VACETS Regular Technical Column

"Everyday Engineering"

"Everyday Engineering" was a technical column posted regularly on the VACETS forum. The Chair of of this column is Dr. Hoang Viet-Dung. For more publications produced by other VACETS  members, please visit the VACETS Member Publications page or Technical Columns page.

The VACETS Technical Column is contributed by various members , especially those of the VACETS Technical Affairs Committe. Articles are posted regulary on [email protected] forum. Please send questions, comments and suggestions to [email protected]

Mon, 28 Nov 94

The Limits of Nautical Charts

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 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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