I don't actually know this to be the intended purpose, but to me, from just a visual inspection, it looks like it would be primarily an angle measurement tool. If you notice, the point at the center of the map has lines coming off of it at different angles, primarily one lines going North, North North East, North East, East North East, East, East South East, etc... all the way around the center of the map in a compass rose type fashion. If you follow these lines out, each of them connect to one of the intersection points you are seeing in a ring around the map. Similarly, it looks like each of these intersection points have all of those same lines drawn from them in the same directions, which is evidenced if you look at the compass rose on the left side of the map. I don't know exactly how these would have been used in their time, but my guess would be that it would help in measuring distance through some calculations as well as the obvious assistance in measuring angle/direction of the map. It also due to the E/W running lines and the N/S running lines creates a grid effect which may or may not have been of some significance.
Again, I don't know why specifically these were put on there or what specifically they would have been used for, but as to what governs where they were placed, it at least appears like it is all basically a big compass rose of compass roses... just my observations. Hope that helps.