A sentence needed your help
Also beginning in the thirteenth century, there were new maps refined by precise calculations and the reports of sailors that made it possible to trace one's path with reasonable accuracy.
People say in the begining of the thirteenth century. So if I invert the word beginning，why shouldn't it be “also of beginning of .” Where is the preposition？
And in the above sentence，what does “that” modify
1.maps and the reports of sailors
The last question :Does "of sailors" modify the reports only. or " new maps refined by precise calculations and the reports " or " precise calculations and the reports "?
Thanks for your help