Saturday, March 8, 2008


A fix is defined as a point of intersection of two or more simultaneously obtained LOPs. The symbol for a fix is a small circle around the point of intersection. It is labeled with the time expressed in four digits. Fixes may be obtained using the following combinations of LOPs:

A line of bearing or tangent and a distance arc.
Two or more lines of bearing or tangents.
Two or more distance arcs.
Two or more ranges.
A range and a line of bearing or tangent.
A range and a distance arc.

Since two circles may intersect at two points, two distance arcs used to obtain a fix are not undesirable. The navigator in making his choice between two points of intersection may consider an approximate bearing, sounding, or his DR position. When a distance arc of one landmark and a bearing of another are used, the navigator may be faced with the problem of choosing between two points of intersection at the same location.

Three considerations in selecting landmarks or other aids for obtaining LOPs are:
Angle of intersection.
Number of objects.

Two LOPs crossing at nearly right angles will give you a fix with a smaller amount of error than two LOPs separated by less than 30° . If there is a small compass error or a slight error is made in reading the bearings, the result will be less in the case of the fix produced by widely separated LOPs than the fix from LOPs separated by only a few degrees.

If only two landmarks are used, any error in observation or identification may not be apparent. With three or more LOPs, each LOP acts as a check. If all intersect in a pinpoint or form a small triangle, you can generally rely on the fix. Where three LOPs are used, a spread of 60° would result in a better accuracy.

When selecting landmarks or other aids, preference should be given to permanent structures such as lighthouses. Buoys, are less permanent and may drift from their charted position because of weather and sea conditions.

The navigator sometimes has no choice of landmarks or their permanency, number, or spread. In this case you must use whatever is available, no matter how undesirable. In the evaluation of his fix, the number of landmarks, their permanency, and their spread should be consideration. When three LOPs cross forming a triangle, it is difficult to determine whether the triangle is the result of a compass error or an erroneous LOP. The plotting of four LOPs usually indicates if a LOP is in error.