Chart Minutes and the Decimal Degrees a Plotter Wants
Navigation is the last trade that still counts angles in minutes as a matter of routine. Latitude and longitude are ruled off a paper chart in minutes, a GPS shows a fix as degrees and decimal minutes, magnetic variation is printed on the compass rose in degrees and minutes, and a hand bearing taken across the water is read to the nearest whole degree. The moment any of that has to be typed into route-planning software, a spreadsheet of waypoints or a passage log, it has to become a single decimal number.
Where Minutes Turn Up on Passage
Positions off the chart
Variation and deviation
Waypoint entry
Distance run
Working a Fix or a Bearing Through the Fields
Convert the minutes part on its own and add it back to the whole degrees — that keeps the sign and the hemisphere in your hands rather than in a formula.
Enter only the minutes
For 51° 27.5′ type 27.5 in the left field, not the whole coordinate. The decimal degree result appears as you type; a comma is accepted as the decimal mark, which helps when the figure was copied from a European almanac.
Add the whole degrees back on
Add the result to the degrees part, then apply the hemisphere: north and east stay positive, south and west take a minus sign. Getting that sign wrong is the error that puts a waypoint in the wrong ocean.
Reverse it to read a plotter figure onto paper
When the number came from software as 51.458333 and you want it on the chart, press swap (↔) and feed in the fractional part — 0.458333° comes back as 27.5′, ready to step off with the dividers.
Copy the plain number into the log
The copy button on either field gives the bare digits with no symbol attached, so a passage log, a customs form or a routing file gets a clean value that will not be rejected by a strict field.
Chart Minutes, Decimal Degrees and Distance Run
Measured along a meridian, a minute of latitude is a nautical mile by definition — 1852 metres exactly since the international agreement of 1929. That makes the middle two columns interchangeable in a way no other pair of angle and distance units manages.
| Arc minutes (′) | Decimal degrees | Along a meridian (nmi) | Metric distance |
|---|---|---|---|
| 1′ | 0.016667° | 1 | 1.852 km |
| 5′ | 0.083333° | 5 | 9.26 km |
| 10′ | 0.166667° | 10 | 18.52 km |
| 15′ | 0.25° | 15 | 27.78 km |
| 30′ | 0.5° | 30 | 55.56 km |
| 45′ | 0.75° | 45 | 83.34 km |
| 60′ | 1° | 60 | 111.12 km |
The nautical-mile column holds only for latitude. Run the same minutes along a parallel and the distance shrinks by the cosine of the latitude: 0.866 of a mile at 30°, 0.707 at 45°, and exactly half a mile at 60°.
What Helps at the Chart Table
Both halves of a coordinate in one sitting
Values update on every keystroke, so latitude minutes and longitude minutes can be worked one after another by overtyping the field rather than starting again.
Plotter figure back onto the paper chart
The swap button turns the pair round so a decimal fix from software becomes the minutes you actually step off with dividers along the side scale.
Tenths of a minute stay visible
Results run to eight decimals, so a fix quoted to a tenth of a minute — roughly 185 metres — is not quietly rounded away before it reaches the log.
Values a routing file will accept
Copy hands over digits alone, with no degree sign or hemisphere letter to strip out before the number goes into a waypoint list or a spreadsheet.
Questions from the Chart Table
Why does my GPS show 51° 27.500′ rather than a single decimal number?
That layout is degrees and decimal minutes, the default on marine plotters and most aviation receivers, and it exists so the display matches the chart in front of you. Because the minute is also the sea mile, a navigator can subtract two readings and get a distance without any further arithmetic. Decimal degrees, the third common format, is what software wants; degrees-minutes-seconds is the older survey-style notation. All three describe the same point, and mixing them up in a waypoint list is a classic source of grounding reports.
Is a minute of longitude also a nautical mile?
Only on the equator. Meridians converge towards the poles, so the ground distance covered by one minute of longitude is the cosine of the latitude times a mile: 0.87 miles at 30°, 0.71 at 45°, 0.50 at 60°, and nothing at all at the pole itself. Navigators call the east-west distance departure and compute it from the difference in longitude and the mid-latitude. Latitude is unaffected, which is the whole reason the mile was tied to it.
How far off track does a one-minute steering error put me?
Very little on a coastal leg, and rather more on an ocean crossing. The old one-in-sixty rule says one degree of error over sixty miles run puts you one mile off; a single minute is a sixtieth of that, so after 60 miles you are about 32 metres to one side. Carry the same error for 600 miles and it becomes roughly a third of a mile. What matters more in practice is that a hand bearing compass is rarely better than a degree or two, so minutes of steering error are noise beside the instrument itself.
Should variation be applied in minutes or as a decimal?
Whichever you use, do the arithmetic in one unit. Charts print variation as, say, 6° 15′ W with an annual change of a few minutes east, and rolling those minutes forward for the chart's age is where the fractions matter. Turning 6° 15′ into 6.25° before adding or subtracting it from a true course is usually less error-prone than juggling degrees and minutes separately — and it is the form a plotter or routing program will expect anyway.
Why is distance always measured off the side of the chart, never the top?
The side scale is latitude, and on the Mercator projection a minute of latitude is a mile at every point on the sheet. The top and bottom scales are longitude, where a minute is worth less than a mile everywhere except the equator — stepping dividers along them would overstate the distance. There is a second subtlety: because Mercator stretches towards the poles, the correct practice is to take the mile off the latitude scale at the same height on the chart as the leg being measured, not from the corner.
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