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Part A: Classification Concepts – The Conceptual basis
2.5.2 The Occurrence of Mixed Mapping Units
In the classification system, all classes are unique and no Mixed Mapping Units are
considered. Mixed Mapping Units are cartography related. However, the user can go from a
more general to a more detailed level of definition of a class. If for instance the classifier
Woody is used, this implies that an intricate mixture of trees and shrubs is present in which
neither is clearly dominant. This results in a mixture of two life forms but not in a mixed
land cover class. A Mixed Mapping Unit in the legend is always characterized by two or
more (to a maximum of three) separate single land cover classes as defined in the
classification system. The conditions governing the utilization of mixed mapping units are
that within the minimum mapable area, two or more land cover classes are present, which
can be:
(1) in a spatially separate entity (e.g., patches of agriculture fields inside a forest); or
(2) in an intricate mixture (e.g., rainfed cultivated fields with Baobab woodland).
The latter is applicable only if a more general definition of the class (as explained above) is
not appropriate.
In the case of spatially separate entities of two or more classes, the general criteria proposed
is that the cover of each one of the class considered must be more than 20 percent (and
consequently less than 80 percent) of the mapping unit. The limit of 20 percent is thus the
threshold of “visibility” of a class in a Mixed Unit. The only exception to this rule is in the
major land cover type of Cultivated Areas, where the use of the option Scattered Isolated of
the classifier Spatial Distribution goes from 10 to 20 percent (see Section 2.4.2).
The sequence of the class names in a mixed mapping unit represents the dominance (e.g.,
for Forest/Cultivated Areas, Forest is more than 50 percent and less than 80 percent,
whereas Cultivated Areas is less than 50 percent but more than 20 percent). A Mixed
Mapping Unit can contain a maximum of three classes.
In the particular case of classes belonging to the major land cover categories Cultivated and
Managed Terrestrial Area(s) (A11) or Cultivated Aquatic or Regularly Flooded Area(s)
(A23), the user has an additional means to create a Mixed Mapping Unit. The classification
system offers the possibility to create a so-called “Temporal Mixed Unit”. Such a unit is
used to describe the situation were in different years, different types of cultivation are
executed in the same field (i.e. the mapping unit). This is the case when the user has, for
example, a situation with cultivated fields of paddy rice in one year (e.g. when there is
sufficient rainfall) followed by a terrestrial crop in the subsequent year(s) (e.g. when rainfall
is poor). This particular type of Temporal Mixed Unit shows often a cyclic, almost
customary, alternation of different crops in subsequent years (e.g. generally an Aquatic crop
followed by Terrestrial crops, or an Irrigated crop followed by Rainfed crops, etc.). It is
important to note that the alternation of crops should be considered only when this
occurs on an annual basis. The combination of different crops in the same growing period
is an option already considered in LCCS’ class creation (see the classifiers related to Crop
Combination in A11). However, because of the specific nature of this type of Mixed Unit,
that occurs only where crops are growing, the classes composing such a mixed unit can
only be those of Cultivated Area(s).