A Data Flow Diagram (DFD) is a graphical means of describing the flow of data through a system that produces, changes, or stores data. The DFD is a visual depiction of an automated or manual system from the perspective of the data. A DFD differs from a flowchart in its perspective: a flowchart describes a system of programs or manual processes that manipulate the data, while a DFD describes a system from the perspective of the flow of information. A DFD is drawn from most general to most specific in a series of levels of detail. Each page, or diagram, describes a part of the system with the first page (or "Level 0") DFD showing the most general picture of the data and its flow through the entire system. Any further pages (or "lower levels") of the DFD provide progressively more detail about the processes being described.
The numbering of the processes (defined below) is dependent on the "level" of the DFD. All Level 0 DFDs have single digit numbers for the processes. All Level 1 DFDs have two-digit numbers separated by a period (e.g., 1.2, 3.4). Each process of a Level 1 DFD will have the same first number as the higher level process that it describes, followed by a sequential number that is unique for that page or DFD level. For example, a Level 0 DFD has 3 processes; the developer numbers these processes as 1, 2*, and 3. Process 2 has an asterisk following it, which signifies that additional information about process 2 is on a lower level (or more specific) DFD.
Since process 2.3 has an asterisk, it too has a lower level (or more specific) DFD. All processes on this Level 2 DFD will be labeled 2.3.1, 2.3.2, 2.3.3, etc.
The DFD view uses the four graphic shapes defined below to describe the system.
| State Department Transportation | right arrow | ENTITY – This box represents an entity that is of involved in the flow of data. Entities are ordinarily a person, group of persons, an agency, etc. |
| 30 | Local Road Database | right arrow | DATA STORE – The rectangular box represents a data store incorporating the data that are flowing into it. These file(s), or data stores, can e computerized or manual. The number is unique for each data store throughout all of the DFDs to ensure that each data store is specifically identified. |
| 1.9.1* Assign Roadway Location |
right arrow | PROCESS – This oval represents a process or activity that is performed on the data flowing into it. The process is used when the data is actually changed or enhanced in such a way that it is important to show in the DFD. The change in the data can be effected by either an automated or a manual procedure. The asterisk signifies that more specific detail about this process may be found on a lower level page of the DFD. |
| Traffic Counts | right arrow | DATA FLOW – This arrow represents a flow of data into, or out of, an entity, data store, or Traffic Counts process. The designer labels the data flow arrow when necessary to clearly describe the specific type of data in the flow. |