2.2 The Flow Chart Interpreter
The interpreter used to translate processing plans into the op-
erations of the underlying GIS consists of two steps (see Figure
3).
In a first step, the powerful and user-oriented 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. The
definition of those elementary GIS operations plays a key role
and is described in section 2.
VGIS function
of the flow chart
universal
GIS function
Interface ———————————
function of the
underlying GIS
Figure 3. The two-step flow chart interpreter
2.3 The Flow Chart Building Tool
The interface between the user and the system will be further
enhanced by a flow chart generating tool. Here, the user needs
only to describe the results and to name the source data to be
employed. Based on a knowledge base of integrity rules, the
appropriate flow chart will then be generated automatically. An
explanatory component is supposed to help the user to under-
stand how the system derived the flow chart, which may be
subsequently modified.
The flow chart generating tool is based on the principle of
backward chaining. The system checks whether data with the
requested attributes exists already. If it does not find such data,
an appropriate procedure will be started to derive the data. In
case the necessary source data cannot be found, it will recur-
sively call other workflows until the warranted data is either
produced or it will halt with an error message stating that it
cannot be derived from the input specified.
The automatic generation is certainly a very complex task re-
quiring additional funded research. It is therefore only envis-
aged as an extension to the current VGIS project.
4 THE VGIS PROTOTYPE
3.1 Hard- and Software
VGIS has been developed so far using a personal computer that
is connected to a workstation on ethernet via an X™ server.
Limited resources made it necessary to rely as much as possi-
ble on standard software under the UNIX operating system.
The initial implementation uses the public domain package
GRASS as GIS. A commercial program called WiT®, that has
originally been developed for image processing, was used as a
tool for processing plan generation. Additional functions are
implemented in C and C++, while the necessary Motif®-based
windows were developed with a graphical user interface
builder called X-Designer®. :
3.2 Design and Functionality of the Prototype
VGIS starts with a shell script that first calls a C++ program
vgis grass start which allows the user to define - within a
Motif? window - the environment variables required to run
GRASS. Once they are set correctly, a VGIS-adapted WiT?
session is started. WiT® controls the actual functionality of
VGIS. Only when WiT® terminates, this control is returned to
the initial shell script which then calls the program
vgis grass end that deletes all temporary files and ends VGIS.
User
communication with
Motif Windows
WIT Flowchart Tool
à
call of an operation
with specification of
filenames for
input and output
interrogation of
parameters and
return of results
Library of VGIS Functions
call with parameters
and filenames
GRASS Operations
storage
of results
Temporary Files
reading of
files needed
Figure 4. The construction of the VGIS prototype
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996
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