Geographic Phenomenon

A GIS operates under the assumption that the spatial phenomena involved occur in a two- or three-dimensional Euclidean space. Euclidean space can be informally defined as a model of space in which locations are represented by coordinates—(x, y) in 2D and (x, y, z) in 3D space—and distance and direction can defined with geometric formulas. In 2D, this is known as the Euclidean plane. To represent relevant aspects of real-world phenomena inside a GIS, we first need to define what it is we are referring to. We might define a geographic phenomenon as a manifestation of an entity or process of interest that:

• item can be named or described;

• item can be georeferenced; and

• item can be assigned a time (interval) at which it is/was present.

Relevance of phenomena for the use of a GIS depends entirely on the objectives of the study at hand. For instance, in water management, relevant objects can be river basins, agro-ecological units, measurements of actual evapotranspiration, meteorological data, ground\-water levels, irrigation levels, water budgets and measurements of total water use. All of these can be named or described, georeferenced and provided with a time interval at which each exists. In multipurpose cadastral administration, the objects of study are different: houses, land parcels, streets of various types, land use forms, sewage canals and other forms of urban infrastructure may all play a role. Again, these can be named or described, georeferenced and assigned a time interval of existence.

Not all relevant information about phenomena has the form of a triplet (description, georeference, time interval). If the georeference is missing, then the object is not positioned in space: an example of this would be a legal document in a cadastral system. It is obviously somewhere, but its position in space is not considered relevant. If the time interval is missing, we might have a phenomenon of interest that exists permanently, i.e.\ the time interval is infinite. If the description is missing, then we have something that exists in space and time, yet cannot be described. Obviously this last issue limits the usefulness of the information.

Types of geographic phenomena

The definition of geographic phenomena attempted above is necessarily abstract and is, therefore, perhaps somewhat difficult to grasp. The main reason is that geographic phenomena come in different “flavours”. Before categorizing such flavours, there are two further observations to be made.

First, to represent a phenomenon in a GIS requires us to state what it is and where it is. We must provide a description—or at least a name—on the one hand, and a georeference on the other hand. We will ignore temporal issues for the moment and come back to these in Temporal dimension and Spatial-temporal data model, the reason being that current GISs do not provide much automatic support for time-dependent data. This topic must, therefore, be considered as an example of advanced GIS use. Second, some phenomena are manifest throughout a study area, while others only occur in specific localities. The first type of phenomena we call geographic fields; the second type we call objects.