Site hosted by Build your free website today!

A2 - - - CALCULATED MERCATOR SAILING - - - 10/03/2004

At the equator a degree of longitude is equal to a degree of latitude, and they are approximately equivalent to 60 nautical miles but in receding from the equator and approaching the pole, while the degrees of latitude remain always of the same length (but for a slight change due to the fact that the earth is not a perfect sphere), the degrees of longitude become less and less, thus the degree of longitude at 70 degrees North or South of the equator is about 20 nautical miles.

In the Mercator charts the degrees of longitude are made to appear the same length over the whole chart. It becomes necessary in order to preserve the direction between different locations to increase the North-South distance on the chart and such increase must become greater and greater the higher the latitude.

The length of the meridian as thus increased between the equator and any given latitude (L), expressed in minutes of longitude at the equator as a unit, constitutes the number of meridional parts (M) corresponding to that latitude.

The value for M is normally taken from a table of meridional parts, but now that we are in the computer age, the value for M can be calculated by a programmable calculator or computer. For normal navigational purposes the value of M is taken to one decimal place (NNNN.N), but for survey work greater precision may be required.

Meridional parts for the Clark spheroid of 1866.

      M = 7915.704468 * log tan ( 45 + (L/2))
          - 23.268932 * ( sin L )
          -  0.052500 * ( sin L )^3
          -  0.000213 * ( sin L )^5

When sailing from point 1 (L1, LO1) to point 2 (L2, LO2) M1 = meridional distance for L1 M2 = meridional distance for L2 m = (M2 - M1) = (meridional difference) C = course angle DLO' = difference in longitude expressed in minutes D = distance from point 1 to point 2 tan C = DLO' / m D = 60 * ( L2 - L1 ) / cos C (nautical miles)

