ARTICLE TAKEN FROM

ARTICLE TAKEN FROM:

“NATIONAL ASSOCIATION OF CHARTER BOAT OPERATOR’S” NEWSLETTER on July 10, 2004

“On October 22, 2003 a 35 foot vessel owned by the State of Maine and

operated by a seasoned captain ran aground and sank on Seat Ledge in

Penobscot Bay near Vinalhaven Island. During an investigation of the

incident conducted by the US Coast Guard it was determined that the operator

of the vessel was navigating with a recently installed GPS system. The

operator was utilizing the GPS unit's "course to steer" function along a set

of waypoints previously entered into the GPS system.

The waypoints in use, had previously been entered by the operator through a

practice known as “relative navigation,” which involves observing a position

with an electronic navigation system and entering that position as a

waypoint.

Navigators of vessels equipped with GPS units are advised that the current

system accuracies for GPS units are approximately 328 feet (100 meters) when

attempting to navigate to a known geographic point (latitude and longitude

calculated from a nautical chart) and approximately 463 feet (141 meters)

when attempting to return to a position previously observed with a GPS unit.

These distances are 95% accuracies; meaning that 95% of received position

fixes will fall within 100 meters of calculated geographic points (specific

navigation) and within 141 meters of observed geographic points (relative

navigation). More accurate position fixes may be obtained but mariners

should allow for the 95% accuracy values.

Mariners familiar with Loran C navigation systems are specifically cautioned

that the Loran C system provides slightly better repeatability accuracies

when utilizing relative navigation practices. Care should be taken when

switching to GPS units to insure that appropriate system accuracies are

compensated for.

All mariners are advised that when operating in the vicinity of navigational

hazards vessels must be navigated with regard for the vessels position in

relation to the hazard. Any electronic navigation system, including GPS, may

be subject to occasional errors in excess of designed system accuracies and

should not be solely relied upon to determine a vessel's position.

The U.S. Coast Guard advises against the use of relative navigation

practices in the vicinity of navigational hazards. The recommended method

for calculating voyage waypoints is to calculate the positions directly from

a nautical chart.

Capt Rod & Susie Stebbins

8626 Chadwick Drive,

Tampa, FL 33635-6209

813-855-6226 Voice

813-855-1986 Fax