the specification of the AVHRR sensor brings other 
economies in that it only has three spectral bands. On 
the other hand it has very much higher temporal 
resolution in that daily coverage (16-18 times the 
Landsat frequency) is acquired at radiometric resolutions 
four times those familiar on the Landsat sensors (1024 
levels instead of the 256 levels of Landsat MSS). It is 
clear in these comparisons that it is in the spatial domain 
that the largest economies are being made. 
One very important area of research in the past twenty 
years has been into the spectral properties of objects and 
their detectability by remote sensing instruments. The 
research to date into the spectral properties of the 
complex soil, vegetation and crop surfaces which are the 
concern of those who survey and manage renewable 
natural resources shows that spectral information alone 
will not be sufficient to discriminate many types of cover 
of great significance to managers. The basis of this very 
broad statement is the findings of the series of meetings 
convened to exchange information on the spectral 
signature of objects (INRA/CNES 1982 and 1984, ESA 
1986). The results of the ten years of detailed research 
into data derived from ground radiometer, as well as from 
airborne and satellite instruments, show that only major 
categories of cover at what have come to be known as 
Anderson's Level 1 land cover classification for remotely 
sensed data can be reliably detected. Otherwise only 
tracts which have been managed to a high degree of 
homogeneity and which are large in extent, that is many 
times the pixel size of the sensor system, can be 
distinguished from eachother and even some of these 
homogeneous tracts may be difficult to discriminate. 
This is particularly the case when such crops as wheat, 
barley and grass are the crops to be detected. It has 
been conclusively demonstrated that reliable 
discrimination of cover will require multi-date information 
and probably improved radiometric resolution. Happily 
the management of large tracts (that is tracts bigger than 
one hectare) to a high degree of homogeneity of cover is 
normal in a large proportion of the rural areas of the 
world and the major thrust of the argument of this paper 
is that advantage should be taken of this normal 
circumstance when designing a data capture system for 
the detection of agricultural land cover. The discussion 
will be restricted to the utilisation of visible and near 
infrared data of the Landsat MSS/TM types as these 
data have proved to be most useful in rural surveys to 
date. At the same time the rich information environment 
which the MSS and TM types of data represent enable 
the comparison of various sampling strategies in a 
search for an economical approach to the potentially 
prohibitively expensive and data engorged problem of 
acquiring and handling comprehensive information on 
the land surface. 
2 RELEVANT APPLICATIONS AND INSTITUTIONAL 
ENVIRONMENTS 
The majority of the land surface which it is likely to be 
worth surveying for information on soil, water, vegetation 
including forest resources and crops falls into two 
categories. First those tracts which are managed by man 
intensively and secondly those tracts which are managed 
less intensively which grade into the regions enduring 
low rainfall and which are as a consequence of no 
■'terest from a resource management viewpoint unless 
they can be commanded by irrigation water. The first 
type of land, the intensively managed tracts, to which we 
shall refer to as HISTAB Land (high intensity with stable 
parcels) is characterised by: 
• high or at least adequate rainfall for one season or 
perennially. 
• a limited range of crops grown in one or more 
growing seasons. 
• a relatively static arrangement of parcels 
(agricultural fields) in which crops are managed. 
The parcels will generally be more than one 
hectare, but there are many areas, especially in the 
tropical world where parcellation is much more 
fragmented than the one hectare level. 
• a high level of inputs and outputs suggesting the 
usefulness of monitoring activities and an economic 
justification for expenditure on agricultural censuses, 
including remote sensing. 
The second type of land, the less intensively managed 
land, to which we shall refer as LOSTAB Land (low 
intensity wih unstable parcels), is characterised by: 
• low and unreliable rainfall with the intensity of use 
determined by the level of rainfall. There are usually 
very clear gradients in the intensity of use which 
change slowly over long distances. 
t a limited range of crops and livestock usually 
restricted to one season. 
• irregular and ill-defined parcels for crop and 
livestock management except in those regions where 
fencing has been installed. 
• low levels of inputs and outputs and no economic 
justification for the deployment of expensive 
monitoring including high resolution remote sensing 
systems. 
The exact global extents of the above two types of land 
are impossible to estimate but it is suggested that these 
two types of environment include ninety percent or more 
of the agriculturally (including rangeland) managed 
environment. The other ten per cent or so of land falls 
into an intermediate type which for reasons of 
fragementation and low intensity of use falls outside the 
areas susceptible to monitoring by remote sensing. The 
discussion here does not refer to this type of cover which 
albeit comprises a minority of global cover. 
The second type of land, the lostab land, is associated 
with low levels of output and any type of monitoring, 
including remote sensing, can only be justified if it is 
inexpensive. The gradual nature of the changes in land 
cover characteristic of these areas indicate that only low 
resolution systems of the AVHRR type will be appropriate 
and economically viable in these areas Drobably in 
association with low level aenal survey tecnniques. 
(Watson 1981) 
It is in surveys of the the first type of cover that remote 
sensing is most likely to be afforded and where at the 
same time remote sensing is most likely to have the 
greatest impact. This type of cover produces at least 
eighty per cent by value of the world's crop and livestock 
production although by area it probably comprises only 
twenty per cent of total agricultural and rangeland cover. 
One matter that has become clear as a result of the 
research of the past twenty years is that because of the