This text is made of extracts from the following paper:
A pedotransfer rules database to interpret the Soil Geographical Database of Europe for environmental purposes.
Daroussin J., King D.. In: The use of pedotransfer in soil hydrology research in Europe. Workshop proceedings. Orléans, France. 10- 12 octobre 1996, p 25 - 40.

    Problems on land use and soil conservation require increasingly accurate information on soil properties and their geographical location. An important point is to obtain harmonized data over the diversity of the regions of concern. For the territory of the European Union, the Commission has suggested different approaches over the past twenty years. One of these has been the publication of the EC Soil Map at scale 1:1,000,000 (CEC, 1985), and then its computerization (Platou et al., 1989; INRA, 1990; King et al., 1993). The current Soil Geographical Database of Europe version 3 provides some answers to the above problems, thus helping in general decision making, but not sufficiently to answer some specific demands, particularly those concerned by environmental problems. Much of the needed information is missing although it is implicitly present into the actual data.

    The Directorate General for the Environment of the European Union (DGXI) asked to draw up procedures to facilitate the use of the Soil Database. This method is based on the concept of pedotransfer function (Bouma and Van Lanen, 1986) but adapted to interpret the  qualitative information available in the database to data needed for such environmental purposes. A European working group of soil scientists reached some consensus in the definition of a set of pedotransfer rules that are grouped into a knowledge database, and tools were developed to manage and make use of this knowledge to interpret the Soil Database. Advantages of the method are that interpretations are explicit and can themselves be updated whenever necessary either if the Soil Database or the interpretation knowledge are improved.
 

Definitions

    Most of the attributes presently input to the pedotransfer rules are those of the Soil Geographical Database of Europe. Its structure is fully described in INRA, 1990. For simplicity we will only say that the soil map is made of polygons grouped into Soil Mapping Units (SMU) (Arc/Info "region" concept). SMUs are complex units of soils which in turn are semantically (not geographically) sub-divided into Soil Typological Units (STU) holding the full description of each soil type present on the map (King et al., 1994).

    Pedotranfer rules are applied at this last STU level. One should note that this database structure implies that a mechanism be planned for to take into account the "complexity" of SMUs whenever a thematic map is to be displayed for attributes of the STU level (concept of "purity" of SMUs). Moreover the internal spatial variability of typological units is described in the Soil Database for some of the attributes but is not considered in the present work. Only the dominant value over the STU is used. In a future work tests of pedotransfer rules' sensitivity to intra-unit variability could be made.

    Input attributes are for example FAO soil name, topsoil textural class, parent material, etc. Output attributes were selected on the basis of the environmental parameters needed for the problems faced, e.g. hydrology of soil types for predicting catchment response to rainfall and standard percentage of run-off, location and sensitivity of wetlands, soil buffering capacity for predicting soil susceptibility to pollution, ecosystem and surface water deposition, vulnerability of ground -and surface- water to pollution by agrochemicals and farm waste, soil erosion potential, etc.

    The output attributes are grouped into four classes that respectively correspond to attributes of biological, chemical, mechanical and hydrological nature. Some of them can be derived directly from the Soil Database via pedotransfer rules, others need previously derived attributes as input. Each output attribute are classified fixed in a rather broad manner, in view of the low level of precision in the input attributes. The thresholds selected for class intervals are resulting from a compromise between currently established values in the Soil Science, and the possible level of precision at this scale. The adopted values may not correspond to the thresholds necessary for environmental problems. However, multiplication of the number of classes certainly would have reduced the reliability of the pedotransfer rules and thus the system would become unusable.
 

Methodology

    Soil Science experts of the working group provide the system with pedotransfer rules. These rules, using expert knowledge, permit to derive new needed information from the existing factual information called a "fact", describing an object of the dataset; e.g. the soil depth of a particular soil type can be inferred from both its known soil name and its parent material. A rule can be seen as a statement of the form:
              IF <available information is ...> THEN <new information is ...>
              ELSE IF <available information is ...> THEN <new information is ...>
              ...
              ELSE IF <available information is ...> THEN <new information is ...>

Each line in this statement is called an "occurrence" of the rule.
An occurrence is stored as a line or record in a rule Info file.

An occurrence can be seen as a statement of the form:
              IF (or ELSE IF)
              <factual value for attribute i is w
              and factual value for attribute j is x
              ...
              and factual value for attribute n is y>
              THEN <inform the object with value z for a new attribute m>

where attributes i to n provide the factual information (values w to y of an object), and attribute m provides the new -inferred- information (with value z). Attributes providing the factual information are the "input attributes" to the rule. The attribute providing the new -inferred- information is called the "output attribute" from the rule.

Example:
              IF <soil name is "eutric Cambisol" and parent material is "450">
              THEN <soil depth is "Medium">
              ELSE IF <soil name is "eutric Cambisol" and parent material is "700">
              THEN <soil depth is "Medium">
              ELSE IF <soil name is "dystric Cambisol" and parent material is "500">
              THEN <soil depth is "Deep">

    Therefore pedotransfer rules tables are describing the link, established through expert knowledge, between input attributes from the Soil Database and output attributes.
    The results provided by the application of these rules are only qualitative estimates. At the 1:1,000,000 scale it is difficult to provide accurate information from the few data contained in the Soil Database, and care is taken to point out the methodological limitations of our approach. Rules are applied to Soil Typological Units (STU) but their results have to be displayed as maps to Soil Mapping Units (SMU). Therefore, each map of the results of a rule inference represents the dominant value of the output attribute over the polygons.  So purity of the SMU has to be accounted for. This can be computed from an indicator of the surface percentage of STUs within each SMU which is provided in the database (see also Understanding purity maps).

    Click here for a detailed description of the pedotransfer rules project specifications.