This trophy is pretty self-explanatory. Climb all the points on your map that have the eagle icon, then press circle to synchronize at the top.
The sneaky part of this trophy is that you also have three viewpoints in the World War 1 storyline.
To access this memory chain, complete sequence 6 and there will be a new icon on the very far right of the Thames River. From there you have to complete at least the first memory to unlock the viewpoints.