Charts Flashcards
What is a Mercator projection
The principle of Mercator projections is that all rhumb lines are represented by straight lines.
A line which cuts all the meridians at the same angle is called a rhumb line!
• Used for normal navigation.
• Land features distorted as one moves north or south from equator.
• Latitude scale used for measuring distance, and because of distortion care should be taken to measure distance using scale corresponding to position on chart.
• Parallels of longitude equally spaced.
• Rhumb lines (course lines) are straight lines, Great Circles are curves.
• How to correct a chart, old style with Cumulative notice to mariners etc
• Look up current Cumulative List to check latest edition date against date of chart.
• Check for corrections that apply to the chart, listed in the Cumulative List.
• Check Weekly Notices to Mariners published since the Cumulative List for any more
recent corrections and any new editions.
• Look at bottom left hand corner of the chart and see which corrections have or have
not been applied to the chart.
Transverse Mercator
• Used for large scale charts, harbour plans.
• Drawn as a Mercator but turned through 90 degrees, thus parallels of latitude now
equally spaced.
• Parallels of longitude not equally spaced, land would become distorted as one moved
east or west from centre of chart – due to this only used for large-scale charts covering
small geographic areas.
• Distance measured on side as normal.
• What’s in the weekly notice to mariner
I Explanatory Notes. Publications List
II ADMIRALTY Notices to Mariners. Updates to Standard Nautical Charts
III Reprints of NAVAREA I Navigational Warnings
IV Updates to ADMIRALTY Sailing Directions
V Updates to ADMIRALTY List of Lights and Fog Signals
VI Updates to ADMIRALTY List of Radio Signals
VII Updates to Miscellaneous ADMIRALTY Nautical Publications
VIII Updates to ADMIRALTY Digital
Explain this light
Fl (3) WRG.15s21m15-11M
• Class of light: group flashing repeating a group of three flashes;
• Colours: white, red, green, exhibiting the different colours in defined sectors;
• Period: the time taken to exhibit one full sequence of 3 flashes and eclipses: 15
seconds;
• Elevation of light: 21 meters;
• Nominal range(s): white 15 M, green 11 M, red between 15 and 11 M, where “M”
stands for nautical miles.
You are required to fully understand all aspects of a light!
What is Nominal range
Nominal range is maximum distance at which a light can be seen for a meteorological visibility of 10 miles.
Details of these ranges are given in the Admiralty List of Lights.
Ways to find position
• GPS
• 3 radar ranges
• 3 point Fix
• Radar Range and bearing
Explain a VSA
• Take a bearing of the object that you can see on the chart and out of the window.
• Measure the angle between the top of the object and the waterline.
• Use Norie’s tables to calculate the range
• Hence I have a range and bearing and thus a fix.
• The height is measured from MHWS so for all other states of tide, I will be further away.
The More Complicated Information
• Find out HOT and the difference from MHWS and add this value to the Structure height.
• Don’t forget to adjust for Index error.
• Plus, correction for height of eye as if using Norie’s tables to be aware that the tabulated information is calculated for a height of eye greater than 12 meters, so if under this a correction for dip would be necessary as found in the nautical almanac.
Explain a HSA
The angular separation between three points is measured by using the sextant horizontally to give us the angle.
As an example, the HSA between A and B was measured to be 30o and the HSA between B and C to be 50o.
A line is drawn on the chart between A and B. Then draw a line from A which is (90o − HSAo) from this line so in this case a line is drawn at 90o − 30o = 60o from the line AB to seaward with the aid of a protractor.
Similarly draw another line 60o from BA. These lines intersect at D as shown in Figure 2 forming the triangle ABD.
A drawing compass is now used, by placing the center of the compass at D and a full circle is drawn with a radius AD or DB which are equal, see Figure 3. The circle will intersect A and B. This is known as the ship’s position circle.
The ship could be anywhere to seaward on this blue circle however it is not known at this stage where on this circle the ship is located hence the name position circle.
This method is now repeated by measuring the HSA between lighthouses B and C.
A HSA of 50o is measured between B and C and the second position circle is now plotted using the above method as shown in Figure 4. As before the ship must be anywhere to seaward on this green position circle for a HSA between B and C to be 50o. Since the ship must also be on the blue position circle as well, the only position where this qualifies is where both position circles intersect: the ship’s position is now plotted on the chart
Leeway
Leeway
The sideways drift caused by wind on the side of the vessel is given in degrees, calculated by looking at the wake. If the wind is from the north, called a northerly and the vessel is on a course of due East then the vessel would be pushed south. If working out an EP or position, then apply leeway to the course steered.
Explain course to steer
Think of a dog chasing a stick in a fast-flowing river. It does not know to aim upstream to get across in a straight line. When determining a course to steer, the tidal stream, current and the effect of wind/leeway must be predicted. Unlike the EP, the
leeway is not applied first but last. A line is drawn on the chart from the proposed start location to the desired destination this line extends past the end point. This is the ground track, we then can determine the likely duration of this leg and from this we can work out the amount of tide to be applied. Tidal information can be found from tidal diamonds, tidal streams or in the case of an exam it will be given in the question!
From the last known position lay of the intended ground track, extend this past the destination.
Apply the tidal vector from the same last known position. Then using a pair of dividers or compass mark out the distance you will travel in one hour across the intended ground track, this could be before or after the destination.
Draw a line from the end of the tidal vector to the point the arc made on the ground track. This is the water track.
Work out the leeway to be applied, add or subtract it depending on the direction of the wind. Once you have this course to steer then convert to compass.
Explain Running fix
A running fix is a method of fixing your position if only one or spaced apart reference points are available. First take a bearing of the object and note the time and log speed. Convert this to True bearing using the compass error for the ships head. Then run for a fixed amount of time take a second bearing of the same object and plot this on the chart. Using any start point on the first bearing line and lay of your heading including leeway for the time period measured. Then add in the tidal vector to find an EP. The final stage is to transfer the first position line across to run thorough the EP and extend it till it crosses the second position line. The resultant position is a fix.
