Fields as a framework for integrating GIS and environmental process models. Part 2: Specifying field variables

In Part 1 of this paper, the concept of a field variable was described. In Part 2, the way in which a field variable might be specified is outlined. Field variables can be declared like other data types in any standard computing language. Specifications of field variables must include several special properties which are used to express the relationship between the physical field and the discrete data model. Field variable properties determine how the spatial data can be manipulated. For example, specifying a cellgrid spatial data model requires information about grid orientation and origin and the size of cells, while specifying a polygon data model requires information about the minimum distance over which variation is recorded. In either case, this information can be used to select appropriate conversion procedures between spatial data models using similarly specified conversion matrices and rules. The use of field variables has pedagogic as well as practical value. New operations allowed include field functions such as those for determining integrals, slope, and aspect, and reserved variables such as latitude and longitude.