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TIDAL
DATUMS |
In general, a
datum is a base
elevation used as a reference from which to reckon heights or depths. A
tidal datum is a standard elevation defined by a certain phase of the
tide. Tidal datums are used as references to measure local water levels
and should not be extended into
areas having differing oceanographic characteristics without
substantiating measurements. In order that they may be recovered when
needed, such datums are referenced to fixed points known as bench
marks. Tidal datums are also the basis for establishing
privately owned land, state owned land, territorial sea, exclusive
economic zone, and high seas boundaries. Below are definitions of tidal
datums maintained by the Center for Operational Oceanographic Products
and Services.
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MHHW*
Mean Higher High Water
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The
average of the higher high water height of each tidal day observed over
the National Tidal Datum Epoch. For stations with shorter series,
comparison of simultaneous observations with a control tide station is
made in order to derive the equivalent datum of the National Tidal
Datum Epoch. |
MHW
Mean High Water
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The
average of all the high water heights observed over the National Tidal
Datum Epoch. For stations with shorter series, comparison of
simultaneous observations with a control tide station is made in order
to derive the equivalent datum of the National Tidal Datum Epoch.
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DTL
Diurnal Tide Level
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The
arithmetic mean of mean higher high water and mean lower low water. |
MTL
Mean Tide Level
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The
arithmetic mean of mean high water and mean low water.
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MSL
Mean Sea Level
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The
arithmetic mean of hourly heights observed over the National Tidal
Datum Epoch. Shorter series are specified in the name; e.g. monthly
mean sea level and yearly mean sea level.
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MLW
Mean Low Water
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The
average of all
the low water heights observed over the National Tidal Datum Epoch. For
stations with shorter series, comparison of simultaneous observations
with a control tide station is made in order to derive the equivalent
datum of the National Tidal Datum Epoch. |
MLLW*
Mean Lower Low Water
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The
average of the lower low water height of each tidal day observed over
the National Tidal Datum Epoch. For stations with shorter series,
comparison of simultaneous observations with a control tide station is
made in order to derive the equivalent datum of the National Tidal
Datum Epoch. |
GT
Great Diurnal Range
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The
difference in height between mean higher high water and mean lower low
water.
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MN
Mean Range of Tide
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The
difference in height between mean high water and mean low water. |
DHQ
Mean Diurnal High Water
Inequality
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The
difference in height of the two high waters of each tidal day for a
mixed or semidiurnal tide.
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DLQ
Mean Diurnal Low Water Inequality
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The
difference in height of the two low waters of each tidal day for a
mixed or semidiurnal tide. |
HWI
Greenwich High Water Interval
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The
average interval (in hours) between the moon's transit over the
Greenwich meridian and the following high water at a location. |
LWI
Greenwich Low Water Interval
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The
average interval (in hours) between the moon's transit over the
Greenwich meridian and the following low water at a location. |
Station Datum
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A fixed base
elevation at a
tide station to which all water level measurements are referred. The
datum is unique to each station and is established at a lower elevation
than the water is ever expected to reach. It is referenced to the
primary bench mark at the station and is held constant regardless of
changes to the water level gauge or tide staff. The datum of tabulation
is most often at the zero of the first tide staff installed. |
National Tidal Datum
Epoch
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The specific 19-year
period adopted by the National Ocean Service as the official time
segment over which tide observations are taken and reduced to obtain
mean values (e.g., mean lower low water, etc.) for tidal datums. It is
necessary for standardization because of periodic and apparent secular
trends in sea level. The present NTDE is 1983
through 2001 and is actively considered for revision every 20-25 years.
Tidal datums in certain regions with anomolous sea level changes
(Alaska, Gulf of Mexico) are calculated on a Modified 5-Year Epoch. |
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*Some locations have diurnal
tides--one high tide and one low tide per day. At most locations, there
are semidiurnal tides--the tide cycles through a high and low twice
each day, with one of the two high tides being higher than the other
and one of the two low tides being lower than the other.
References
1. (Get PDF reader)
Tide
Datums and Their Applications - NOAA Special Publication NOS CO-OPS 1
2. (Get PDF reader)
Computational Techniques for Tidal Datums Handbook - NOAA Special
Publication NOS CO-OPS 2
3. (Get PDF reader)
Tide and Current
Glossary (Get PDF reader)
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![Datums page at http://tidesandcurrents.noaa.gov](https://webarchive.library.unt.edu/eot2008/20080916125809im_/http://www.co-ops.nos.noaa.gov/images/datumpage.jpg)
Click the link at the
top of the page for a
printer-friendly version of the page. Scroll down the page to see all
the datums available for the station. Link to other types of data for
the same station using the links on the left. Links are highlighted
only for data types that are available for the station.
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The Datums page (example at
left) provides access
to established datums for the station indicated in the upper left
corner of the page. The tidal datums on this page are referenced to an
arbitrary station datum. In order to apply these datums for surveying
or coastal management they must be reduced
to Mean Lower Low Water (MLLW), which is the reference datum for
predictions, bench
mark publication and nautical charting. Other tidal (Mean High Water
(MHW)) and geodetic (North American Vertical Datum 1988 (NAVD 88))
datums may also be used for specific reasons. The time meridian (TM) is
the reference meridian
used to calculate time. An epoch is a 19-year tidal cycle used to
calculate
datums. The present National Tidal
Datum Epoch (NTDE) is 1983
through 2001. Tidal datums in certain regions with anomolous sea
level changes (Alaska, Gulf of Mexico) are calculated on a Modified
5-Year Epoch.
![A screenshot of the feet/meters select button](https://webarchive.library.unt.edu/eot2008/20080916125809im_/http://www.co-ops.nos.noaa.gov/images/apply.png)
The
data can be displayed in either feet or meters. Click the Apply Change
button to apply the selected unit to the table of values for tidal
datums. The heading of the table will reflect the units currently
applied to the table.
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GEODETIC DATUMS
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The National Geodetic Survey (NGS) defines a geodetic datum as: 1. "A set of constants used for calculating the coordinates of points on the Earth." Generally a datum is a reference from which measurements are made. In surveying and geodesy, a datum is a reference point on the earth's surface against which position measurements are made, and an associated model of the shape of the earth for computing positions. Horizontal datums are used for describing a point on the earth's surface, in latitude and longitude. Vertical datums are used to measure elevations or underwater depths.
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North American
Vertical Datum of 1988 (NAVD
88)
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A fixed reference
for elevations
determined by geodetic leveling. The datum was derived from a general
adjustment of the first-order terrestrial leveling nets of the United
States, Canada, and Mexico. In the adjustment, only the height of the
primary tidal bench mark, referenced to the International Great Lakes
Datum of 1985 (IGLD 85) local mean sea level height value, at Father
Point, Rimouski, Quebec, Canada was held fixed, thus providing minimum
constraint. NAVD 88 and IGLD 85 are identical. However, NAVD 88
bench mark values are given in Helmert orthometric height units while
IGLD 85 values are in dynamic heights. See International Great Lakes
Datum of 1985, National Geodetic Vertical Datum of 1929, and
geopotential difference. NAVD 88 should not be used as Mean Sea Level.
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National Geodetic
Vertical Datum of 1929 (NGVD
29) |
A fixed reference
adopted as a standard geodetic datum for elevations determined by
leveling. The datum was derived for surveys from a general adjustment
of the first-order leveling nets of both the United States and Canada.
In the adjustment, mean sea level was held fixed as observed at 21 tide
stations in the United States and 5 in Canada. The year indicates the
time of the general adjustment. A synonym for Sea-level Datum of 1929.
The geodetic datum is fixed and does not take into account the changing
stands of sea level. Because there are many variables affecting sea
level, and because the geodetic datum represents a best fit over a
broad area, the relationship between the geodetic datum and local mean
sea level is not consistent from one location to another in either time
or space. For this reason, the National Geodetic Vertical Datum should
not be confused with mean sea level. See North American Vertical Datum
of 1988 (NAVD 88). NGVD 29 should not be used as Mean Sea Level.
NGVD 29 is no longer supported by NGS.
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References
1. Frequently
asked questions at http://www.ngs.noaa.gov/faq.shtml
2. NGS
Publications at http://www.ngs.noaa.gov/PUBS_LIB/pub_index.html
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TIDAL AND GEODETIC RELATIONSHIPS
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Geodetic datum
relationships to tidal datums are established at tide stations by
connecting tidal bench mark networks to the National Spatial Reference
System (NSRS) maintained by NGS. There are two survey procedures used
to make this connection. The first is to connect the tidal bench marks
with traditional differential levels to nearby geodetic bench marks
with known geodetic elevations. The second is to occupy the tidal
bench marks using a static GPS survey to determine the geodetic
elevations of the bench marks directly. In all cases it is advised to
make the connections to more than one bench mark, preferably to three
marks, in order to confirm the connection and identify unstable bench
marks. The elevation relationship between geodetic datums and
tidal datums should not be extrapolated away from a particular location
without correction or interpolation as the relationships vary with
parameters such as variations in range of tide, bathymetry, topography,
geoid variations, and vertical land movement. Any interpolation should
be done carefully, and where possible guided by the use of the National Ocean Service
VDatum tool which can be obtained at: http://www.nauticalcharts.noaa.gov/csdl/vdatum.htm.
![CO-OPS Representation of Tidal Datums](https://webarchive.library.unt.edu/eot2008/20080916125809im_/http://www.co-ops.nos.noaa.gov/images/Slide4.png)
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