When does the time for shortening prayers (qasr) begin, and is it permissible to shorten prayers upon arriving at one's family's home if one did not intend to shorten and combine prayers (qasr and jam')?

A traveler is permitted to shorten the prayer as soon as he leaves the populated area of his village or town of residence, provided the travel distance is 83 kilometers. However, once the traveler arrives in his hometown, he must pray Dhuhr as four rak'ahs without shortening it.