Module 6 - Position, Course & Speed
Latitude and Longitude
What you will learn
Transfer a position in degrees and decimal minutes between independent sources without confusing coordinate precision, chart scale and real positional accuracy.
Latitude and longitude identify an angular position. A safe coastal plot also records hemisphere, time, source, datum or reference system and the uncertainty hidden by the displayed decimals.
Latitude is angular position north or south of the Equator, from 0° to 90°N or S. Longitude is angular position east or west of the Greenwich meridian, from 0° to 180°E or W. Nautical charts commonly show degrees, minutes and decimals of a minute, for example 50°24.30'N 001°12.80'W. The N/S and E/W suffixes are part of the position, not optional decoration.
Read latitude from a side border and longitude from a top or bottom border. Transfer the latitude horizontally and the longitude vertically; their intersection is the plotted coordinate. Label its time and source so it remains distinct from a visual fix, DR, EP or another sensor position.
The international nautical mile is defined as exactly 1,852 metres. For practical Mercator chartwork, one minute on the nearby latitude scale represents one nautical mile. That chart convention must not be restated as a claim that every geodetic minute of latitude has exactly the same length on the real ellipsoid.
Decimal places show resolution, not trustworthiness. A coordinate can still be displaced by a wrong datum, antenna offset, old chart, user entry, poor signal or shared system failure. Compare sources that do not all depend on the same position feed before the apparent precision becomes a decision.
Worked example
At 1040 the plotter shows 50°24.30'N 001°12.80'W. At 1050 it shows 50°24.80'N at the same longitude.
- 1Subtract the latitude minutes: 24.80' − 24.30' = 0.50'.
- 2On the nearby Mercator latitude scale, 0.50' represents 0.50 M.
- 3The second coordinate is about 0.50 M north of the first because its northerly latitude increased and longitude is unchanged.
- 4This is displacement between two displayed coordinates; it does not prove either sensor position is correct.
Sense check: Half a minute on the nearby latitude scale is half a nautical mile, not 5 M; an east-west movement would also change longitude.
| Plot label | What it preserves | Failure it exposes |
|---|---|---|
| Time | When the position applied | Movement or stale data |
| Source | GNSS, visual, radar, DR or EP | Common-source dependence |
| Datum/reference | How coordinates relate to the chart | Systematic offset |
| Uncertainty | How much margin the evidence deserves | False precision |
Challenge a coordinate before it enters the pilotage plan
A handheld GNSS and the fixed plotter differ by 0.12 M beside a harbour entrance, but both show many decimal places.
1. Compare references
Check time, datum, antenna or receiver position, chart scale and whether both displays use the same underlying signal or waypoint data.
2. Seek independent evidence
Use a charted transit, radar range, visual bearing or depth evidence that does not simply repeat the same GNSS solution.
3. Protect the decision
Use the danger-side uncertainty and increase margin or pause the approach until the disagreement is explained.
Sense check: Two displays fed by the same antenna are two presentations, not two independent positions.
Common mistake or limitation
- Reading 24.30 minutes as 24 minutes 30 seconds; 0.30 minute is 18 seconds, not 30 seconds.
- Using the longitude border as a nautical-mile ruler because it is closer to the route.
- Copying a coordinate without its time, source or datum and creating false precision.
Recap
- Latitude is north or south; longitude is east or west.
- The nautical mile is exactly 1,852 metres; the nearby latitude scale is the practical chart-distance convention.
- Time, source, reference and uncertainty make a coordinate usable.
- Precision on screen is not proof of position accuracy.
Optional quick check
Section 1 of 8
Two plotters share one GNSS antenna but display slightly different chart positions. What is the strongest next check?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
- RYA Coastal Skipper/Yachtmaster Offshore Theory Course
- MCA MGN 610 Amendment 1 — navigation and SOLAS Chapter V
- MCA MGN 379 Amendment 1 — electronic navigational aids
- IHO S-4 chart specifications, edition 4.10.0
- BIPM SI Brochure, ninth edition
- USCG Boat Crew Handbook — Navigation and Piloting
- GPS Standard Positioning Service Performance Standard, fifth edition
Continue studying Position, Course & Speed
This public lesson is part of Revision Module 6. Full revision access adds the guided Learn, Practise and Test flow, flashcards and revision progress.