If you only have 20 feet of water then you can narrow it down to only 15 feet from you and you can tell if it is rigrt under you by looking as to where it starts,for instant,if a stump at the bottom of it is at the top of bottom coverage you are right over it but if it starts below the top of bottom then it is that much to one side and if it is way down to the top of bottom it is much to left or right,i agree to others that it is a detail to the 2d and helps give a pic to see what 2d is showing and as for reading from top to bottom it would not be as easy to read for it would look side ways,sounds like you need si or learn your di better

