138
Chapter 15
However, the comparison of land-cover classifications for different dates
does not allow the detection of subtle changes within land-cover classes.
Even if some of the attributes of one class have changed, the magnitude of
these changes will not always be large enough to justify a shift from one
land cover category to another, unless the vegetation classification identifies
a very large number of narrowly defined categories. Therefore, monitoring
land-cover changes can only be achieved through repetitive measurements of
biophysical attributes which characterize the land cover.
2.
DISCRETE VERSUS CONTINUOUS
REPRESENTATIONS OF LAND COVER
The land surface can be represented as a set of spatial units, each
associated with one or more attributes. These attributes are either a single
land cover category (i.e., leading to a discrete representation of land cover)
or a set of values for continuous biophysical variables (i.e., leading to a
continuous representation of land cover) (DeFries et al., 1995). The
correspondence between these two representations can be established
through a table which associates to each land cover category the average
range of values for the biophysical variables.
In the continuous representation of land cover, the biophysical
variables vary continuously not only in space but also in time, at the seasonal
and inter-annual scale. By contrast, in the discrete representation of land
cover, each spatial unit is represented by a single categorical value which is
stable over a season. Inter-annual changes in the values of the biophysical
attributes of the surface are described, in the discrete representation of land
cover, as land-cover conversions only if the changes exceed the range of
values which is characteristic of the land-cover category (i.e., if the
magnitude of the change is such that the values of all biophysical attributes
falls within the range of another land cover class during the entire seasonal
cycle). By contrast, a land-cover modification—which is not detectable with
the discrete representation of land cover—implies that variations in the
values of the biophysical attributes remain within the range of values which
is characteristic of the land-cover category. Processes leading to changes in
the seasonal dynamics or in the fine scale spatial variability of biophysical
attributes would also be described as land-cover modifications. Changes that
would only affect the values of some of the biophysical attributes, for just
part of the seasonal cycle would probably also enter in that category.
This suggests that monitoring land-cover changes by remote sensing
requires the measurement of a set of indicators of the biophysical attributes
of the surface, the seasonality of these attributes and their fine scale spatial
Chapter 15
However, the comparison of land-cover classifications for different dates
does not allow the detection of subtle changes within land-cover classes.
Even if some of the attributes of one class have changed, the magnitude of
these changes will not always be large enough to justify a shift from one
land cover category to another, unless the vegetation classification identifies
a very large number of narrowly defined categories. Therefore, monitoring
land-cover changes can only be achieved through repetitive measurements of
biophysical attributes which characterize the land cover.
2.
DISCRETE VERSUS CONTINUOUS
REPRESENTATIONS OF LAND COVER
The land surface can be represented as a set of spatial units, each
associated with one or more attributes. These attributes are either a single
land cover category (i.e., leading to a discrete representation of land cover)
or a set of values for continuous biophysical variables (i.e., leading to a
continuous representation of land cover) (DeFries et al., 1995). The
correspondence between these two representations can be established
through a table which associates to each land cover category the average
range of values for the biophysical variables.
In the continuous representation of land cover, the biophysical
variables vary continuously not only in space but also in time, at the seasonal
and inter-annual scale. By contrast, in the discrete representation of land
cover, each spatial unit is represented by a single categorical value which is
stable over a season. Inter-annual changes in the values of the biophysical
attributes of the surface are described, in the discrete representation of land
cover, as land-cover conversions only if the changes exceed the range of
values which is characteristic of the land-cover category (i.e., if the
magnitude of the change is such that the values of all biophysical attributes
falls within the range of another land cover class during the entire seasonal
cycle). By contrast, a land-cover modification—which is not detectable with
the discrete representation of land cover—implies that variations in the
values of the biophysical attributes remain within the range of values which
is characteristic of the land-cover category. Processes leading to changes in
the seasonal dynamics or in the fine scale spatial variability of biophysical
attributes would also be described as land-cover modifications. Changes that
would only affect the values of some of the biophysical attributes, for just
part of the seasonal cycle would probably also enter in that category.
This suggests that monitoring land-cover changes by remote sensing
requires the measurement of a set of indicators of the biophysical attributes
of the surface, the seasonality of these attributes and their fine scale spatial