Basics of Chart Work – Terrestrial Navigation 1

Chartwork is the foundation of traditional marine navigation. Even in the age of GPS, every deck officer must understand how to plot a ship’s position using symbols like Dead Reckoning (DR), Estimated Position (EP), Fix, Course Steered, Course Made Good (CMG), and Current (Set & Drift). These symbols help you track where the vessel should be, where it actually is, and how wind or current is affecting its movement.

Symbol	Meaning	Description
Horizontal line with vertical tick + time (e.g., 0830)	Dead Reckoning Position (DR)	A predicted position based on course and speed over time, without external fixes. Used when no fix is available.
Triangle + time	Estimated Position (EP)	A position based on dead reckoning plus additional information (like wind, current), but still not confirmed by a fix.
Circle with dot + time	Fix (Known Position)	A confirmed position determined by visual bearings or celestial observations. Most reliable.
Square with crosshairs + time	Electronic Fix	A position obtained using electronic navigation systems like GPS, radar, or echo sounder.
Single arrow + direction (e.g., 095°T)	Course Steered	The direction the vessel is intended to travel through the water, based on compass heading.
Double arrow + direction	Course Made Good	The actual direction traveled over ground, considering effects like wind and current.
Triple arrow + direction	Current (Set)	The direction in which the water is moving, affecting the vessel’s movement over ground.

Dead reckoning position (DR)

Symbol: Straight line track with a short vertical tick and time (e.g., 0830).

Meaning: A DR position is where you predict the ship to be based only on:

• Last known position
• Course steered
• Speed through water
• Time run

No allowance for wind, current, steering error, or leeway is made, so it’s approximate.

Example (practical):

• At 0800 your fix is: 20∘00′N,088∘00′E.
• You steer course 090∘T at 10 knots.
• After 30 minutes (0.5 hours), DR distance = 10×0.5=5 nautical miles.
• On the chart, you measure 5 NM along 090°T from the 0800 position.
• That point is marked with the DR symbol and time “0830”.

This is your theoretical position if nothing else (wind/current) acted on the vessel.

Estimated position (EP)

Symbol: Triangle with time.

Meaning: EP is a refined DR that includes your best estimate of errors due to:

• Wind
• Current (set and drift)
• Leeway
• Helm/steering error

But it’s still not a true fix – it’s your best guess, adjusted using whatever info you have.

Example:

• From DR, at 0830 you should be 5 NM due east of your 0800 fix.
• You know there is a current setting 180∘T at 2 knots.
• In 0.5 hours, current carries you 1 NM south.
• On the chart:
  • Plot DR position (5 NM east).
  • From that DR point, plot 1 NM south for current.
• Mark that resulting position with a triangle and “0830”.

That triangle is the EP – more realistic than DR, but not confirmed.

Fix (known position)

Symbol: Circle with a dot in the center + time.

Meaning: A fix is a position determined from actual observations, not just calculations. Traditionally:

• Intersection of 2–3 visual bearings to shore objects (lighthouse, tower, buoy).
• Or celestial observations (sun, stars) reduced to lines of position.

It’s considered a reliable, known position (more reliable than DR or EP).

Example:

• At about 0830, you take bearings:
  • Lighthouse A: 045°T
  • Buoy B: 120°T
• You plot both lines of position on the chart.
• Where they cross is your fix.
• Mark it with the circle-and-dot symbol and write “0832”.

This is your actual position at that time.

Electronic fix

Symbol: Square with crosshairs + time.

Meaning: A fix obtained from electronic means like:

• GPS/GNSS
• Radar ranges and bearings
• Echo sounder cross-checks with contour lines
• Loran/Decca (historically)

Electronic fixes are usually very accurate but must be checked against chart datum and system errors.

Example:

• GPS at 0830 gives: 20∘00.5′N,088∘05.2′E.
• You plot that point directly on the chart.
• Mark it with a square-with-crosshairs symbol and write “0830”.

If you also have a visual fix nearby, you might compare them to see if GPS/chart are aligned correctly.

Course steered

Symbol: Single arrow along your intended track, marked “095°T”.

Meaning: This is the direction you’re steering through the water, based on your compass/gyro.

• Expressed in degrees True on the chart (hence “T”).
• It’s the intended or ordered course – what the helmsman is holding.

Example:

• Master orders: “Steer 095°”.
• You convert from compass/gyro to True and get 095°T.
• On the chart, from your last fix (0800), draw a line in direction 095°T.
• Mark it with a single arrow and label “Cst 095°T”.

That line is your planned track through the water, not necessarily what you actually make over ground.

Course made good (CMG)

Symbol: Double arrow along the actual track, marked “095°T” (or whatever the result is).

Meaning: CMG is the actual direction over the ground between two known positions (usually fixes).

• It includes all effects of current, wind, leeway, steering errors, etc.
• Calculated from the line joining the earlier fix to the later fix.

Example:

• Fix at 0800.
• Next fix at 0830 (from GPS or visual).
• Draw a straight line from 0800 fix to 0830 fix.
• Measure the bearing of that line: suppose it’s 100°T, not 095°T.
• Mark that line with a double arrow and label “CMG 100°T”.

You see that although you steered 095°T, the ship actually traveled along 100°T due to current/wind.

Current (set)

Symbol: Triple arrow showing direction, marked “095°T” (or whatever the set is).

Meaning: In plotting, the triple arrow represents current:

• Set: Direction the water is flowing (in degrees True).
• Drift: Speed of current (in knots) – usually written nearby.

Together, they explain the difference between course steered and course made good.

Example (linking all three):

1. At 0800: Fix at point A.
2. Course steered: 095°T at 10 knots.
3. At 0830: Fix at point B.
4. You find:
   • Distance from A to DR position (based on 095°T, 10 knots, 0.5 h) is 5 NM.
   • Actual fix at 0830 is, say, 1.5 NM south of the DR position.
5. You plot a vector from DR position to actual fix:
   • Direction of that arrow = set of current, say 180°T.
   • Length represents drift, e.g., 3 knots (over 0.5 h gives 1.5 NM).

You draw that as a triple-arrow vector from DR to fix and label “Set 180°T, Drift 3 kn”.

How they all work together in one scenario

Imagine you are teaching this in class:

1. 0800 fix: Mark with circle-and-dot.
2. Course steered: Draw a line 095°T, 10 knots. Mark single arrow.
3. 0830 DR: 5 NM along that line → DR symbol with “0830”.
4. Current known from tide tables: Set 180°T, 3 knots.
5. EP at 0830: From DR, 1.5 NM south (due to current) → triangle, “0830”.
6. Electronic fix at 0831 via GPS: Slightly different again → square with crosshairs, “0831”.
7. CMG: Connect 0800 fix and 0831 electronic fix, bearing maybe 098°T → double arrow.
8. Current vector: Triple arrow from DR to actual fix showing the set/drift.

This is the classic vector triangle: Course steered + current = course made good.

🧭 Exercise Set 1: DR, EP, Fix, CMG & Current (Beginner Level)

Exercise 1 — Basic DR & Fix

Given:

• 0800 Fix: 20∘00.0′N, 088∘00.0′E
• Course steered: 090°T
• Speed: 10 knots
• Next visual fix at 0830 from bearings:
  • Lighthouse A: 045°T
  • Buoy B: 120°T

Tasks:

1. Plot the 0800 fix.
2. From the fix, draw the course steered 090°T.
3. Calculate DR distance for 30 minutes and plot 0830 DR.
4. Plot the 0830 fix using the two bearings.
5. Draw the CMG between 0800 and 0830 fixes.
6. Compare DR vs Fix and comment on possible current.

🧭 Exercise 2 — DR, EP & Known Current

Given:

• 1000 Fix: 19∘50.0′N, 087∘40.0′E
• Course steered: 135°T
• Speed: 12 knots
• Known tidal current:
  • Set: 090°T
  • Drift: 2 knots
• Time of EP: 1030

Tasks:

1. Plot the 1000 fix.
2. Plot DR for 30 minutes (distance = 6 NM).
3. From the DR point, apply current vector (1 NM east).
4. Mark the resulting point as EP 1030.
5. Label all vectors clearly.

🧭 Exercise 3 — DR, EP, Electronic Fix & Current Vector

Given:

• 1200 Fix: 18∘40.0′N, 087∘20.0′E
• Course steered: 210°T
• Speed: 8 knots
• Known wind causing 2° leeway to port
• GPS electronic fix at 1230: 18∘35.5′N, 087∘18.0′E

Tasks:

1. Plot the 1200 fix.
2. Apply leeway: new course steered = 208°T.
3. Plot DR for 30 minutes (distance = 4 NM).
4. Apply estimated wind/current to get EP.
5. Plot the 1230 electronic fix.
6. Draw CMG between 1200 and 1230 fixes.
7. From DR to Fix, draw the current vector (triple arrow).
8. Label set & drift.

🧭 Exercise 4 — Full Vector Triangle (Advanced)

Given:

• 1400 Fix: 18∘10.0′N, 087∘00.0′E
• Course steered: 300°T
• Speed: 14 knots
• At 1430, radar ranges give a fix at: 18∘13.0′N, 086∘58.0′E

Tasks:

1. Plot the 1400 fix.
2. Plot DR for 30 minutes (distance = 7 NM).
3. Plot the 1430 radar fix.
4. Draw CMG between 1400 and 1430 fixes.
5. From DR to Fix, plot the current vector.
6. Determine:
   • Set (direction of current)
   • Drift (speed of current)
7. Label all symbols correctly.

🧭 Exercise 5 — Mixed Fixes & Error Analysis

Given:

• 1600 GPS fix: 17∘50.0′N, 086∘40.0′E
• Course steered: 045°T
• Speed: 9 knots
• Visual fix at 1630 from:
  • Beacon X: 300°T
  • Tower Y: 020°T
• Known current from tide tables:
  • Set: 180°T
  • Drift: 1.5 knots

Tasks:

1. Plot the 1600 electronic fix.
2. Plot DR for 30 minutes (distance = 4.5 NM).
3. Apply current to get EP 1630.
4. Plot the 1630 visual fix.
5. Compare EP vs Fix and comment on:
   • Accuracy of tide table current
   • Steering error
   • Possible gyro error