From Glossary of Meteorology
A system that divides a Mercator chart of the world into squares of 10° latitude by 10° longitude.
Each square is numbered and then subdivided into 100 one-degree squares numbered from 00 to 99. These 100 one-degree squares are applied to the eight octants of the globe. Marsden squares are mainly used for identifying the geographic position of meteorological data over the oceans.