Boat navigation for the rest of us : finding your way by eye and electronics by Brogdon Bill

Author:Brogdon, Bill
Language: eng
Format: epub
Tags: Navigation -- United States, Electronics in navigation, Pilots and pilotage -- United States, Navigation, Pilots and pilotage, United States
ISBN: 0070081646
Publisher: Camden, Me. : International Marine/McGraw-Hill
Published: 1995-10-15T00:00:00+00:00

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Approximate position

Table captionFigure 5-13. The tank on Great Hill is shown by a small dot inside a circle, indicating that its position is accurate. A small circle without a dot indicates an approximate position.

60 feet of water can easily move 30 to 90 yards. In addition, it has a certain position tolerance since the buoy tender cannot place the sinker (anchor) in the exact same spot each time. Buoy symbols on charts include a small circle similar to the position-approximate symbol to emphasize that the buoy position is not precise. Storms, ice, strong currents, and passing vessels also move buoys from their correct positions. I don’t hesitate to use buoys for bearings, but the bearings to them aren’t as accurate or as reliable as bearings to lights.

Sometimes it is handy to take a bearing to the end of a point or the side of an island. Such bearings are called tangents since the line of bearing is tangent to the object (in the language of geometry). Tangent bearings may be good or lousy. If the side of the island is steep rock, a tangent bearing is a good one. On the other hand, avoid using tangents to any land that slopes gently into the sea; a beach extends farther out at low tide than at high tide, and shifts with storms and seasons.

Small, isolated rocks also give good bearings. Hydrographic surveyors find these positions with great accuracy, and it is easy to take a bearing to the center of the rock.

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