When an SMU is composed of several STUs, it cannot be depicted on a thematic map -such as a soil name map - without dropping part of the information stored in the database to characterise that SMU for that particular theme. Only the value of the dominant STU or, better, the dominant value is shown on the map. The purity map associated with that particular thematic map is there to convey an indication of the amount of information lost through the thematic mapping process. It shows the proportion (as a percentage computed from the proportion of each STU composing the SMU) of the area of the SMU that is effectively covered with the corresponding thematic value shown on the corresponding thematic map. A purity map is thus an indicator of the quality of a thematic map against the database from which it was drafted. It is not an indicator of the quality of the data itself (for this, see the Confidence Level attributes and corresponding Confidence Level maps). Purity maps are not standalone, but are always paired with a corresponding thematic map.
Confidence Level attributes and maps
A Confidence Level is an indicator of the quality of an attribute value describing an STU. There are two types of Confidence Level attributes. One, named CFL, is an overall confidence level provided by the Contributor to globally qualify the STU description he made. It applies to all original STU attributes. All other Confidence Level attributes are named <attribute>.CL where <attribute> stands for the name of an attribute inferred through a Pedotransfer Rule. It was computed through the inference of that Pedotransfer Rule and applies only to that corresponding output attribute. Confidence Levels are data quality indicators. They can be thematically mapped to convey an indication of the estimated quality of the value shown on the corresponding thematic map for each SMU. Confidence Level maps can be standalone, but they are generally paired with a corresponding thematic map.