For this achievement the easiest way to get it is to grab a helicopter
A very consistent spawn location for the helicopter would be the noose headquarters pictured here
https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcTb1gtE... Simply climb to the roof (on the left side of the building there is a ladder) and most times there will be a buzzard attack chopper.
Once you have all of the land marked, if it still hasn’t popped fly around the eastern coastline. Marking each of the white circles on your map. If it still doesn’t pop just fly around the island and search on the map for any small transparent spots on the map.