Harvesters take the shortest route... by first going diagonally towards the destination until they hit its X or Y axis, then they travel horizontally or vertically along the X or Y axis until they reach the destination.

So if Source is 0,0 and Destination is 3,10 the route would be...

0,0
1,1
2,2
3,3
3,4
3,5
3,6
3,7
3,8
3,9
3,10

Make sense?