Sophisticated methods of prediction, such as kriging, disjunctive kriging or geostatistical simulation produce large sets of values with spatial co-ordinates. To comprehend such data good graphical representation is essential. Appraising columns of numbers provides no clues as to what spatial patterns are present within the data, but these can be portrayed succinctly on maps.
We illustrate the importance of mapping with soil organic matter data. Soil organic matter is an important component of the soil and it is involved in many soil processes. In agriculture soil organic matter influences soil fertility, water holding capacity and the vulnerability of soil to erosion. It is also affects the degradation of pesticides and limits their movement through to the ground water. Soil organic matter also has an important role in removing carbon dioxide from the atmosphere. Evidence has shown that soil carbon sequestration is increasing with increased atmospheric CO2. Nevertheless, present levels of organic matter are much lower generally than when the soil was first used for cultivation and, therefore, interest in the amount and changes in organic matter content are increasing.
Interest in soil organic matter ranges over several spatial scales from a whole country to a single agricultural field. Large scale patterns allow regional trends in declining or increasing soil organic matter content to be identified. This enables particular regions to be targeted for remedial action. At a field scale a farmer might adjust management practices to account for the variation in soil organic matter content, for example the application of pesticide organic fertilizers. Regardless of the scale of variation of interest it is important that high quality graphics are used for mapping because they enable us to identify the variation accurately and reliably. In addition comparisons between different methods of analysing the data and between different sampling intensities are often made visually and therefore depend on the quality of graphics.