The user outlines the workflow interactively by combining 
these operators to form processing plans. Selecting the WiT^ 
menu item Run starts the processing of the flowchart. WiT® 
calls for each operator a C program with a corresponding name. 
Input and output filenames, as well as operator parameters if 
necessary, are transferred. The C program processes them in 
such a way that one or more GRASS commands can be issued. 
GRASS carries them out and saves the results on the hard disk. 
The names for these temporary files produced by GRASS are 
automatically generated. The next VGIS operator uses the tem- 
porary file as input. In case that not all necessary parameters 
have been provided, the C program calls a C++ module that 
prompts for the required data in a Motif” window. Reclass is 
an example for such an operator requiring further information 
from the user. 
By employing several display- and output operators, interme- 
diate results can be displayed and compared. Figure 7 depicts 
the previous example with the display of intermediate results. 
  
  
  
  
  
  
Output #1 
distance: 100 
  
  
  
  
  
  
  
  
  
  
Figure 7. Display of provisional results in VGIS 
5 FIRST RESULTS 
User resonance to the first VGIS prototype is quite positive. 
The interface is easy to handle and the learning curve excep- 
tionally low. 
A few problems, mainly resulting from the use of WiT®, 
should nevertheless be mentioned. Direct querying of parame- 
ters in self-defined windows is not supported and can even 
with tricks only be accomplished while processing a flow chart. 
During the creation of flow charts only some very simple 
WiT®-provided windows can be called. This is an enormous 
obstacle. 
In addition, it is not acceptable for a GIS application that the 
number of inputs of an operator needs to be fixed and cannot 
be parameterized. There is an unnecessary large set of overlay 
operators only because the number layers needs to be defined 
by the developer rather than the user. C is the only language 
86 
supported by the WiT® APL In general, WiT ® offers too few 
means for customer-driven extensions. 
The probably biggest handicap of the current VGIS prototype is 
the lack of a real interface between its modules. Data keeps 
being written to disk and read from there, causing the perform- 
ance to drop dramatically. The original concept of WiT® allows 
for images to be transferred from one operator to the next one 
directly. GRASS (and other GIS) however, do not accepts other 
input and output media than the hard disk. 
The development of this prototype proved the correctness of 
the underlying concept. The current development of an 
Arc/Info? interpreter for VGIS will show that true data struc- 
ture-independence is feasible. The authors came nevertheless 
to the conclusion that due - to the restrictions caused by em- 
ploying standard software packages - some major changes to 
the current path of development are necessary. 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996