Abstract:
The Global Soils database resident at UNEP/GRID was digitized by the ESRI
Corporation in Redlands, California (USA) from the 1974 FAO-Unesco Soil Map of
the World at 1:5,000,000 scale. The legend of the Soil Map of the World
comprises an estimated 5000 different mapping units, consisting of soil units
or associations thereof, occurring within the limits of a mappable
physiographic unit (Africa
... alone in three map sheets has a total of 1509 map
units). The number of soil unit classes which compose the FAO-Unesco Soil Map
of the World legend is 106. In the UNEP/GRID version, this has been increased
to 129 to allow the identification of soils where insufficient information
existed to specify the dominant soil unit.* Instead, the dominant soil GROUP is
identified (e.g., 'Acrisols' when the dominant Acrisols UNIT is not known).
When a map unit is not homogeneous, it is composed of a dominant soil and
associated soils, with the latter covering at least 20 per cent of a given
area. Important soils covering less than 20 per cent of a given area are added
as inclusions.
The original FAO-Unesco Soil Map of the World, published in a total of 19 map
sheets (including the legend), was in the Bipolar Conic Conformal Projection
for the Western Hemisphere, and in the Miller Oblated Stereo- graphic
Projection for the Eastern Hemisphere. In order to have a global digital data
set that was consistent and uniform, the vector-based data files were first
rasterized and then re-projected at GRID-Geneva into the Plate Carree or Simple
Cylindrical Projection (a particular form of the Equirectangular). The original
digitized polygon vertices in table coordinates were rasterized and
georeferenced using the ELAS (NASA's Earth Resources Laboratory Applications
Software) programs on a Perkin- Elmer 3241 minicomputer, with an output cell
(pixel) size of 30 seconds latitude/longitude spatial resolution (approx. 925
meters at Equator), or 0.86 square kilometers/pixel. The resulting database
was also generalized (resampled) to two minutes latitude/longitude resolution
(approximately 3.7 km. on a side), yielding an output cell size of 13.7 square
kilometers. This is the spatial resolution which UNEP/GRID ( - Geneva)
normally recommends to users and most frequently distributes.
The original FAO-Unesco Soil Map of the World included information on texture,
slope and phase, but separate data layers depicting these variables have not
been developed by UNEP/GRID.
The proper reference or original source document for this data set is as
follows: "FAO-Unesco Soil Map of the World, 1:5000000, ten volumes,
Unesco-Paris 1974."
The FAO-Unesco Soil Map of the World data file consists of 5400 lines by 10800
elements, with an origin of 180 degrees West longitude and 90 degrees north
latitude. At two minutes lat./long. spatial resolution, (approx. 3.67 km. or
30 per degree), the data file covers the entire globe to 180 degrees East and
90 degrees south, and comprises 58.32 Mb.
There are 106 original Soil Units, and 27 ("Great") Soil Groups; of these, 23
were composed of various soil units and four had ONLY the Group name
designations. Therefore, it was only necessary to add 23 new classes to the
UNEP/GRID version legend, for 129 in total (106 + 23 = 129). There are three
additional categories in the GRID legend for Rock, Salt and Water, or 132. 
