Detailed Operational Example: Navigation With The Compass And Rangefinding Scales - Tasco OffShore Manual De Instrucciones

Detailed Operational Example:
Navigation with The Compass and Rangefinding Scales
The following examples use nautical situations to explain the use of the compass and Rangefinding scale features.
However, the same principles apply to navigation on land. Please refer to the illustrated figures on the next page
as you read the section below:
Used by itself, a compass will only indicate direction to or from an object but not relative position. To determine
position, you need a map or chart and a protractor of some type. Figs.1 and 2 illustrate this point. In Fig.1, the arrow
shows your general direction of travel but not your course or position. Fig.2 shows that you have taken a compass
reading from your boat to the buoy and that it is on a bearing of 190°. You now know your course is 190°, but you
still do not know your position.
To determine position, you should now lay in your course line FROM the buoy. Since your direction of travel is
190°, direction FROM the buoy will be 10° (this is known as back azimuth. Back azimuth is compass reading mi-
nus 180°). The next step is to take a bearing on a second fixed object. Fig.3 shows that the lighthouse lies
on a bearing of 300° from your position. Laying in the back azimuth of 120° from the lighthouse, your position will
be indicated by the intersection of the two lines.
NOTE: the illustration does not take into account errors in compass readings due to the earth's magnetic field or
errors induced by the effect of nearby metal objects.
The amount of correction required for errors due to the earth's magnetic field (know as variation) will be indicated
on your charts. Local disturbances from metal objects, such as engines, fittings, etc. (known as deviation) should
be determined from accurately known fixed positions.
Fig.4 illustrates the use of the rangefinding reticle. You are on a known course of 180° and you know that when you
are abeam of the lighthouse you should be 2 ½ miles from it. The lighthouse is known to be 86' high, and a range
reading shows that it occupies one division. Using the rangefinding calculations, you determine that you are 3.25
miles east of the lighthouse. You now know that you are east of your intended course by ¾ of a mile.
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