Marsden square


Marsden square mapping or Marsden squares is a system that divides a chart of the world with latitude-longitude gridlines between 80°N and 70°S latitudes into grid cells of 10° latitude by 10° longitude, each with a unique, numeric identifier. William Marsden, when first secretary of the Admiralty, conceived of this method for collecting and combining information respecting the oceans.

Structure & Design

On the plate carrée projection the grid cells appear square, however if the Mercator projection is used, the grid cells appear “stretched” vertically nearer the tops and bottoms of the map.
On the actual surface of the globe, the cells are approximately “square” only adjacent to the equator, and become progressively narrower and tapered as they approach the poles, and cells adjoining the poles are unique in possessing three faces rather than four.
Each one of the 540 10°x10° squares is allocated a number from 1 to 288 and from 300 to 551, plus the sequence extends to 936 in higher latitudes, and then can also be subdivided into 100 one-degree squares numbered from 00 to 99 in order to improve accuracy.

Use

Marsden squares have mostly been used for identifying the geographic position of meteorological data, and are described further in various publications of the World Meteorological Organization. The 10°x10° square identifiers typically use a minimal number of characters which was/is an operational advantage for low bandwidth transmission systems.
However the rules for allocating numbers to squares do not follow a consistent pattern, so that reverse-engineering the relevant square boundaries from any particular Marsden Square identifier is not particularly straightforward.
Slightly confusingly, an alternative, four-digit notation for global 10°x10° squares is actually known as World Meteorological Organization squares but does not seem to be actively promoted by the WMO itself.