There are numerous ways that a vessels position may be fixed on the chart, and the mariner should use as many methods as possible to determine the vessels position at all times.
Some examples of position fixing methods are give below:
1. Two or more simultaneous visual bearings of charted objects
2. Ranges of two or more charted objects
3. Visual bearing and range of a charted object
4. Horizontal sextant angles of three charted objects
5. Running fix off one or more charted objects
6. Celestial observations of heavenly bodies
7. Latitude and longitude readout from a position sensor such as GPS or Loran
When deciding on the method of position fixing to be used the mariner always has to take into account the accuracy of each method and wherever possible should not rely on a single method to determine the vessels position.
If at all possible the position obtained by one method should be checked using other methods.
Failure to do so has lead to countless groundings and strandings in recent years and this it is extremely dangerous. The MCA has repeatedly emphasised this point in M Notices and Casualty reports issued by the Marine Accident Investigation Branch. (MAIB)
(these reports can be found in the College Library and are well worth reading).
All of the above methods are liable to errors and the mariner must also be aware of what these errors are and which method is appropriate in different circumstances.
In this particular unit we are concerned with navigating a vessel in coastal waters, where we can expect to be close to land and to either see the land visually or at least on radar.
In this situation it has been suggested that the most accurate method of fixing the vessels position is as follows:
5. Radar bearings of two or more charted objects
6. Running fix off one or more charted objects
i.e. wherever possible the mariner should attempt to get two or more
visual bearings of charted objects.
Note the bearings may be Compass bearings, Magnetic bearings or Gyro bearings, however only TRUE Bearings must be laid off on the chart, therefore any errors in the compass must be applied correctly before being laid off on the chart.
The following visual bearings were obtained
MPC Buoy brg 275o (T)
Sandettie Lt Buoy brg 020o (T)
Dunkerque Lt Buoy brg 117o (T)
Using the compass rose on the chart (remember to use the outer scale for true bearings) and parallel rulers we lay off the bearings through each of the charted objects and we can see that all three of the bearing lines (position lines) cross at a point.
The point of intersection must be where the vessel is.
We can read the latitude and longitude of the vessels position off the lat and Long scales on the chart.
51 05.3’N , 001 44.7’E
A vessel makes the following observation of Dunkerque Light Buoy
Bearing 070o (T)
Radar range of buoy from radar 5.5 nm
Find the vessels position?
Using the compass rose on the chart lay off the true bearing through the light buoy.
Draw a circle of radius 5.5 nm from the light buoy.
Where the bearing line and the circle intercept is where the vessel will be.
Position = 51o 01.2’N , 001o 42.9’E
A vessel obtains the following ranges from the radar
MPC buoy 4.9 nm
Sandettie Buoy 5.2 nm
Centred on Sandettie light buoy, draw a circle of radius 5.2 nm.
Draw a similar circle of radius 4.9 nm centred on the MPC buoy.
Where the two circles intersect will be the position of the vessel.
Position = 51o 04.3’N , 001o 44.6’E
The other methods of position fixing are the subject of later lessons and will be dealt with then.
Lesson 3.doc Fixing Position DGR1999
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dariusz.lipinski