Coring site positioning |
In recent years GPS (Global Positioning Systems) instruments have been used widely for locating and documenting sampling sites in the field. Suffice to say that this tool has made accurate positioning on the water much simpler and more convenient. However, those accustomed to routine positioning with sub-meter accuracy on land have to adopt a somewhat different perspective when using GPS on small boats. While it is possible in theory (and sometimes in practice) to position a small boat on a station with that kind of precision, it is more often not possible for very practical reasons. Even when a small pontoon boat is securely anchored at all four corners, its anchor lines are elastic and it will still move constantly in wind, waves and currents -- sometimes for yards (meters) in all directions. At best, an average position can be determined with GPS to some measured degree of accuracy. When navigating to and anchoring at locations pre-determined with GPS, it is most helpful to have a visual marker -- a bouy -- to steer by. The usual 3-point anchoring method requires setting anchors well beyond the station in three different directions, so that all three lines can be tightened to hold the boat securely over the station bouy. Minor corrections can be made by further adjusting the lines. It is important to maintain this position over the work while vibrating the tube in and pulling it out. |
