REGULATIONS OF THE IHO FOR INTERNATIONAL (INT) CHARTS AND CHART SPECIFICATIONS OF THE IHO
The standard units for depths and heights must be metres (m) and decimetres (dm). The standard units for positional accuracy must be metres (m). The standard units for distance ‘on the ground’ must be nautical miles (M) and cables, or metres (m).
The standard units for dimensions of charts
must be millimetres (mm). The standard units for time must be hours (h), minutes (min or m)
and seconds (sec or s), referred to Universal Time Co-ordinated (UTC). The standard units for speed must be knots (kn). The standard units for geographical positions should be degrees (°) minutes (') and decimals of a minute. Degrees (°), minutes (') and seconds (") may be used if appropriate. The standard units for bearings, such as for a recommended track or magnetic variation, should be degrees (°) and decimals of a degree. Degrees (°) and minutes (') may be used if appropriate.
B-131 GEOGRAPHICAL POSITIONS
Geographical positions quoted on charts and in related publications should be:
• expressed in degrees, minutes and decimals of a minute
• with a single
space between the coordinates and no other spaces
• without punctuation
• with a decimal separator according to national practice (comma, decimal point or full stop – comma is the preferred ISO sign)
• with leading zeros for single
number minutes, but not for degrees
• with the minute tick following the fractional part eg: 51°42,03'N 5°07,14'E 51°42·03'N 5°07·14'E 51°42.03'N 5°07.14'E
• Degrees, minutes and seconds may be used if the graduation of the chart concerned is in that format, to avoid confusion.
The four cardinal points must be denoted by the following abbreviations whenever their names are not inserted in full:
North = N
South = S
East = E
West = W
2. And the ISO says (ref. ISO 19018:2004; https://cdb.iso.org/cdb/termentry!di...857&language=1
1 NM = 1852 m. The nautical mile is not an SI-unit. This definition was adopted by the First International Hydrographic Conference in 1929
(see ISO 31-1).
International symbol for unit: NM
International symbol for unit in charts: M [Symbol M is to be used in Charts according to the "Chart Specifications of the IHO" which came into force at the XIIth International Hydrographic Conference 1982 in Monaco.]
So my conclusion is:
For chart data M should be used, in all other cases (e.g. the status line of OpenCPN) NM should be used.