HND Nautical Science
OBSERVED POSITION:- The position of the vessel as marked on the Chart and is the ACTUAL position of the vessel on the Earths Surface, subject to the accuracy of the method used.
Written as Latitude and Longitude or Bearing and Distance from a known point
An observed position is commonly known as a FIX
Can be taken from: visual bearings
Echo sounder
Radar ranges / bearings
Electronic navigational aids(GPS, Loran etc)
Astronomical Navigation
Or a combination of any of the above.
EG: Your vessel obtains a GPS position of 40° 30’N 005° 30’ W at 0900.
0900
40° 30’N
005° 30’W
requires : a known starting point
a method of determining direction travelled (Compass)
a method of determining distance (log)
EG From the harbour your ship travels at a speed of 10 knots on a compass heading of 090° , where will the vessel be in 1 hour?
Distance = speed x time > 1hr x 10 kts = 10M
On the chart from the harbour, lay off a distance of 10M in a direction of 090°
N
time
Vessels DR position in 1 hours time
10m
090°
EG Your vessel is steering 090° at 10 kts and has taken a series of fixes by GPS, at 1000 the GPS breaks down, what is the EP.
DR positions( Co + Spd)
0800
1000
Estimated position at 1000
The Solid line is the vessels projected Track based on the vessels course and speed, the dotted line is the vessels ACTUAL TRACK based on the GPS observations, to find the estimated position for 1000, the actual direction and speed of the vessel ,( the Dotted line), is projected on from the 0900 Fix position, to obtain the ESTIMATED POSITION.
Note: the above is a good example of how an observed position(gps) can be compared with a DR, such a difference means there is another factor influencing the vessel, other than course and speed.
Finding your Estimated position using tidal information will be covered later.
Maintaining a DR or EP plot is vitally important as it allows the vessels future positions to be predicted.
It also acts as a check on your other position fixing methods, no single position fix should be relied on 100%, always cross check with another method if available - the means to provide a DR position should always be available.
If there is a large difference between the OBSERVED position and the DR or EP, then there must be a reason for it.
Eg Faulty plotting, electronic Aid faulty, faulty compass, unaccounted tide and or wind etc
With good experience of the sea, weather and the vessels characteristics an EP can be extremely accurate, prior to the adoption of Electronic nav aids, EP’s used in conjunction with Astro Nav were the only way to Navigate when out of site of the coast.
They should still be used by the Prudent Navigator.
Always cross check your position.
If sailing in the vicinity of Natural Geographical Features or Visible Navigational Aids, as well as using different fixing methods, LOOK OUT OF THE WINDOW, look at the Radar to see if the picture agrees with the chart.
Check your Echo Sounder to see if the actual depth agrees with the Chart.
For example if your GPS or Electronic Chart shows a Light buoy on your Starboard side, look to see if it actually is!! If you cannot see it or you see it in the wrong place then something must be wrong. Either the buoy is out of position or your fix is incorrect.
Over reliance on Electronic nav aids has been the cause of many groundings, when the simple check of looking out of the window would have indicated the vessels proximity to danger.
DO NOT PLACE ALL YOUR FAITH IN A SINGLE SYSTEM, PARTICULARLY ELECTRONIC AIDS, THEY ARE NOT INFALLIBLE
SHIPS HEAD,WATER TRACK AND GROUND TRACK
The Ships Head is the direction the ship is POINTING as indicated on the Compass.
Ships Head is expressed in Degrees,(either True, Compass or Gyro, these will be discussed later)
The Water Track is the direction the ship is MOVING THROUGH THE WATER
Water Track is expressed in Degrees True.
The Ground Track is the direction the ship is MOVING OVER THE SURFACE OF THE EARTH.
Ground Track is expressed in Degrees True
These three elements may be the same but are often different, depending on the effect that the wind and tide have on the vessel.
THEY SHOULD ALWAYS BE TREATED AS THREE SEPARATE ELEMENTS.
WATER TRACK
Visual Bearing: the best method to be used when near the coast.
A Compass bearing (several if possible) of a conspicuous object(s) is taken, and then plotted on the chart, the vessel must then be somewhere on that line.
Bearings are FROM the vessel TOWARDS the object.
Provided the Bearing is accurate, the Observer, i.e. you, must be somewhere on that line.
Plot the following example on the Chartlet
St Kirk church is bearing 090° . Using the compass rose on the chart, line up your rules with 090 and then move them to the church, draw a line from the church, you are somewhere on that line.
Mark the time down next to the line
If more than one bearing is plotted then a cross bearing position is obtained.(Three if possible)
Do the same with the other Objects.
Eilean Begg Brg 045°
Sore Head Lighthouse Brg 000°
Bearings are obtained ether from : Azimuth Ring on the compass Repeaters, Hand Held Bearing compass, Pelorus or Radar.
It is important to remember that every line that is drawn on the chart or taken off the chart is TRUE, this will be discussed in more depth later.
Range and Bearing: A Compass bearing combined with a Range from an Object to give a Position Fix.
See Chartlet
Sore Head +Lighthouse is Bearing 045° at 5M
Plot the Compass bearing as you did for the previous example.
Using your dividers measure a distance of 5 M, put the point of the divider on the object(the lighthouse) and draw an arc so that it cuts the bearing line, this is the vessels position.
Several ranges can be used in a similar way to cross Bearings, again it is best to use three if possible.
Eg: High Point 4.5M
Sgeir More 6.0M
Eilean Begg 12.5M
Ranges can be obtained from: Radar, Measured Distance, Range Diagrams, Sextant and Trigonometry
Transits:
When two objects are observed in line, that is, one behind the other, they are said to be in Transit.
If these two objects are marked on the chart then a position line can be obtained without using a compass.
Transit as seen from the ship:
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dariusz.lipinski