When I want to design a software I usually think about the use-cases of my software, but I have heard some sort of analyzing the existing (non-automated) system is required.
To be specific, is analyzing the existing system through a use-case diagram a prerequisite to find the use-cases of a software system (automated system)? or we can directly start from the use cases of the intended software? If it depends on the project, which projects require that step and which don't.
If that is not a prerequisite, is there any other diagram which can help us in drawing the use-case diagram of the new system? I know some analysis of the existing system is required but I would like to know is there some standard diagrams or steps for such analysis?