Each of those commands must be issued individually. The user 
is responsible for administering the names of input files and 
output files. The example shows that the user has to provide 
unique names for each intermediate result so that it can be cor- 
rectly referenced in subsequent procedures. This task is a 
common source of errors. Commonly, users create a processing 
plan themselves to avoid loosing track. 
Employing VGIS, the user only has to create the work flow 
depicted in Figure 8 and to start its execution. While process- 
ing the flow chart the user is then prompted by the system to 
enter the rules for reclassification interactively. The result is 
displayed on the screen. 
7 FUTURE RESEARCH 
VGIS does not yet include conditional and iteration operators 
as they are used in formal programming languages. The 20 
universal analytical GIS operations employed do not account 
for any three-dimensional or temporal reasoning. The formali- 
zation given in (Albrecht 1996) can only be a preliminary one 
as the universal data model that underlies the definition of 
operations is based on an older draft of OGF's geodata model 
(OGF 1993) which has in the meantime been considerably al- 
tered. Finally, it would be extremely useful if not only the 
services-directed operations on the application level were stan- 
dardized, but the low-level functions, that are actually used by 
OGIS-conform systems, as well. Research into this direction is 
currently pursued by the authors both within OGF and ISO's 
technical committee 211 (Geoinformation). 
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