 cm accuracy to the NAVD 88 datum
However, the absolute accuracy of NAVD 88 datum is unknown WRT to the “true” geoid
Need to study the relationship between gravimetric geoid, MSL, and
NAVD 88 at TBM‘s
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.
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 good agreement. Many of the remaining
features are likely artifacts from the rough data and require further processing.