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I've been using QGIS for a little while now, but I am still a bit confused about coordinate systems. There is plenty of guidance about how to change project and layer CRS, on-the-fly transformation etc, but I can't find anything which explains fairly simply why you should use a particular CRS in preference to another. Does it depend on the data sources (if any) which I intend to use? Or the size of the area to be covered (eg a few square kilometres vs a whole country)? Or the locale?
I've tried searching for tutorials etc which can shed a bit more light, but haven't come up with anything yet.