Diving  is a  potentially  hazardous  activity.  The materials  contained within  this  magazine  are for informational purposes only and are not intended as a substitute for proper and appropriate training.

Underwater Navigation

A diver's ability to know  precisely where they are in relation  to the shore or  boat is fundamental to every  good  dive  plan.  The  greater  the  diver's proficiency  in   the   techniques   of   underwater navigation the easier it becomes for them to know not only  where  they are  at any point during the dive, but  also  where  they're  going!   It's a  skill that, once  mastered, helps  moderate the  subtle stress and higher  breathing rate often associated with  disorientation; one  that  gives  the  diver  a

greater  degree of  control over  proper gas  management,  that reduces  the need for lengthy - and potentially exhausting - surface swims, and that adds to the safety and pleasure of the dive.

With these  considerations in mind, all  divers should  have  an  understanding  of  the problems posed  by the environment  and a  basic  knowledge  of  standard navigation techniques using both natural aids and instruments.

Environmental Considerations Inspired  by gravity  and the two-dimensional  comfort  of land, most  of  us have  no difficulty in navigating around our neighbourhood, especially on a clear, sunny day.  In the denser, three-dimensional  environment of  the ocean accurate natural navigation over even short distances often becomes fraught with difficulties. Because  water is a  denser  medium  than  air,  one  that  readily  holds  organic  and inorganic particles  in suspension, the  divers  range of  vision  is limited.  Even  in the clearest  of waters  it's difficult  to  clearly  distinguish  objects  little  further  than  a hundred metres  or so away.  (Sadly, it's rare to find a location where the underwater visibility is  constantly  that good!  More  often than  not the  visibility  is measured in single  digits.)  Coupled  with  this  is  the  angle  at which  the  objects  are  viewed.

On land, with our feet firmly anchored  to the ground, we tend  to  view  our  surroundings  from  the  same perspective.  If  we   know  the  area  well  then  it's relatively  easy   to  distinguish  one   landmark  from another.  At a pinch we can even - depending on the extent   of   any  cloud   cover  -   orient   ourselves according to the position of  the sun or stars and the time of day!

Beneath the  surface things  change.  Near  weightless and  now able to move up and down, as well as  forward, backward or from  side to  side, the  diver is subject to the influences of an  aquatic environment.  Currents  and tidal flows - even those with no more force  than a gentle  surface breeze - impact  on the  diver; apparently  familiar features like rock and reef formations assume different  and sometimes unrecognisable characteristics when viewed from the angle of a  different depth; turbid water affects vision and the diver's ability to gauge direction by focussing on distant objects.  Even determining direction by the position of the sun  becomes a poor guide in all but clear, calm  and  shallow  waters.  That  light  which  is  not  reflected  back  from the sea's surface  is  diffused  by  the  denser  medium,  while  ripples  and  waves  cause  light refraction and constantly shifting shadows.

Research Regardless of  the complexity of the dive  and the  required  navigational  skills, begin building a mental  picture of the  dive site  by studying  marine charts and  noting the depth  contours of  the  seafloor as  well as  its composition.  Water  movement  or  a careless  fin kick  can easily  disturb ooze  and fine silt deposits, making navigation by visual reference difficult or obsolete.  Consult tide tables and, where  possible, determine  in advance of a dive the strength of the tidal flow  and the  times of high  and low tide.  Pay  similar  attention  to  local water  circulation patterns, the  types of current common to  the  area, their strength and direction.  These are often determined by the shape and nature of a nearby shore as well  as the  prevailing wind  and weather  patterns, forces  that  can  combine  to produce local variations in water movement.

Talk with other  divers familiar with  the area and check out the mud  maps that many dive  stores  compile  on  popular  dive  sites  in  their  locality.  Although  lacking  the precision, scale and accuracy of a  chart, mud maps will  usually contain a lot of detail on  features  and   physical  characteristics  of   the  site  to  help  verify  the  diver's approximate position.

Navigating without a compass Although an imprecise art and one that's really only appropriate to shallow waters, (as from a shore  dive) having an overall appreciation  of the dive site  is an important first step when using natural aids to navigation. The more familiar a diver becomes with the dive site, the easier it becomes to navigate around it  without reference  to a compass.  Apart from the ability to recognise certain prominent features of their surroundings, they  become more  attuned to  the tidal flow and its direction.

On a featureless seafloor a small handful of ooze or  fine silt will, when released, form a small  cloud  that  tends to  follow the  direction  of an  incoming  and  outgoing tide or prevailing current.  A coarser sandy bottom will, especially in shallower waters, take on a ripple pattern  that always runs parallel  to the shoreline.  Formed by water deflected back  from the land, the  tops of  these  sand ripples  have a  tendency  to curve over towards the shore. Water movements and the  actions of the tides have  a similar affect on seaweeds and grasses.  Swaying towards and away from  the shore, the length  of time that they will point in any one  direction is governed  by the water flow; longer towards the shore on an incoming tide and longer away from it on an outgoing one.

Depth is  another indication  of position.  Broadly  speaking, the  deeper  the water the further the diver  is away from  land.  Ledges, drop-offs and steeper  slopes provide an obvious  visual reference, but  where  increasing  depth occurs  more  gradually  -  and again, especially in shallower  waters - the diver can gauge the extent of their descent without reference  to instruments, (but  why bother?) by  whether or not they have to clear their ears - and how frequently. Sunlight,  while an  often-quoted aid  to natural navigation, is  a poor reference source unless the waters are exceptionally clear and calm, especially where heavy cloud cover exists!  Even in  those situations when  the sun is shining  brightly, the  amount of light penetrating the surface will be governed by the time of the day, the sun's angle to the horizon and surface ripple and wave patterns. Having  knowledge of their underwater  swim speed, or the number  of fin kicks it takes to cover a  measured  distance, can  also be used  as a rough guide in  determining the diver's position in relation to the surface.

Navigating with a compass When diving in areas  of  poor visibility, new  and unknown  locations, or  where  strict accuracy is required, a compass becomes an essential item - as does the ability to use it properly! A good  underwater compass for use by  divers differs slightly from  those used on land and may  consist of  either a  magnetised  needle or  a free swinging  magnetised  dial. Etched into  the surface window of  both is a  straight  line, known  as the  'lubber line' that  effectively bisects  the compass in two.  To  be of use  to the  diver, both  types have a rotating outer bezel to which is attached two  parallel line markers that sit over the window.

Although  there  are variations in  the  manner in  which  the  degrees appear  on  the compass, they  tend  to  be shown  in increments  of 5°.  In  the  case  of  a  popular magnetised dial brand, the  dial itself shows  the four  cardinal points  with bolder lines used to mark north and south.  Painted  clockwise onto the  dial in 30° gradations with 10° line indicators, the way in which the degrees are displayed has been reversed from the normal; north is  shown as  180° while the  360°/home position  is in the  south, a feature  that allows the diver  to read  his true heading through a small, side-mounted window in the casing.  On the  rotating outer bezel, the  degrees, marked clockwise in increments of 5°, appear in the normal fashion. A simple illustration  of the use of a compass is to  point the lubber line in the direction in  which the  diver wishes  to travel.  The bezel is then  rotated until  the north arrow rests between the parallel line markers.  Maintaining  the position of the north-pointing arrow between the  two parallel markers and  following the  direction  indicated by the lubber line allows the diver to move forward in a constant direction.

To return  back to their  starting point on a  reciprocal course, the  diver turns around until the south-pointing arrow on  the compass dial rests  between the parallel markers and north lines up with the opposing pointer. Using a compass  in the weightless  water environment  sounds easier  than is actually the case and preserving  a desired direction is  only possible when the  diver holds and reads the instrument  correctly.  This is first accomplished by the diver positioning him or herself properly in relation to the lubber line and visualising that line extending down the entire length of  their body, neatly  dividing it in  to two equal  halves.  Something that is best achieved by holding  the compass  as far away from the  head as possible and by looking across it, rather than down on to it.

For  this  reason  a  compass  worn on  the  left  wrist and  brought  into  position  by extending the right arm straight out in  front and bending the left elbow until the diver is able to grasp  the right arm  on and above the  elbow, provides  better  accuracy in holding a heading than attempting to read a console-mounted compass. Swimming  more  complex  patterns  can  be  as  simple  a  matter  as  stopping  at  a pre-determined  point  and, while  still facing the direction of travel, rotating the bezel clockwise  by  the  required  number of  degrees if  the turn  is to the  left - counter- clockwise if  it's to the right - and  moving the  body in that  direction until the north- pointing arrow is again aligned between the parallel markers.

Subject to  other  influences,  however, the  diver's  use  of  a  compass  alone  is  no guarantee of navigational  accuracy.  Swimming  headlong into a  current, or travelling with  its flow, will affect  the distance  covered  while the  subtle effects  of a  cross- current will  carry an unprepared  diver holding a constant  compass heading easily off course.  In those  instances, the diver  must ascertain  the direction  and  strength of any  current in  advance of  the  dive and be  able to factor  those variables  into the planned route using a chart, a protractor and - if necessary - a calculator! In that regard - and  when navigational accuracy  is paramount - the  use of an easily made swim-board becomes invaluable.  A two-handled board constructed of marine ply or  plastic  and  small  enough  to   avoid  unnecessary  drag   but  large   enough   to accommodate  a centrally  mounted  compass, depth  gauge, timing device  and slate, the  swim-board offers many  advantages over a  wrist-mounted compass.  When held out in  front of the diver, it  effectively extends  the lubber  line, a characteristic that permits even greater accuracy in maintaining a heading.

Previously  armed  with  knowledge  of  his  or her surface  air  consumption  rate  and swimming speed  underwater, information  on the details  of depths  to  be maintained, compass headings  to be followed, distances to be  travelled, times  and at what point changes to the course direction are to be affected, can be translated to the slate and seen by the diver at a glance. Something  that is best  learned through  constant practice, proficiency in underwater navigation is a confidence building skill that adds immeasurably to  the safe enjoyment of every dive.

Measuring current strength From an anchored vessel, the diver drops  a small low-profile marker buoy attached to 50-metres of light line over  the side and notes the time taken for it to travel to the limit of the line.  He also takes a compass bearing on its direction of movement - which should be the same as the boat  swinging on its  anchor and which, for the purpose  of this  exercise, is  due south at 180°.  If  it takes the buoy 3 minutes to travel 50 metres, then it will take 6 minutes to travel 100-metres, or 60 minutes to travel 1  kilometre.  The current's  strength is, therefore, approximately 1 kilometre per hour.  (To  be strictly  accurate, wind and wave  action and their effect on the buoy's movement also needs to be factored in to the calculation!) Compensating for currents Given  the  speed  of  the  current,  the  diver  plans  a  compass  swim  towards a submerged  object lying in an easterly direction, 500 metres  away.  He knows that his normal swimming speed underwater is, say, 2 kilometres per hour; a speed that, barring  the  external  influence  of  the  current,  would  allow  him  to  cover  the straight-line  distance  in 15 minutes.  Setting  his proposed  compass heading due east at 90  degrees, he begins his swim unaware of the influence that  the current moving  southward  at 1  kilometre  per hour  has on  his  progress.  After covering 500-metres while maintaining a  constant compass heading, the diver finds that he is almost 250-metres downstream of where he intended to be! In this  hypothetical example, the  choice faced  by the diver is either to re-anchor the boat 250-metres  upstream of his  target and, while still swimming on the same compass  heading  of 90°, allow  the current  to  carry  him down  to  his  intended objective,  or -  and less satisfactory  in terms  of effort  and energy  expenditure, (especially  in  stronger  currents)  -  to  take  a  bearing  on  a  spot  250-metres upstream of the target and swim slightly into the current.