equipment,   your   unit   can   be   susceptible   to   electrical interference. Interference    produces    random,    false    depth    readings which   bear   no   relation   to   the   true   depth.      It   can   be identified   readily   since   it   disappears   as   soon   as   the equipment  causing  the  interference  is  shut  off.    Below are some checkpoints. Indicator  and  transducer  cable  may  be  too  close  to other electrical wiring on board.  Try moving them. In  some  instances  interference  will  be  greater  when the  unit  is  powered  from  the  boat's  batteries  than from a separate battery source. High    speed    and    high    engine    rpm's    can    cause considerable  interference.    The  transducer  picks  up mechanical vibration caused by the engine, propeller,   shafting,   etc.   thus   producing   simulated 200 KHz echo signals that may false the depthfinder. NAVIGATING AND PILOTING Every government navigation chart-be it a Coastal Chart, Smalicraft     Chart,     Lake     Survey     Chart,     Intracoastal Waterway   Chart   or   River   Chart-gives   hundreds   and sometimes    thousands    of    representative    and    useful soundings.  Government survey teams have taken great pains  to  create  a  highly  reliable  underwater  topography (or    mapping    of    bottom    contours)    for    almost    every sizeable, navigable body of water. Knowledgeable    boatsmen    use    quality    depthfinders, along  with  their  compass  and  charts,  primarily  for  the safe  piloting  of  their  boats  in  unfamiliar  waters.    It  tells them  how  much  water  is  under  their  transducers  at  any given moment, and enables them to explore shoal areas without  running  aground.    To  repeat,  it  is  important  to remember,   however,   that   every   depthfinder   has   one critical  limitation:  it  cannot  make  predictions.    In  other words,  you  can  have  15  feet  of  water  under  you  one minute  and  be  hard  aground  the  next  because  of  rapid shoaling.    But  avoiding  groundings  is  only  one  useful-if obvious-function   of   your   digital   depthfinder.      Another, which   we'll   go   into   at   length,   takes   advantage   of   the many  hundreds  of  accurate  soundings  provided  on  your chart. Determining position Your   digital   depthfinder   is   probably   the   world's   most under-rated  or  under  utilized  electronic  navigational  aid. This    is    because    very    few    non-commercial    owners understand   and   take   advantage   of   its   position-finding potential. If  you  study  a  chart  you  will  see  that  it  not  only  has  an enormous number of soundings or indicated depths, but it  also  has  "elevations"  or  "bottom  contour  lines"  which outline areas of equal or nearly equal depth.  Your digital depthfinder is the only practical means of learning when you are crossing these lines.  It is therefore an enormous help in determining your position relative to your chart. Following contours Northern-bound  commercial  ships  approaching  the  port of New York will be on the continental shelf off the New Jersey coast.  When their depthfinders suddenly indicate much   deeper   water   they   know   they've   reached   the Hudson   River   Canyon.      They   then   turn   toward   the northwest   and   begin   their   approach   up   the   Narrows. Most    contour-following,    however,    is    the    other    way around. For example, say you're approaching a coastline in poor visibility,   carefully   comparing   depth   readings   to   your chart.        Your    Aqua-Probe    digital,    which    .has    been indicating   depths   of   60   to   70   feet,   begins   indicating sudden  shoaling  and  soon  is  registering  depths  of  30 feet;  your  shallow  alarm  (set  to  30  feet)  sounds.    Your chart  confirms  that  your  estimated  course  cuts  rapidly across    several    successive    fathom    curves    that    run exactly parallel to the shore half a mile out.  You may not know quite where you've crossed this shoaling area, but you do know you're half a mile off shore.  In other words, your  digital  depthfinder  has  given  you  a  pretty  firm  "line of    position."    (A    simultaneous    visual    bearing    on    a landmark   ashore   or   buoy,   or   a   simultaneous   bearing using   a   radio   direction   finder,   would   then   give   you  a "fix"...  that is, establish your position.) 16