The sea is an international unit of measurement. 1 nautical mile =1.852km (km) (China standard). It is equal to the arc length corresponding to 1 minute latitude on the earth's elliptical meridian (once equal to 60 minutes, a circle is 360 degrees). Because the meridian circle of the earth is an ellipse with different curvatures at different latitudes, the corresponding arc lengths of latitude 1 are not equal.

The average radius of the earth is 637 1 300m, and1nautical mile calculated from the average radius of the earth is:

1 nautical mile = 2×π× 6371300/360/60 =1853.3m.

On the equator, the arc length corresponding to longitude 1 is equal to the arc length corresponding to latitude 1, so the corresponding longitude value can also be used to calculate the distance between the two places in nautical miles.

1 minute latitude (longitude) = 1 nautical mile length, from the south pole to the north pole is two 90 degrees, that is, 180 degrees, so when calculating the meridian length between the north pole and the south pole, it is necessary to multiply 60 points on 180 degrees, that is, 1 degree.

Calculating the length of the equator is the same, except that the equator is a circle, which is 360 degrees. Of course, the unit of counting is also the sea.