altitude corrections

11

Altitude Altitude CorrectionsCorrections

Junior Junior NavigationNavigationChapter 4Chapter 4

22

Learning ObjectivesLearning Objectives Understand how the following factors Understand how the following factors

affect altitude observations: affect altitude observations: DipDip RefractionRefraction ParallaxParallax SemidiameterSemidiameter

Apply altitude corrections from the Apply altitude corrections from the Nautical AlmanacNautical Almanac to observations of the to observations of the sunsun

Celestial ToolsCelestial Tools

33

Comparing hs to Ho and HcComparing hs to Ho and Hc

Earth Celestial

Horizon

Visible Horizon

True Horizontal

Apparent

PositionTrue Position

hs – altitude measured with sextantha – hs corrected for ‘IE’ and ‘dip’ and used to enter Altitude Correction TablesHo – observed altitude after correctionsHc – altitude calculated using LHA, Dec, and Latitude

Ho or HC

hahs

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Sight Reduction FormSight Reduction Form

LAST

WEEK

LAST

WEEK

NEXT WEEK

NEXT WEEK

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Sequence of CorrectionsSequence of Corrections Record height of eyeRecord height of eye Sextant altitude (Sextant altitude (hshs)) Index correction (Index correction (ICIC)) Dip Correction (Dip Correction (DipDip))

Apparent altitude (Apparent altitude (haha) (Used to ) (Used to enter altitude correction enter altitude correction tables)tables)

Correction from tables (Main)Correction from tables (Main)

Observed altitude (Observed altitude (HoHo))

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Nautical AlmanacNautical Almanac Tables Tables Altitude Correction table for Sun, and Dip

correction for a natural horizon - Appendix B Sun table includes corrections for

refraction, semidiameter, parallax, and other minor items

“Altitude Correction Table” and most of “Dip Table” are critical tables

DO NOT require interpolation Extreme right-hand column of “Dip Table”

correction (less than 8ft or greater than 70ft)

DOES require interpolation

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Nautical AlmanacNautical Almanac Tables Tables

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Dip on Natural HorizonDip on Natural Horizon Correction based on

height of eye (HE) above water line where sight taken

For HE <8ft or >70ft <155ft use right column and interpolate

For HE between 8ft-70ft use left column and no interpolation required

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Dip on Natural HorizonDip on Natural Horizon How to interpolate:

HE is 7.5ftHE is 7.5ft From table correction is 2.7 for From table correction is 2.7 for

8ft and correction is 2.4 for 6ft8ft and correction is 2.4 for 6ft Change in correction is 0.3 in 2ft Change in correction is 0.3 in 2ft Your HE from 8ft is -0.5ftYour HE from 8ft is -0.5ft 0.5ft is 25% of 2ft0.5ft is 25% of 2ft 25% of 0.3 is 0.07725% of 0.3 is 0.077 Correction is 2.7 - 0.08 = 2.6 Correction is 2.7 - 0.08 = 2.6

1010

Dip on Natural HorizonDip on Natural Horizon How to select correction:

HEHE is 14.6ft is 14.6ft 14.6ft is 14.6ft is more thanmore than 14.1ft 14.1ft

but but not more thannot more than 14.9ft 14.9ft so correction is 3.7so correction is 3.7

Sight taken with HE of 10.6 ft, across a distance of 670 yards.

(Dip to NH would be -3.2’) Use Almanac tables if:

Height of Eye

Distance to Horizon at least

Feet nm sm yds3 2.0 2.3 41035 2.6 3.0 52967 3.1 3.6 62679 3.5 4.0 7106

10 3.7 4.3 749015 4.5 5.2 917420 5.2 6.0 1059

3

Distance to shore in

Height of Eye in

Dip Short (Ds) Formula d = distance & h = height of eye

Yards Feet Ds = (0.0002052 x d) + [1146 x (h/d)]

Meters Meters Ds = (0.0002244 x d) + [3438 x (h/d)]

Nautical miles Feet Ds = (0.4156 x d) + [0.5658 x

(h/d)]Statute miles Feet Ds = (0.3611 x d) + [0.6511 x

(h/d)]1111

Dip Short (Appendix A)Dip Short (Appendix A)

Ds = (0.0002052 x d) + [1146 x (h/d)]Ds = (0.0002052 x 670) + [1146 x (10.6 / 670)]

Ds = 0.137484 + (1146 x 0.015820896) Ds = 18.268’, rounded to 18.3’

Dip correction is -18.3’

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Find HoFind Ho LL Sun sight is taken on 29 Feb.

HE = 9.4 ft NH hs is 17° 26.5´ IE is 1.7’ off the arc

9 417 26 5

1 73 0

17 25 2

1 7 3 01 3–

13 2

13 2+ 13 2

0 0

17 38 4

?

17 25.2

?

1313

Find HoFind Ho UL Sun sight is taken on 25 Jun. UL Sun sight is taken on 25 Jun.

HE is 6.9 ft NHHE is 6.9 ft NH IE is 3.1´ on the arc IE is 3.1´ on the arc hs is 56° 31.2´hs is 56° 31.2´

Use excerpts (Appendix B), pg 212

LL Sun sight is taken on 10 May. LL Sun sight is taken on 10 May. HE is 5.5 ft Ds 345 ydsHE is 5.5 ft Ds 345 yds IE is 0.5´ off the arc IE is 0.5´ off the arc hs is 43° 50.0´hs is 43° 50.0´

15 13 11 9 7 5 4 3 2 1READY FOR YOUR ANSWERS?

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Find HoFind Ho UL Sun sight is taken on 25 Jun. UL Sun sight is taken on 25 Jun.

HE is 6.9 ft NHHE is 6.9 ft NH IE is 3.1´ on the arc IE is 3.1´ on the arc hs is 56° 31.2´hs is 56° 31.2´

6 956 31 2

3 12 5

56 25 6

0 0 5 65 6–

16 5

0 0– 16 5

16 5

56 09 1

HE is 6.9ftHE is 6.9ft From table correction is 2.7 for From table correction is 2.7 for

8ft and correction is 2.4 for 6ft8ft and correction is 2.4 for 6ft Change in correction is 0.3 in Change in correction is 0.3 in

2ft 2ft Your HE from 8ft is -1.1ftYour HE from 8ft is -1.1ft 1.1ft is 55% of 2ft1.1ft is 55% of 2ft 55% of 0.3 is 0.16555% of 0.3 is 0.165 Correction is 2.7 - 0.2 = 2.5 Correction is 2.7 - 0.2 = 2.5


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LL Sun sight is taken on 10 May. LL Sun sight is taken on 10 May. HE is 5.5 ft Ds 345 ydsHE is 5.5 ft Ds 345 yds IE is 0.5´ off the arc IE is 0.5´ off the arc hs is 43° 50.0´hs is 43° 50.0´

Find HoFind Ho5 5

43 50 0

0 518 3

43 32 2

0 5 18 317 8-

15 0

15 0+ 15 0

0 0

43 47 2


Ds = (0.0002052 x d) + [1146 x (h/d)]Ds = (0.0002052 x 345) + [1146 x (5.5 / 345)] Ds

= 0.070794 + (1146 x 0.015942029) Ds = 18.34036’, rounded to 18.3’

Dip correction is -18.3’

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‘‘Celestial Tools’Celestial Tools’

Sight Planner – calculate the twilight times for a selected date and location and find the azimuth, altitude, and magnitude of the visible navigational bodies at a selected time, as a list or a star chart. It will also find the times of moonrise and moonset, the phase of the Moon, and the availability of acceptable Sun-Moon fixes.

Sight Averaging – analyze a run of sights and calculate the average watch time and sextant altitude.CTS/SOA – calculate the “Course to Steer” and “Speed of Advance” for a course and speed affected by current.Arc <=> Time – convert an angular value to its equivalent time and

vice versa. Distances – a distance to the natural horizon calculator, a geographical range of visibility of an object calculator, and three distance by vertical angle calculators.

Interpolation – will do single or double interpolation, and includes a sexagesimal-to-decimal converter. 60D=ST – calculate speed, time, or distance when two of the values are

known.Yellow Pages – will produce the values of the Nautical Almanac Increments and Corrections table.

Dist. by 2 Brngs – calculate the distance to a fixed object (and related quantities) by taking two bearings on the object from a moving vessel.

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Using ‘Celestial Tools’Using ‘Celestial Tools’

Sight Reduction and Fix – completely reduce a sight to intercept and azimuth by the Law of Cosines method, allowing students to check the quality of their sights while still “at the beach”. After a minimum of two sights are reduced and saved, you have the option of establishing a fix using sights selected from the list of up to ten.

Noon Sight – calculate the time of Local Apparent Noon (LAN) for a selected date and longitude, and then calculates latitude from a noon sight. It will also calculate longitude from the observed zone time for LAN.

LoC/NASR and Fix – allows the user to enter data from the upper part of a sight reduction form (derived from data extracted from the Nautical Almanac) and reduce the sight using the Law of Cosines and Nautical Almanac Sight Reduction methods, giving the same intermediate and final values as would be obtained with a manual reduction.

The Sailings – calculate, using mid-latitude or Mercator sailing, or the method used in VPP2, course and distance from initial and final positions, final position from initial position, course, and distance, and set and drift from DR position, fix position, and elapsed time. It will also calculate, using great circle sailing, initial course and distance or final position, plus maximum latitude, final course, and points on the great circle route.

TVMDC – will update the values of the compass variables (True, Variation, Magnetic, Deviation, and Compass) when any one is changed. It will also calculate the value of variation for a selected year based on the data provided on a chart.

MoBoard – perform several calculations typically done on a maneuvering board, e.g. Closest Point of Approach between two moving vessels (or between a moving vessel and a stationary object), course and speed of contact vessel, true wind from apparent wind.

Favorite Places – save the latitudes, longitudes, range of useful azimuths, type of horizon, and dip short distance for locations that will be reused for sight-taking sessions. Automatically loads this date into the Sight Planner.

Length of Degree – calculate the length of a degree of latitude and longitude in various linear units for both a spherical earth for a specified latitude, north or south. The WGS84 spheroid output duplicates Bowditch 2002 Table 7.

Although the data produced by Celestial Tools is fairly accurate, the program should

not be used for navigation. Its primary purpose is instructional: to aid a student in

planning sights; to allow a student to quickly check the quality of sights; and to help a

student find errors in his/her work. It is not intended to do the work for the student.

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30 June 201212-58-29

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1. Dip correction is the angular difference between the visible horizon and a true horizontal.

a. Trueb. False

QuizQuiz

2222

2. A ship's captain takes a sight from the ship's rigging with a HE of 54 feet. His mate takes a sight from the deck of the ship with a HE of 18 feet. For which HE will the dip correction be greatest?

HE of 54 feet

QuizQuiz

2323

3. Find the dip correction:

HE Correction 18 ft ____22 ft ____ 72 ft ____

- 4.1'- 4.5'- 8.2'

QuizQuiz

2424

4. For the following data, determine main sextant altitude corrections:

Body Date ha Sun UL 21 June 17° 32.1' Sun LL 29 Sept 43° 46.0' Sun UL 29 Sept 43° 46.0' Sun LL 10 Mar 61° 41.6'

Main -18.8'

+15.0'

-16.8'

+15.7'

QuizQuiz

2525

BRING INFORMATION

ON ANY SIGHTS YOU

HAVE TAKEN.

Next classNext class

2626

Altitude CorrectionsAltitude Corrections

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Chapter 4Chapter 4

altitude corrections

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