From AMS Glossary
A system introduced by Marsden early in the nineteenth century for showing the distribution of meteorological data on a chart, especially over the oceans.
A Mercator map projection is used, the world between 80°N and 70°S latitudes being divided into Marsden squares each of 10° longitude. These squares are systematically numbered to indicate position. Each square may be divided into quarter squares, or into 100 one-degree subsquares numbered from 00 to 99 to give the position to the nearest degree.