Although I
always love fishing, I must admit that water was not and probably will
never be my favorite subject. But having to deal on a weekly basis with
the US Coast Guard, you are bound to get some appreciation of our world
biggest natural resource.
When you work
in the computer field for as long as I have, where time is measured in
nanosecond and distance in micron or less, where state-of-the-art technology
span lasts but a year, getting exposed to the art of hydrographic surveying
(mapping and charting of oceans, lakes and rivers) was an eye opening,
to put it mildly.
The Exxon
Valdez disaster in Prince William Sound in 1989 and, more recently, the
Queen Elizabeth II grounding near Buzzards Bay in 1992 created a great
deal of concern on the adequacy and accuracy of the US coastal nautical
charts.
A survey team
found that the depth of the spot where the QE II went aground was around
33 feet; but the largest and best chart of the area showed at least 39
feet. As it turned out, the area was last surveyed in 1939. In fact, more
than 60% of the charts rely on data collected prior to 1940, and only 25%
of the charts have been produced with modern hydrographic survey equipment
(post-1960). It is not unusual for a hazard on a harbor approach chart
to be located as much as 75 meters from its indicated position. Furthermore,
it is difficult to measure points on the chart to an accuracy better than
about 1/50th of an inch (0.5 mm). Thus on a 1:40,000 scale chart, the mariner
must allow at least a 40 yards of uncertainty. It is estimated that to
survey all of the US coastal areas will require 100 years. Yet, our (US)
charts are models of accuracy and thorough coverage, as compared with other
nations. That is a scary thought, isn't it?
In simplistic
terms, hydrographic surveying consists of two major functions: the measurement
of the ocean bottom depth and the position determination of this location.
To appreciate the difficulties of this work let us consider one aspect
of charting the depth of the ocean.
Measuring
the depth is a relatively simple process. A soundwave is transmitted by
a depthfinder to the bottom of the ocean. When the pulse hits the bottom,
it bounces back. The depthfinder senses its return. Knowing the soundwave
velocity, its round trip time, the depth measurement can be easily calculated...in
theory!
One must realize
that not every square yard of the ocean bottom is measured: typically survey
ship makes passes (tracks) some 50 meters (165 feet) apart, but a good
depthfinder can only cover a path of about 12 to 15 feet wide with any
survey accuracy. In other word less than 10% of the area was surveyed,
to be charitable. To be fair, not every square yard of land is position
determined, before our road maps are made either. But, most obstacles on
land can be seen by drivers, while those rocks at the bottom of the ocean
are not.
Since soundwave
has a spherical shape, it can only accurately indicate the depth directly
under it. Furthermore, without accurate positioning, these depth measurements
are relatively worthless. Just picture yourself trying to keep your row
boat moving in a straight line. Of course survey ship is no row boat. But
with the wind and the current and the waves, and with no point of reference,
it is probably safe to say that for a pilot to keep the boat close to a
desired track is a chore. With the new dGPS technology, chart accuracy
will improve. Furthermore, sound velocity varies significantly with temperature.
Draft and height of tide and other weather related also play a big part
in the accuracy of the depth measurements. And schools of fish often cause
false depth indications.
It is amazing
that we can chart almost an exact course for our satellites to Saturn,
yet our nautical charts are so outdated they should be in museums. To be
fair, the ocean bottom probably takes centuries to change. On the other
hand, hydrographic survey equipment of the 1940's was not very accurate.
Necessary funds will be required to complete this all important task of
nautical charting. It is a wonder that we do not have more groundings.
Viet-Dung
Hoang, Ph.D.
[email protected]
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Copyright
© 1994 - 1997 by VACETS and Viet-Dung Hoang
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