• Platform independence
Since VGIS is developed independently from the underlying GIS, users may exchange workflows without
need to adapt them to particular environments.
• Self-documentation through storage of processing plans that capture lineage
Processing plans can be saved and modified anytime. The user may enhance results by iterative changes of
the workflow depicted in the processing plans.
Flow charts are a standard process-oriented tool in visual programming (Chang 1990, Glinert 1988,
Monmonnier 1989). Such a modeling flow chart allows the user to "play" with the data flow. Within VGIS, it
is easy to test the result of new routing paths within the flow chart so that different hypotheses can be tested by
adding or changing a connection between operators. A similar reconfiguration of a conceptual model would
require substantial GIS expertise if it were attempted in a vendor GIS.
3.2 The Flow Chart Interpreter
The interpreter used to translate processing plans into the operations of the underlying GIS consists of two
steps (see Figure 3).
In a first step, the powerful and user-onented VGIS functions are dissected into elementary GIS operations. In
a second step, these elementary GIS operations are then translated into the proprietary functions of the
underlying GIS.
The advantage of this two-step approach is the independence of the first phase from whatever GIS will be used
in the second phase. The second step makes the actual interface between VGIS and the proprietary GIS. That
way, both the custom-made user interface and processing plans can be exchanged across platforms. The more
powerful VGIS functions should ideally be developed by a GIS manager who builds them from elementary
GIS operations to fit a particular application.
Figure 3. The two-step flow chart interpreter