CWO Zeezeilschool | RYA Training Centre | Jachtverhuur
cumlaudesailing.com | Est. 2000

Lesson 6 Position fixing

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 bearing. 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 bearing is not reliable. Even better is to make 3 bearing. If the 3 lines on the map are not about the same point, something is wrong. Always make the bearing 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 bearing, 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!

kruispeiling

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?

Explanation:

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°

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.

Luminous range diagram

The nominal range of a lighthouse can be found in the list of lights and in the nautical chart. That nominal range applies at a sight of 10 Nm. If the visibility is better, then of course a light can be seen from a greater distance. That distance / range that is determined by the sight is called the "luminous range" and we can look it up in the graph below.

If you would draw a vertical line at the nominal range of 12Nm to the graph of 10Nm visibility and from that intersection a horizontal line to read the luminous range, then it is immediately noticeable that the luminous range is equal to the nominal range, 12Nm. This is because the nominal range applies at a normal sight of 10Nm.

luminous range

Example 1: Luminous range

Suppose a lighthouse has a nominal range of 18 Nm. Visibility is 20Nm. What is the luminous range?

Answer: 30Nm

Example 2: Luminous range

Suppose you want to use a light to make a bearing and therefore need a luminous range of 5Nm. The nominal range of light is 6Nm. How much visibility do you need to be able to make a bearing on the lighthouse?

Answer: At least 7.5 Nm

Bearings to stay clear of dangers

Background bearings

For example: if you can no longer see between two islands, then you know on which line in the nautical chart you are. You also could use a "leading line" of two objects.

NMT or NLT bearings

We can also draw a bearing line on 1 object in the nautical chart that separates a safe and a dangerous sector. T stay in safe water, we then determine whether the bearing:

  • should be no more than x degrees (No more than)
  • must be no less than an x ​​number of degrees (no less than)

We note NMT or NLT at the bearing line.

Running fix

  1. Take a first bearing, calculate the true bearing, draw in in the nautical chart. Write down course, time and log.​
  2. Draw a second true bearing line if the angle is more or less 90 degrees. Write down the course, time an dlog again.​
  3. Draw from any point on the first bearing line, your dead reckoning construction. ​
  4. Move the first bearingline to the end of that dead reckoning construction to find your position fix.​

Snellius method


We use the snellius method if we want to determine our position very accurately in case of an unknown error of deviation and variation. This method uses the angles between the bearings, so the bearings themselves do not have to be exactly correct (so an unknown error of the compass does not affect the fix. There is a very complicated construction for this, but we can also draw the three bearings on a transparent piece of paper and slide it over the nautical chart. The bearings only fit the objects in one way. The intersection of the three bearing lines is of course the fix.

Heeling error


The heeling error is the deviation that the compass gets from the heeling of the ship. The vertically placed magnetic objects normally do not disturb the compass because the magnetic force is exactly downwards. When the ship starts heeling, that magnetism will start to change the compass needle because the magnetic force is also sidewards. To calculate the heeling error, determine the heeling error on the compass heading 0 ° = north. Then determine the course factor (with graph or by entering the course on your calculator and then pressing COS). Determine the heeling factor, this is the heeling error / heeling error used at Compass heading (KK) 0°. The heeling error = heeling error at kk 0 ° x course factor x heeling factor. The heeling error doubles as the heeling doubles. When heeling over the other bow, + becomes - and vice versa.

Dip error


The magnetic fields come upwards from the South Pole, at the equator, the magnetic fields follow the curvature of the Earth, then the magnetic fields go downwards into the North Pole. The compass needle follows the up- or downward direction of the magnetic fields. If the compass is constructed for the Northern Hemisphere, where a downward direction applies, then that compass may deviate in the Southern Hemisphere, where the magnetic fields go upward.

Download and print the nautical chart to make the following excersizes. You are starting from 54° 5,0’N en 008° 05,0’E and you are sailing a true course of 320°. Theere is a western wind and you estimate the leeway 8°. The boat speed is 7 knots. After one hour you make a crossbearing and calculate the true bearings as follows. Lighthouse of Zeedam 54° true Church of Zoutdam 102° true. There is an unknown tidal current.

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