Either someone (possibly Luke, possibly someone else) knew where Luke was going and put an incomplete version of the map in R2 (why?). Or someone tracked Luke’s journey, created the map, and put it in R2 without telling anyone else in the Resistance (again, why?).
Yes, Luke knew where Luke was going (all the Jedi Temples) and put an incomplete version of the map in R2 so that when people actually needed him they would be able to put the pieces back together. If he’d just left the map intact, anyone could walk up to R2 and find out where he was headed. The fact that no one knew R2 had the rest of the map is kind of weird, because it’s Luke’s best friend, of course he’d know what was happening. Unless he told R2 to lie about it or stay dormant until they found the other piece, which he apparently did.