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| Vertical
Datum Adjustment |
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Postprocessed
lidar data are provided in the UTM coordinate system using the
NAD83 datum. Vertical measurements are expressed as heights above
the reference ellipsoid (HAE). Using the G99 model, HAE heights
are converted to orthometric heights relative to NAVD 88, which
approximates mean sea level (MSL). A local MSL correction is then
applied to the orthometric height based upon vertical information
from the bay-side tide gauge at Port Bolivar. Water levels at
the time of the survey from the Galveston Entrance Channel North
Jetty tide gauge are compared with heights from the Digital Elevation
Model (DEM) as a check upon the conversion from HAE to height
above MSL. Below is the formula for the HAE to MSL correction:
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Hmsl
= HAElidar + NG99SSS + MSL correction
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| Ellipsoid
and Geoid Heights |
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Schematic
of relationships between ellipsoid and geoid heights.
Click
on images to enlarge.
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| GEOID99
Model |
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| GEOID99
model along a portion of the Texas Gulf of Mexico shoreline. The
geoid model is used to convert HAE to NAVD 88 orthometric heights. |
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| Water
level at Galveston Entrance Channel North Jetty tide gauge,
July 16-18, 2001. The water level is indicated by a dashed
line showing the time the aircraft passes over the north jetty.
A sampling of water levels from the DEM matches the water
level at the tide gauge. |
Shoreline
levels and horizontal shifts on beach profile BEG02 if a
local mean sea level or alongshore geoid anomaly gradient
were not applied.
Click
on images to enlarge.
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