Latitude and Longitude

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.