TIDAL DATUMS


   The computation  of topographic datums using tidal station data provides
VCR-LTER researchers with standard topographic levels for use in
plant zonation and geology surveys, tidal flooding frequency studies etc.

   Tidal datums are being provided for Hog Island and Quinby
(at the mainland). Red Bank is not tied to the USGS net yet. However,
Quinby datums could be used as a good aproximation for those working
at Bronwsville and Phillips Creek.

   The barrier inslands benchmarks were tied to mainland USGS
benchmarks using Survey Grade GPS. In the specific case of Hog Island,
benchmark FARL was tied to USGS Kiptopeake benchmark. The calculation
of the USGS level of FARL uses the USGC level of Kiptopeake adding the
relative difference between Kiptopeake and FARL.  The USGS level for
FARL, which is being used for all the Hog Island calculations, is
1.230 meters.  Because their relative difference did not change with
the most recent adjustments, the above datum is still valid.

   The tables below list levels and ranges. A level is a topographical height
(in this case, height above the USGS 1967 reduction level). The level is
calculated adding (or subtracting) half of a selected amplitude to the
Half Tide Level(field data based Half Tide level). A range is the
amplitude between the selected high and low tide levels.

   Two categories of levels and ranges are listed. The first is comprised
by those tidal datums calculated directly from field tidal heights
(field data from the installed maregraphs and indicated in the table as FD).
The second category is made by the datums that were estimated using harmonic
constants (H) (which were also calculated from field tidal heights).

   Datums computed from harmonic constants estimate a tidal level which
is determined only by the gravitational effects of moon and sun. In other words,
they do not take into account daily and seasonal changes in sea level or wind
effects. In the other hand, the datums based on field data do take into account
meteorological and sea-level changes effects.

    Hourly tidal data from September through December 93 was used in the
datum calculation for both sites.

    All figures are in meters.



                                   HOG ISLAND DATUMS



Mean Tide Range (FD) (m)					        1.190
Mean Tide Range ( H)  (m)					        1.050



Mean Neap Tide Range (H)  (m)					        0.819

Mean Spring Tide Range (H) (m)					        1.263


Mean Apogean Tide Range (H) (m)					        0.897

Mean Perigean Tide Range (H) (m)					1.235


Mean Tide Level (FD) (m)	          		                0.282


Halt Tide Level   (FD) (m)					        0.256
Halt Tide Level (H)  (m)					        0.279

Mean Low Tide Level (FD) (m)					      - 0.339
Mean High Tide Level (FD)  (m)					        0.851

Mean High Water Neap Tide Level (H) (m) -				0.665


Mean High Water Apogean Tide Level (H) (m) -				0.705


Mean High Water Spring Tide Level (H) (m) -				0.887


Mean High Water Perigean Tide Level (H) (m) -				0.873


Mean Higher High Water Spring (H) (m)					0.990

Mean Extreme Astronomical High Tide Level (H) (m)			1.165


Mean Low Water  Neap Tide Level (H) (m) -				-0.153


Mean Low Water  Apogean Tide Level (H) (m) -				-0.193


Mean Low Water  Spring Tide Level (H) (m) -				-0.375

Mean Low Water  Perigean Tide Level (H) (m) -				-0.361


Mean Lower Low Water Spring Tide Level(H) (m)				-0.478

Mean Extreme Astronomical Low Tide Level (H) (m)			-0.499





				QUINBY	DATUMS




Mean Tide Range (FD) (m)					        1.479
Mean Tide Range ( H)  (m)					        1.274

Mean Neap Tide Range (H)  (m)					        1.009

Mean Spring Tide Range (H) (m)					        1.525


Mean Apogean Tide  Range (H) (m)					1.024

Mean Perigean Tide Range (H) (m)					1.541


Mean Tide Level (FD)  (m)					        0.276


Half Tide Level (FD) (m)					        0.233
Half Tide Level (H)  (m)					        0.265

Mean Low Tide Level (FD) (m)					       -0.507
Mean High Tide Level (FD)  (m)					        0.972


Mean High Water  Neap Tide Level (H) (m) -				0.737


Mean High Water  Apogean Tide Level (H) (m) -				0.745


Mean High Water  Spring Tide Level (H) (m) -				0.995


Mean High Water  Perigean Tide Level (H) (m) -				1.003


Mean Higher High Water Spring Tide Level (H) (m)			1.104

Mean Extreme Astronomical High Tide Level (H) (m)			1.237


Mean Low Water  Neap Tide Level (H) (m) -				-0.272


Mean Low Water  Apogean Tide Level (H) (m) -				-0.280

Mean Low Water  Spring Tide Level (H) (m) -				-0.530


Mean Low Water  Perigean Tide Level (H) (m) -				-0.538


Mean Lower Low Water Spring Tide Level (H) (m)				-0.639

Mean Extreme Astronomical Low Tide Level (H) (m)			-0.653