Chart Work & Navigation

Every position on the Earth’s surface can be described using two coordinates: latitude (how far north or south of the equator) and longitude (how far east or west of the Prime Meridian at Greenwich).

Latitude is measured in degrees from 0° (the equator) to 90°N (North Pole) or 90°S (South Pole). Lines of latitude run east–west and are called parallels.

Longitude is measured from 0° (Greenwich) to 180°E or 180°W. Lines of longitude run north–south and are called meridians. They converge at the poles.

Reading coordinates: Positions are written as degrees, minutes, and decimals of minutes. For example: 50° 43’.5N 001° 18’.2W (approximately the Needles, Isle of Wight).

• 1 degree = 60 minutes of arc
• 1 minute of latitude = 1 nautical mile (1852 metres)
• Always read latitude from the side scale and longitude from the top/bottom scale of the chart.
• Use dividers placed on the latitude scale at the same level as your position to measure distance.

Admiralty charts use a standardised set of symbols and abbreviations (detailed in Chart 5011 / NP5011). Knowing the most common ones is essential for safe navigation.

Depth & seabed:

Hazards:

Buoys and lights: Chart symbols show the shape, colour, and topmark of buoys. Light characteristics are abbreviated — e.g. Fl.R.5s means “Flashing Red every 5 seconds.”

When navigating, you need to convert between different “norths.” The chart shows True bearings (relative to the geographic North Pole), but your steering compass shows Compass bearings, which are affected by variation and deviation.

The five stages — TVMDC:

Applying the corrections:

• Chart to compass (True → Compass): Start with True, apply Variation to get Magnetic, apply Deviation to get Compass.
• Compass to chart (Compass → True): Reverse the process.
• Mnemonic: “Timely Viking Made Delicious Cakes” or “True Virgins Make Dull Companions”
• Going down (T→C): add westerly errors, subtract easterly errors.
• Going up (C→T): add easterly, subtract westerly — “Error West, Compass Best; Error East, Compass Least.”

Knowing where you are is the most fundamental task in navigation. There are several methods to fix your position, each with different levels of accuracy.

Methods of fixing position:

Three-point fix technique:

1. Identify three charted objects spread at least 30° apart (ideally ~120° between each).
2. Take compass bearings to each in quick succession.
3. Convert to true bearings (using TVMDC).
4. Plot the reciprocals on the chart — where the three lines meet (or form a small triangle, the “cocked hat”) is your position.
5. If the cocked hat is large, your bearings are unreliable — retake them.

Once you know your position and destination, you need to work out the Course to Steer (CTS) — the compass heading that accounts for tidal streams and leeway so you actually arrive where you intend.

Key terms:

• COG — Course Over the Ground: the actual track your boat follows across the seabed.
• SOG — Speed Over the Ground: your speed relative to the seabed (includes tidal effect).
• CTS — Course to Steer: the heading you point the boat to achieve the desired COG.
• Boat speed — Speed through the water (from log or estimated).
• Tidal set & drift — The direction (set) and speed (drift) of the tidal stream.
• Leeway — Sideways drift caused by the wind pushing the boat downwind.

Plotting a CTS using a tidal vector triangle:

1. Draw the ground track — a line from your start position (A) to your destination (B).
2. From A, plot the tidal vector — the direction and distance the tide will carry you in one hour. Mark this point C.
3. Set your dividers to your boat speed (distance through the water in one hour).
4. With one point on C, swing an arc to cut the A–B line. Where it crosses is point D.
5. The direction from C to D is your water track — this is the CTS (in true). Convert to compass using TVMDC.
6. The distance A to D is your SOG for that hour.