Geoscience Reference
In-Depth Information
Table 1 Definition of International Hydrographic Organization (IHO) tidal datum (International
Hydrographic Bureau
2010
)
Level
Definition
LAT (HAT)
LAT (HAT) is defined as the lowest (highest) tide level which can be predicted
to occur under average meteorological conditions and under any combination
of astronomical conditions. It is recommended that LAT and HAT be
calculated either over a minimum period of 19 years using harmonic
constants derived from a minimum of 1 years observations or by other
proven methods known to give reliable results. Tide levels should, if
possible, reflect the estimated error values obtained during the determination
of these levels
datum analysis and the use of Lowest Astronomical Tide (LAT) and Highest
Astronomical Tide (HAT) to generate a littoral zone for marine cadastre.
The objective of this chapter is to present some fundamental principles that can
be applied to produce the tidal line and analyze the consistency of LAT to use as a
reference level in marine cadastre. In this chapter, we propose a computational
technique of tidal datum to obtain the water level value based on LAT. In order to
delineate the littoral zone, several of datasets have been identified to derive the
tidal line.
2 Tidal Datum
Tides are the periodic vertical movement of water on the Earth's surface. It is also
defined as the periodic variation in the surface level of the oceans and of bays,
gulfs, inlets, and estuaries, caused by gravitational forces of the moon and sun. It
also can be defined as the rise and fall of sea level. The measurement of the tide is
called tide observation and the equipment used for the observation is called tide
stations. Tide gauge is the device that we use to measure the changes in sea level.
A tidal datum is a standard elevation defined by a certain phase of the tide.
Tidal datum is used as a reference level for measuring local water levels and
should not be extended into areas having differing oceanographic characteristics
without substantial measurements. The definition of tidal datum for Lowest
Astronomical Tide and Highest Astronomical Tide is given in Table
1
.
IHO resolves that Lowest Astronomical Tide (LAT) shall be adopted as chart
datum where tides have an appreciable effect on the water level. A tidal datum is
essentially the average of all the water elevations (high, low or mean) over an
18.6 year period. This eliminates most meteorological effects on water level. It is
defined more simplistically as averaged stages of the tide such as MSL, MHW and
MLLW. Tidal datum elevations vary significantly with geographic distance
especially in shallow water. They are used as references to measure local water
levels and should not be extended into areas having differing oceanographic
