Setting Up the Frame - Sampling Spatial Units for Agricultural Surveys - page 99

statistical units and other concepts like a reference nomenclature and the
dissemination of the results.
The set up of this land partition, whose accuracy is of such fundamental
importance, is often very difficult, so as to give often serious consideration to
the possibility of defining macro-units with the only purpose of avoiding its
definition in detail but only for large aggregates. In agriculture, the establish-
ment of a hierarchical structure of this partition (through the use of multiple
layers) could solve some operational problems.
It might also be appropriate to require that the units that are defined in the
spatial frame should respect some particular constraints:

1. The available domain boundaries that should be quite stable over time, and
used in the data disseminated by other surveys.
2. A criterion

of internal homogeneity with regard to most

geo-morphological characteristics or land use.
3. Delimitation by natural boundaries easily detectable by the enumerator on
topographic maps (roads, railways, slopes, rivers, river beds etc.).
4. The average size in agriculture is usually of 50 or 100 ha with a variability
that should be as small as possible.

From a conceptual point of view the set up of this spatial frame is
equivalent to the design of a common geographical nomenclature obtained
through a micro-partition for areas in an integrated way that will satisfy the
needs of all future censuses and form a common basis for geocoding the
statistical units, for data collection and for data dissemination.
Among the problems arising from the use of a list of farms, it is important
to consider that according to any definition of farm is used in the survey to
identify the statistical unit, the population will never covers exhaustively all
the crops area. What follows is thus a systematic underestimation of agricul-
tural production. In fact, although well articulated and complex from both a
legal and a statistical point of view, the definition of the units always and
necessarily involves constraints on the type of conduction, on the size that
should exceed a certain threshold, or the economic activity based on the sale
of the farm products on the market. The fragmentation of the phenomenon
could justify the assumption that the majority of the production is concen-
trated in a few percentages of units. However, although this assumption can
be considered as true, the amount not included in the target population is
absolutely not negligible.
Further problems arise when trying to extend to agriculture the concept of
local unit , widely used in surveys of industry and services, in order to
georeference the units. From a logical point of view, we could consider the
set of portion of land and buildings adjacent to each other and having the
same owner as a subdivision of the farm that respects the territorial limits.

(continued)