My sympathies... the real world meets our processes!
There are a couple of things to mention. On is the inherent difficulty of
replicating a non-documented system. It is really fraught and you may end up
pleasing no one. I would strongly recommend that you make very clear the
success criteria (obviously in a measurable way) before you go too far.
Also, as mentioned below, your analysis may uncover errors/issues in the
current system and then the decision needs to be made whether to fix the old
system, fix the new system, or replicate the issue in the new system.
Now as to your question... and a diagram being worth a thousand words. Well,
IMHO diagrams are great for overview and "picturing" what is going on.
However, I do not find them very useful for analytical work. They are hard
to maintain, difficult to spot changes and in general lead to a lot of
arm-waving when applied in areas where they are inappropriate.... as in
My recommendation, from the outside of course, would be to use high-level
Context Diagrams to get the overall picture, but to resort to tabular
representations for input/output/analysis. These, together with Decision
Tables, can give a clear analytical view of the processes. When I have done
this in the past it has practically always uncovered some unknown processing
or errors in the existing system ... and this is down to the existing system
having grown by accretion rather than by design, so you might need to be
prepared for this.
If you follow my suggestion, then you will end up with a hierarchy of
diagrams and tables. The top level will be the introductory Context Diagram
( in Business-speak). The next level should be a tabular version of that,
and from then on down, into more and more detail expressed in a tabular
form, with a numbering notation for each diagram which indicates ownership.
This is a kind of hierarchic set of documents. I suppose this could be
represented in a form of the Product Breakdown Structure diagram in PRINCE2
(although that generally does not allow reuse of elements... probably not a
Cross-linked to this (presumably) could be a set of System (or Product in
Volere-speak)Use Cases which identify the functionality from the User
Perspective. Once again, you may find that the Use Case Diagram is too large
for comfort and that a tabular form is preferred. I am assuming ( big
assumption) that the Business Use Cases will be very nearly identical to
those in current use? If not, then we will probably end up with some mapping
from old to new System Use Cases. Either way, again we end up with a
hierarchical view from diagram/table to Use Cases.
Summary, Divide and Conquer!
I hope this helps
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