Lesson 6: Lines of Position Fixes

1
Lesson 6 Lines of Position / Fixes

• Learning Objectives
• Comprehend how sources of visual ranges, visual
  bearing, and distance LOPs are selected acquired
  and plotted.
• Comprehend which combinations of LOPs used in
  determining a fix are most reliable and
  desirable.
• Apply correct procedures to become proficient in
  plotting and labeling fixes.
• Comprehend the use of radar in piloting.
• Comprehend the use of danger bearings and apply
  correct procedures to plot and label such
  bearings.
• Comprehend the difference between a fix and an
  Estimated Position (EP) and know the proper
  labeling procedures for EPs.
• Applicable reading Hobbs, pp. 128-132, 140-141,
  143-145, 174-175.

2
Humor

• "Good Things About Rooming With the President's
  Daughter"
• As presented on the 09/10/97 broadcast of LATE
  SHOW with DAVID LETTERMAN
• 10. Bitchin' motorcade from history class to
  language lab
• 9. She shows up with beer coasters hand-knit by
  Betsy Ross
• 8. Your summer job next year Ambassador to
  Belgium
• 7. Secret Service guys always available to buy
  you beer
• 6. Her care packages always include a tray of
  dad's "special" brownies
• 5. You become fourth in line for Presidency
• 4. At some point, you find yourself playing
  "quarters" with Ted Kennedy
• 3. When ordering from Domino's, you can take
  advantage of the President's volume discount
• 2. If you receive poor mark on test, you can have
  professor slapped around by Janet Reno
• 1. Somehow, you're not so embarrassed about your
  own father

3
Lines of Position / Fixes

• The major concern of navigators at sea is the
  accurate determination of position. They must
  be able to determine present position, and also
  predict future ship positions.
• Determining the Fix
• Fix The initial element of the ships DR plot.
  It is the ships position on the earths surface
  at some given point in time.
• A fix is determined by the intersection of at
  least two simultaneous lines of position (LOPs).
• Line of Position (LOP) The locus of points along
  which a ships position must lie.
• Although the intersection of two LOPs would be
  sufficient to obtain a position, standard
  practice is to obtain three lines of position in
  order to guard against the possibility of one or
  more being in error.
• The most accurate LOP possible is obtained by
  visually observing two objects in a line
  (usually called a range)

(Overhead 6-1)
4
Lines of Position / Fixes

• LOPs of this type are known as visual ranges.
  They are plotted on a chart by placing a
  straightedge along an imaginary line drawn
  through the objects sighted in line and drawing a
  short segment of the line near the approximate
  position of the ship on the chart at the time of
  observation (the dashed line is included for
  clarity and should not appear on the chart in
  normal practice). LOPs are never extended to the
  navigation aid in order to avoid erasing the aid
  with repeated use of the chart.
• All LOPs as well as all others are labeled with
  the time of observation above the line segment.

5
Lines of Position / Fixes

• A range can seldom be observed at the moment when
  a fix is desired, so a visual bearing LOP is
  plotted by observing a bearing to a single object
  by means of a gyro repeater and a bearing circle.
  The true bearing from the object is sighted
  (shot) and plotted on the chart. If it is not
  possible to obtain the true bearing to an object
  from a gyro repeater, the bearing taker may shoot
  a relative or magnetic compass bearing to the
  object. The navigator must convert the relative
  or magnetic bearing into a true nearing before it
  can be plotted.

6
Lines of Position / Fixes

• A Distance Line of Position can be obtained by
  use of a stadimeter or radar. A ship must lie
  somewhere on the circle centered on the object,
  with the radius equal to the distance measured.
  Distance LOPs are usually referred to as ranges.
  Ranges are plotted by placing the pivot point of
  a drawing compass on the object or landmark shop
  and swinging an arc with the point.
• If two simultaneous lines of position are plotted
  (it does not matter how they were obtained - i.e.
  a range and a visual can be used together) their
  intersection represents the ships position.
  However, as previously stated, standard practice
  is to use at least three LOPs as this will
  eliminate ambiguity. Consider the situation
  below. Is the ship at point A or B?

. Tower
7
Lines of Position / Fixes

• It is difficult to tell where the ship is at, and
  another LOP is needed to be sure of ships actual
  location.
• Note If a navigator is unable to determine the
  ships exact location, it must be assumed that
  the ship is at the point closest to danger until
  another round of bearings is obtained and actual
  position is determined.

Point
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Lines of Position / Fixes

• In order to minimize the effect of possible
  errors in observed bearings, navigators should
  attempt to optimize the angular spread of objects
  shot.
• If two objects are used, they should be as close
  to 900 apart as possible. If three objects are
  shot, they should optimally be 1200 apart.
• The following illustration shows the reasoning
  behind this rule of thumb by showing the effects
  of a /- 5 error in the bearings of two objects
  300, and 900 apart

9
Lines of Position / Fixes

• The estimated position It is sometimes
  impossible to obtain more than a single LOP
  within the recommended 30-minute interval. In
  this event, it may be possible to obtain an
  estimated position based on whatever incomplete
  information is available. Examples of this type
  of information may be a bearing to an aid to
  navigation, a distance to land, or a series of
  echo soundings which coincide with a certain area
  depicted on the chart.
• One fairly well-established method of obtaining
  an estimated position when a single LOP is
  available is to draw a construction line from the
  DR position corresponding to the time of the LOP
  to the closest point on on the LOP. In the case
  of a straight LOP, such as a line of bearing,
  this construction line would be a perpendicular
  drawn from the DR position to the LOP, as shown
  below
• Note A new track is not plotted from an
  estimated position.

10
Lines of Position / Fixes

• A questionable fix, or a fix based on LOPs of low
  confidence, can be treated as an estimated
  position.
• On the reliability scale, an EP is normally
  considered to be about midway between a good
  running fix and an unsubstantiated DR position.
• Danger Bearings In conjunction with plotting
  the new track, the navigator should clearly mark
  the safe limits of navigable waters on either
  side of a channel by means of a pre-computed
  visual bearing to a prominent landmark or
  navigation aid known as a danger bearing.
• Hatching is always applied to the hazardous
  side of the bearing, and the side on which the
  hazard exists is indicated by labeling the
  bearing NLT for not less than, or NMT for not
  more than, the indicated bearing

(Overhead 6-2)
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Lines of Position / Fixes

• In this example, the navigator has laid a track
  down the center of a narrow channel and has drawn
  danger bearings marking the shallow water to the
  right of the 0750 leg and the shoal to the left
  of the 1000 leg.
• If the bearing to the first light were anything
  less than 1150 as the ship approached the 1000
  leg it would be in danger of running through the
  shallow water.
• As the ship approached the 1000 leg, a bearing to
  the second light greater than 0820 would indicate
  that the ship is in danger of running onto the
  shoal.
• When danger bearings are drawn in relation to an
  intended track, those markings ahead and to the
  left of track are always labeled NMT, and those
  to the right are labeled NLT.

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Lines of Position / Fixes

• Three and Six Minute Rules
• Three Minute Rule
• Six Minute Rule

Distance traveled in yards in three minutes
Ships speed in knots x 100
Distance traveled in miles in six minutes
Ships speed in knots x 1/10
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Homework

• Chapter 7 Section 1- 1,2,3,4,
• Section 2- All ODD s