• 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