Position fixing
Position Lines
A
position line may be defined as a line on the earth’s surface on which
the vessel lies. A single position line does not give a position. When
we have two lines taken at the same time, the vessel is on each line -
it’s the intersection of the two lines. This is a position. Three or more position lines increases the accuracy of the position.
Position lines may be obtained by various means:
- Terrestrial
- Compass bearing line
- Relative bearing line
- Transit
- Horizontal sextant angles
- Range from vertical sextant angle and raising and dipping of lights
- Electronic
- Radar
- eLoran (hyperbolic navigation systems)
- Line of soundings
- Celestial
- Astronomical observations of heavenly bodies.
Electronic
- Radar
The electronic bearing line (EBL) of a radar gives a bearing which may be 'compass' or 'relative'.
- Hyperbolic Navigation Systems
Hyperbolic
navigation systems use ground stations which transmit signals to a
shipboard receiver from which a LoP (line of position) is obtained.
- Line of Soundings
A vessel crossing two depth zones - this gives an approximate position line, by using the depth sounding instrument.
Positions
A
ships position consists of the geographical coordinates of latitude and
longitude as corrected for the datum in use; e.g. WGS834. The position
of a ship is classified into three types on the basis of accuracy.
- Dead Reckoning Position (DR)
- Estimated Position (EP)
- Observed Position
Dead Reckoning Position (DR)
It
is the position of a vessel obtained only from plotting the true course
steered and the speed through the water. No other factors are
considered. It is marked with a small cross or a small line on a track.
Estimated Position (EP)
It
is more accurate than a DR position and is obtained by calculating the
DR position and then adjusting it to take into account known effects of
leeway, set and drift. It may be indicated by a cross within a circle or
a triangle
Observed Position
The
actual position of the ship obtained by the intersection of at least
two position lines or from a GPS/DGPS instrument. It is shown on the
chart as a dot within a circle.
Position Accuracy
It
must be always borne in mind that a position is only as good as its
accuracy. While DR and EPs are identified as positions which lack
accuracy, an observed position also has varying levels of accuracy.
While it is difficult to grade observed positions with regard to their
accuracy a loose list can be made in descending degrees of accuracy.
- 3 – 4 satellite DGPS position
- 2 satellite GPS position
- Hyperbolic eLoran position
- Radar range and visual/racon bearing
- Radar range and bearing
- Cross bearings
- HSA and VSA
- Running fix
- Celestial observations
- Sounding line and bearing line
Cross
bearings of all types are susceptible to an error caused by the
vessel's run, in between the time taken to take the set of bearings. It
is evident when three bearings are crossed and gives rise to a triangle
within which the ship lies called a 'cocked hat'. The larger the time
interval the larger the hat. The point in the hat or triangle is to be
taken as the observed position.
Errors that cause a 'cocked hat' to form:
- Error in the plot
- Too much time between bearings
- Error in deviation and variation
Let us now revise Position Lines, or Lines of Position, and look at fixing by terrestrial and celestial bearings.
POSITION LINES
A
line on which the position of the ship is known to be is called a
position line, or a line of position. The bearing of an object will
therefore be a position line.
A
single position line does not give a position. When we have two lines
taken at the same time, the vessel is on each line - it is at the
intersection of the two lines. This is often referred to as a fix. Three
or more position lines increases the accuracy of the position.
Position lines may be obtained by various methods such as:
Electronic
- Radar
- eLoran (hyperbolic navigation system - awaiting introduction)
- Line of soundings
Position by cross bearing.
Example
From a ship steering 
point A bore 
, point B bore 
and point C bore 
. Draw the ships position.
point A bore , point B bore and point C bore . Draw the ships position.Remember
Always
correct the compass course or bearing of an object and obtain the true
value, as on the chart we only draw a true course or bearing. The thumb
rule for correcting the value of a magnetic compass bearing to true is
EASTERLY ADD and WESTERLY SUBTRACT.
The
error of the magnetic compass is always for a particular ship's head.
Every ship should have a deviation card and the magnetic variation for a
particular place is mentioned on the chart.
If the gyro error is say 
high it has to be subtracted from the gyro course to obtain the true course and vice verse if the gyro error is low.
high it has to be subtracted from the gyro course to obtain the true course and vice verse if the gyro error is low.The ship's position on the chart is always stated in terms of latitude and longitude.
Solution
Draw bearing to point A (this forms a position line). Similarly position lines will be formed by laying and bearing
to points B and C respectively. The intersection of the three lines is
the position of the ship. The position of the ship may now be read off
in terms of latitude and longitude with the help of a parallel ruler and
a divider.
bearing to point A (this forms a position line). Similarly position lines will be formed by laying and bearing
to points B and C respectively. The intersection of the three lines is
the position of the ship. The position of the ship may now be read off
in terms of latitude and longitude with the help of a parallel ruler and
a divider.
The above example demonstrates the interception of the position lines obtained from terrestrial objects.
No comments:
Post a Comment