Physical DFD to Logical DFD

How to Derive Logical DFD from Physical DFD?

Physical DFDs are a means to an end, not an end in themselves. They are drawn to describe an implementation of the existing system for two reasons:

·   to ensure a correct understanding of the current implementation (users are generally better able to discuss the physical system as they know it through people, workstations and days of the week.)

·   the implementation itself may be a problem or limiting factor; changing the implementation, rather than the system concept may provide the desired results.

A logical view steps back from the actual implementation and provides a basis for examining the combination of processes, data flows, data stores, inputs and outputs without concern for the physical devices, people or control issues that characterise the implementation.

A logical data flow diagram is derived from the physical version by doing the following:

·   Show actual data needed in a process, not the documents that contain them.

·   Remove routing information; that is, show the flow between procedures, not between people, offices or locations.

·   Remove references to physical devices.

·   Remove references to control information

·   Consolidate redundant data stores.

·   Remove unnecessary processes, such as those that do not change the data or data flows.