Tuesday, April 22, 2008

Navigation Terms

Line Of Position (LOP): The points along which a ship's position must lie. A minimum of two LOP's are necessary to establish a fix. It is standard practice to use at least three LOP's.

Range or Distance LOP: Obtained by using a stadimeter, sextant or radar. A circle equal in radius to the measured distance is plotted about the navigation aid, the ship must be somewhere on this circle.

Running fix: A position determined by crossing lines of position obtained at different times and advanced or retired to a common time.

Dead reckoning: Determining a position by plotting courses and speeds from a known position. It is also used to predict when lights become visible or to determine the set and drift of a current. DR positions are drawn in advance to prevent sailing into danger. A DR position will be plotted:

1. Every hour on the hour

2. At the time of every course change or speed change

3. For the time at which a (running) fix is obtained, also a new course line will be plotted

4. For the time at which a single LOP is obtained

5. And never draw a new course line from an EP position

Estimated position: The most probable position of a craft determined from incomplete data or data of questionable accuracy. Such a position might be determined by applying a correction to the dead reckoning position, as for estimated current; by plotting a line of soundings; or by plotting a LOP of questionable accuracy.

Double angle on the bow: A method of obtaining a running fix by measuring the distance a vessel travels on a steady course while the relative bearing (right or left) of a fixed object doubles. The distance from the object at the time of the second bearing is equal to the run between bearings, neglecting drift.

Four point fix: A special case of doubling the angle on the bow, in which the first bearing is 45° right or left of the bow. Due to angular spread this is the most precise isosceles fix.

Special angle fix: A construction using special pairs of relative angles that give the distance travelled between bearings as equal to the navigation aids' range abeam.

Distance from horizon: The distance measured along the line of sight from a position above the surface of the earth to the visible horizon.

Sensible horizon: The circle of the celestial sphere formed by the intersection of the celestial sphere and a plane through the eye of the observer, and perpendicular to the zenith-nadir line.

Visible horizon: The line where Earth and sky appear to meet. If there were no terrestrial refraction, visible and geometrical horizons would coincide. Also called : apparent horizon.

Geometrical horizon: Originally, the celestial horizon; now more commonly the intersection of the celestial sphere and an infinite number of straight lines tangent to the earth's surface and radiating from the eye of the observer.

Dipping range or Geographic range: The maximum distance at which the curvature of the earth and terrestrial refraction permit an aid to navigation to be seen from a particular height of eye (without regard to the luminous intensity of the light).

Elevation: The height of the light above its chart datum in contrast to the height of the structure itself.

Chart Datum: Officially: Chart Sounding Datum: An arbitrary reference plane to which both heights of tides and water depths are expressed on a chart. In the same chart heights can be related to other datums than depths.

Vertical sextant angle: The method of using the subtended angle of a vertical object to find its range.

Index error: In a marine sextant the index error is primarily due to lack of parallelism of the index mirror and the horizon glass at zero reading. A positive index error is subtracted and a negative index error is added.

Estimation with horizon: Estimation of heights using the horizon: All tops crossing the horizon and with bases at sea level are on eye level.