Module 6 - Position, Course & Speed
Latitude and Longitude
What you will learn
Read and write a position in degrees and decimal minutes, then use the correct chart border to transfer each coordinate without confusing angular position with distance.
Latitude and longitude identify a position by angle. On a Mercator chart, latitude is read at the side and longitude at the top or bottom; the nearby latitude scale also provides the practical nautical-mile distance scale.
Latitude states how far north or south a position lies from the Equator, from 0° to 90°N or S. Longitude states how far east or west it lies from the Greenwich meridian, from 0° to 180°E or W. IHO chart specifications normally express positions in degrees, minutes and decimals of a minute, for example 50°24.30'N 001°12.80'W.
Read latitude from a side border and longitude from a top or bottom border. To plot a coordinate, transfer the latitude horizontally into the chart and the longitude vertically; the intersection is the position. Keep the N/S and E/W suffixes visible, because the digits alone do not identify the hemisphere.
The international nautical mile is defined as exactly 1,852 metres. It was chosen because it is approximately the distance subtended by one minute of angle at the Earth's surface. For practical Mercator chartwork, one minute on the nearby latitude scale represents one nautical mile; that chart convention is not a claim that every geodetic minute of latitude has exactly the same physical length on the ellipsoid.
A precise-looking coordinate is not proof of an accurate position. Its usefulness still depends on the source, datum or reference system, chart scale, chart currency and the uncertainty of the observation or sensor that produced it.
Worked example
Two positions share longitude 001°12.80'W but have latitudes 50°24.30'N and 50°24.80'N.
- 1Subtract the latitude minutes: 24.80' − 24.30' = 0.50'.
- 2On the nearby Mercator latitude scale, 0.50' represents 0.50 M.
- 3Because the second latitude is the larger northerly value and longitude is unchanged, the second charted position is about 0.50 M north of the first.
Sense check: Half a minute on the latitude scale must be half a nautical mile, not five miles or 0.05 M.
Transfer a plotter position to the working chart
At 1040 the plotter displays 50°24.30'N 001°12.80'W. You want an independent chart mark with enough information to compare the next observation.
1. Read latitude
Find 50°24.30'N on the side scale and carry that level horizontally into the chart.
2. Read longitude
Find 001°12.80'W on the top or bottom scale and carry that meridian vertically into the chart.
3. Label the evidence
Mark the intersection using the required exercise convention and add 1040 plus the source, such as GNSS or visual fix.
Sense check: If either coordinate was read from the wrong border or the W suffix became E, the plotted point would not sit near the expected route and should be rejected before it influences a decision.
Common mistake or limitation
- Reading 24.30 minutes as 24 minutes 30 seconds; decimal minutes and seconds are different formats.
- Using the longitude border as a nautical-mile ruler because it is closer to the route.
- Copying many decimal places without recording time, source or reference information, which creates false precision rather than stronger evidence.
Recap
- Latitude is north or south; longitude is east or west.
- Positions normally use degrees, minutes and decimals of a minute with hemisphere suffixes.
- The nautical mile is exactly 1,852 metres; the nearby latitude scale is the practical Mercator chart-distance scale.
- Coordinate precision does not remove source, datum, chart or sensor uncertainty.
Optional quick check
Section 1 of 6
In 50°24.30'N, what does .30 represent?
Sources and factual review
Continue studying Position, Course & Speed
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