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. Lesson 6: Lines of Position / Fixes

Learning Objectives:• Comprehend how sources of visual ranges, visual bearing, and

distance LOP’s are selected acquired and plotted.• Comprehend which combinations of LOP’s 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 EP’s.• Applicable reading: Hobbs, pp. 128-132, 140-141, 143-145, 174-175.

Learning Objectives:• Comprehend how sources of visual ranges, visual bearing, and

distance LOP’s are selected acquired and plotted.• Comprehend which combinations of LOP’s 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 EP’s.• Applicable reading: Hobbs, pp. 128-132, 140-141, 143-145, 174-175.

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. 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

"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

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. 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 ship’s DR plot. It is the ship’s position

on the earth’s 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 ship’s

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)

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 ship’s DR plot. It is the ship’s position

on the earth’s 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 ship’s

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)

Tower .Tower .1500

Range LOP

(Overhead 6-1)

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. 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.

• 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.

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• 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.

• 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.

Light .

1300Visual Bearing LOP

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. 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 ship’s 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?

• 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 ship’s 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?

Point

Distance LOP 1000

. TowerA B

13001300Point

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. Lines of Position / Fixes

• It is difficult to tell where the ship is at, and another LOP is needed to be sure of ship’s actual location.

• Note: If a navigator is unable to determine the ship’s 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.

• It is difficult to tell where the ship is at, and another LOP is needed to be sure of ship’s actual location.

• Note: If a navigator is unable to determine the ship’s 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

A B

13001300

1300. Tower

Ship’s position determined to be point B by use of third LOP.

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• 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:

• 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:

90o30o

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. 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.

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.

. Tank

C080S15

1805. .

.1820

1830 1830.

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. 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:

• 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:

TR 075SOA 10

TR 100SOA 10

. LightShoal

NMT 082

. Light

Shoal

NLT 115(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.

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:

Three and Six Minute Rules:Three Minute Rule:

Six Minute Rule:Distance traveled in yards in three minutes = Ship’s speed in knots x 100

Distance traveled in miles in six minutes = Ship’s speed in knots x 1/10

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. Homework

Chapter 7: Section 1- 1,2,3,4,

Section 2- All ODD #s

Chapter 7: Section 1- 1,2,3,4,

Section 2- All ODD #s