During the initial catalogization of the Hustedt collection (1977-78), a first georeferencing of accessions was done based on the Marsden system. The original Marsden squares divide the globe into "squares" of 10° latitude x 10 ° longitude. This system was further refined as described below, and the refined Marsden coordinates were entered into the collection database during its initial creation, so that the location of most samples in the database is now determined to a 5° latitude times 5° longitude grid cell (1° times 1° in Europe). Finer geographic coordinates are availble for some samples, and entry of latitude-longitude coordinates is ongoing. For new additions, we record the latter whenever possible, but at the moment, only Marsden numbers are available for the majority of accessions. For the map display, all samples from a particular Marsden cell are being mapped to the midpoint of the grid cell in which they are located. This fact explains the regular distribution of records currently often seen on maps generated by database queries.

Refined Marsden coordinates

As the original Marsden system with its 10° times 10° resolution only allowed a rather coarse-grained georeferencing, the system used in the database (and on the original index cards of the collection) is a further refined variant of the Marsden system. The system adopted has four levels of resolution:

1. The first three digits define a 10° latitude times 10° longitude cell on the world map, as shown in Wikipedia's Marsden map. Note that differently from this map, our database always uses three digits (e.g., 001 instead of 1).
2. The fourth digit, ranging from one to four, specifies a quarter (5° times 5° "square") of the Marsden cell. Number 1 is always in the quarter closest to 0° latitude, 0° longitude; 2 is the second quarter closer to the Equator. 3 is the quarter closer to 0° longitude but farther from the Equator, and 4 is the quarter further from 0° lat, 0° long.
3. Within Europe, and when finer scale geographical information was available, two further digits were added specifiying a 1° latitude (digit 5) times 1° longitude (digit 6) subsquare.
4. When finer resolution information was available, these 1°x1° squares were again subdivided into four subsquares which were numbered according to the same system as described under 2.