ECTRONIC TIDE GAUGES have been an industry standard for many years. Most gauges provide reliable real-time digital (or water level gauges)
water level data of a few centimetres, at variable time
increments that can be set to meet project requirements.
The problem is that tide gauges can only report the
tidal condition at the place where they are located and
cannot define swell conditions. If a project is located
near the ocean, or in a river, significant differences will
occur over relatively short distances, depending on the
configuration of the water body. One must be careful
to be sure that the water level conditions at the gauge
match the conditions in the area of the bathymetric
survey. It is a historical fact that the most common
source of error in any hydrographic survey is the
application of the tidal data — with the assumption
that the tidal surface at the site mirrors that at the
tide gauge.
Normally, tide is considered as a nuisance parameter
but it is a critical component of the marine environment.
The key factor for tide measurements is the vertical
reference surface. Development of a stable surface is a
vital step in being able to handle modern bathymetric
depth data and use it to its fullest. Chart datum is the
traditional surface to which depths are referred. However
chart datum is not a seamless reference surface as it
varies from location to location. Chart datum is based on
local water level measurements at discrete locations. It is
selected as a surface that is so low that the tide will not
frequently fall below it, but not so low as to be
unrealistic and only gradually varying between adjacent
datums. Figure 1 demonstrates the common vertical
reference surfaces.
In spite of numerous country-defined designations of
allowable tolerances for vertical control during
hydrographic surveys, the International Hydrographic
Organization (IHO) has developed a set of standards that
have been adopted by many member countries for SOLAS
(safety of life at sea) nautical charting (Table 1). From
this, we can roughly equate a fixed ‘a’ value for each
order to the allowable positioning error.
Using techniques such as RTK-GPS, centimetre level
accuracies are attainable over baselines of 20km length
from the base station. Using RTK-GPS allows the hydrographer to reference soundings directly to the WGS84 GPS
ellipsoid; thereby completely avoiding the tidal reduction process, assuming that
the separation between ellipsoid and chart datum is well known at the survey
site. Transfer of these soundings to chart datum then only requires the application
of the one-dimensional shift, ignoring the time offset. This shift, however, can be
cumbersome and a clear understanding of the vertical datum is required. |
