TheImprovement ofTropical and Subtropical Rangelands

REGIONAL RESOUROE ASSESSMENT 83
important in estimating land capability in terms of indigenous practices.
It also should provide some clues about the acceptability of
proposed changes in management practices.
Interviews may be conducted formally, and may rely on the use of
questionnaires if the objectives of the project call for a quantitative
description of some features of local culture. However, informal
interviews are done more easily and may serve equally well.
Ground Sampling
Much of the information required for the resource 888essment
can be gathered only through direct observation. This information
may be used to develop maps, to develop estimates of the magnitude
ofother characteristics that are not ordinarily mapped (for example,
population), or to describe the composition of mapping units (for
example, vegetation species and cover, and soil type and depth) that
have been recognized by other means (see the following section on
remote sensing).
Maps and estimates of land characteristics may be developed in
two ways. First, ground samples may be gathered in a sampling pattern
such as a grid. Reasonable maps or estimates may be developed
from such data. However, the accuracy of this approach is dependent
upon the complexity of the region and the density of sample points.
At the resource assessment level, it is unlikely that a project could
afford the expense of allocating enough samples to characterize a
large region. Thus, systematic ground sampling is used only for very
intensive studies, such as irrigation soil surveys, or where the features
of interest are 888umed to be poorly correlated with other observable
features, such as archaeological sites.
A second approach employs stratified sampling and is used where
it is possible to 888ume a reasonable correlation between two or more
characteristics. For example, in a landscape approach to survey, it
is possible to stratify an area according to landform and elevation
if good topographic maps of a region exist. Ground samples are allocated
to each stratum, according to its importance or complexity.
Maps and estimates developed in this way are reasonably accurate
and more efficient than a systematic approach. However, dynamic
land characteristics, such 88 land use and vegetation, may be inadequately
sampled because of their high variability. For example,
major changes in vegetation resulting from clearing or fire may be
missed because they are not nece88arily correlated with landform.