Lesson 7 Position fixing | Coastal Navigation

Cross bearing

We can determine our position by means of a cross-bearing. We make a compass bearing of a point that we can recognize in the map as well as on the coast, eventually using the Light list (HP2). We convert this compass bearing as a true bearing. We plot that true bearing into the nautical chart, as a line to the point on the map on which we have made the poll. We know then that we are at least on that line. We just do not know where on that line. So we need a second sounding line at another recognizable point. The crossing of two survey lines is our position. It is important that we allow the angle between the two bearing to be as perpendicular as possible, through the right choice of recognizable bearing objects. Otherwise the poll is not reliable. Even better is to make 3 polls. If the 3 lines on the map are not about the same point, something is wrong. Always make the poll that changes the least. A bearing at a point perpendicular to the direction of the boat will change more quickly to a point at a point straight ahead or right behind the boat. If we make a poll, do not forget to settle the variation and deviation (only if the bearing is made on the steering compass). If we make the bearing with the hand-level compass, then we assume that the deviation is 0 because we do not have a deviation table for the hand-level compass.

We have to lookup the deviation at the compass course, not the compass bearing!


Check your deviation with leading lights

We are able to check the deviation in the deviation table that belongs to our compass course by making a bearing on the leading lights, at the moment that we are exactly on the leading lights. You can now compare the compass bearing with the true bearing that you can find in the nautical chart. The variation is also in the nautical chart. the deviation is the last unknown number in the calculation of "True virgins make dull company".
For example... You make a bearing on the leading lights of Enkhuizen of 36°. In the nautical chart you see that also the true bearing on those leading lights is 36. Your compass course is 270°. In the variation compass in the nautical chart you find a variation of –2°. What deviation belongs to the compass course of 270°?
Next you make another bearing. on a compass course of 90°, you make a bearing of 42° on the leading lights. What deviation is applicable on the compass course?


Compass course 270°

Compass Bearing   36°
Dev +2°
Magnetic bearing 38°
Variation -2°
True bearing 36°

Compass course 90°

Compass Bearing 42°
Dev -4°
Magnetic bearing 38°
Variation -2°
True bearing 36°

From here is for Day Skipper only

Distance to light when rising dipping

In the nautical sea chart it is written how far (in nautical miles) the light of the light house will be visible maximum, depending on how strong is the light. This is also written in the HP 2 light list. In the nautical chart we could find a big M for the number of sea miles we could see the lighthouse from. The small m is the number of meters the light is above mean sea level (most of the times). The elevation, the distance from the light above the mean sea level can be different from the height of the lighthouse, of the building. That is for example the case if the lighthouse is on top of a high cliff.

The curvation of the earth causes us to lose sight of the lighthouse, sooner on small boats. In case we just start seeing a lighthouse for the first time after it was hidden after the horizon, we are able to calculate the distance to that lighthouse with this formula: 2,1 x (nth root of the height of the light + nth root of the height of the eyes above the water). The distance will be in Nautical miles. height of the eyes above the waterline is in meters. The also is a tabel so you can lookup the distance without calculating anything.

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Questions and answers

Download and print the nautical chart. You are sailing from 54° 5,0’N and 008° 05,0’E a true course of 320°. There is a westen wind and you estimate the drift on 8°. The speed trough water is 7 knots. After one hour you make two bearings convert those to true bearings. True bearing 1 on lighthouse of Zeedam 54° and true bearing 2 on the church of Zoutdam: 102°. There is an unknown tidal current.

Question 1: What is the latitude of the cross-fix?

Question 2: What is the longitude of the cross-fix?

Question 3: What would have been your latitude without tidal current?

Question 4: What would have been your longitude without tidal current?

Question 6: What has been the direction unknown tidal current?

Question 5: What has been the unknown tidal current rate?

Question 7: What has been the course over ground?

Question 8: The height of a lighthouse of which the light has just become visible at the horizon, is 49 meter. The height of the eyes is 2 meter. What is the distance to the lighthouse in Nautical miles?

Question 9: After how many miles distance you will lose sight of the light house that is 49 Nm height when your eyes are 4 meters above the water?

Question 10: What is the reason you don't see a light house anymore in the following case. The height of eyes above water is 4 meter and the height of the light house is 49 meter and the light is strong enough to shine 25 Nm?

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