"The parallels of latitude do not converge, so each minute of latitude is always equal to 1 nm."
Wrong way round I think. Cosider parallel of latitude 89deg North. Its a lot shorter than the equator but still equal to 360 deg in circumference. Therefore 1 min in latitude at 89 north would be a lot shorter than 1 min of latititude at the equator. lines of longitude are the same length so i min of longitude at the equator is roughly equal to 1 min of longitude at 89 lat north. having said that 1 min of longitude soes shorten slightly as you near to the poles. The earth is not a perfect spheroid. Due to the fact that the earth is spinning the earth is not a perfect sphere. The equator (line of latitude) is longer than a line of longitude, i.e. the earth is "flatter" at the poles. GPS uses WGS 84 which compensates for the distance difference, which is why old charts which use a different WGS have to be adjusted in order to agree with the GPS standard. Sorry for going off thread but the original assertion is fundamental to navigation. You should always use the longitude lines for distance measuring on a chart, never the latitude lines which change with longitude.