Setting Up the Frame - Sampling Spatial Units for Agricultural Surveys - page 97

fields operated by him. Checking the internal consistency of the cadastre is essen-
tial. By covering each field of the study region, it should be possible to completely
list the holders.
Nevertheless this approach has some disadvantage. In countries where rural
reconstruction and development programs have been initiated, a list of households
may be readily available, and this can be used to screen the farm households. In case
no list of households or holdings is present, it will be necessary to prepare a new list
of households operating some land. The enumerator may be instructed to start from
a fixed point of the village, and number serially every house in which there may be
more than one holding. Only some of the members of the household may be
agricultural holders. The enumerator should proceed from house to house, listing
households along with the information whether its members are agricultural
holders.
The list frames are not generally linked with the territory, except if the parcels of
the households and farms are digitized. This is obviously a very expensive task.
Conversely, if this link exists, the operational sampling frame includes the
geographic dimension of the units, such as farms and households that should be
connected to the land cover and/or land use dimensions. This produces a series of
benefits. Among the others, the link of the farm with its position in the territory
ensures the assessment of the quality of self-reported responses of farmers, and the
use of these measurements for benchmarking. Furthermore, this association facil-
itates agro-environmental analysis. Remote sensing data can help in handling this
problem by adding the geographical dimension to the list frames (Carfagna and
Gallego 2005 ).
In case of spatial units with segments that have recognizable physical bound-
aries, the frame is an ordered list of land areas, called frame units, with their
assigned number of segments, such that they form a complete subdivision of the
total land area of each land use stratum, with no overlap. They provide a clear-cut
means of identifying of each segment, and the number of segments assigned to each
frame unit facilitates the probability sampling of segments (FAO 1995 ).
The preparation of such a frame, to select a sample of segments that have
recognizable physical boundaries, is a very demanding issue. Up-to-date carto-
graphic material (maps, satellite images, aerial photos) are required on which the
land to be included can be visualized. The resolution or detail of the material must
be sufficient to stratify according to intensity of land use and the subsequent
subdivision of these land use strata into frame units also with recognizable physical
boundaries. Land use strata and frame units are identified on satellite images or on a
mosaic of aerial photography and then transferred to topographic charts and
measured. Frame units are constructed generally with maps on which the bound-
aries of the land use strata have been transferred. In each land use stratum, each
frame unit must be measured and assigned to a target number of segments of
approximately equal size. Then, the number of segments assigned to each frame
unit is summed to provide the total number of segments in the stratum, and a sample
of segments is selected from each land use stratum. Each sample segment is
constructed on small mosaics of aerial photography on which the boundary of the