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Altitude Junior Navigation Corrections Chapter 4 1 Learning Objectives Understand how the following factors affect altitude observations: Dip Refraction Parallax Semidiameter Apply altitude corrections from the Nautical Almanac to observations of the sun Celestial Tools 2 Comparing hs to Ho and Hc hs – altitude measured with sextant ha – hs corrected for ‘IE’ and ‘dip’ and used to enter Altitude Correction Tables Apparent Position Ho – observed altitude after corrections Hc – altitude calculated using LHA, Dec, and Latitude True Position ha hs Earth True Horizontal Celestial Horizon 3 Sight Reduction Form 4 Sequence of Corrections Record height of eye Sextant altitude (hs) Index correction (IC) Dip Correction (Dip) Apparent altitude (ha) (Used to enter altitude correction tables) Correction from tables (Main) Observed altitude (Ho) 5 Nautical Almanac 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 6 Nautical Almanac Tables 7 Dip 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 8 Dip on Natural Horizon How to interpolate: HE is 7.5ft From table correction is 2.7 for 8ft and correction is 2.4 for 6ft Change in correction is 0.3 in 2ft Your HE from 8ft is -0.5ft 0.5ft is 25% of 2ft 25% of 0.3 is 0.077 Correction is 2.7 - 0.08 = 2.6 9 Dip on Natural Horizon How to select correction: HE is 14.6ft 14.6ft is more than 14.1ft but not more than 14.9ft so correction is 3.7 10 Dip Short (Appendix A) Sight taken with HE of 10.6 ft, across a distance of 670 yards. (Dip to NH would -3.2’) Use Almanac tablesbe if: Distance to shore in Yards Meters Ds Height Height of of Eye Eye in Feet Feet 3 Distance to Horizon Dip Short (Ds) at least Formula d = distance & h = height of eye nm sm yds Ds 2.0 = (0.0002052 x d) + [1146 x (h/d)] 2.3 4103 Meters 5 Ds x = (0.0002244 x d) x + (h/d)] [3438 x (h/d)] = (0.0002052 + [1146 2.6d)3.0 5296 Nautical miles Feet 7 Ds 3.1 = (0.4156 x6267 d) + [0.5658 x (h/d)] 3.6 Ds = (0.0002052 x 670) + [1146 x (10.6 / 670)] Statute miles Feet Ds = (0.3611 x d) + [0.6511 x (h/d)] 3.5 4.0 Ds = 0.1374849 + (1146 x 7106 0.015820896) 10 3.7 4.3 7490 Ds = 18.268’, rounded to 18.3’ 15 4.5 5.2 9174 Dip correction -18.3’ 20 5.2 6.0is 10593 11 Find 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 4 17 26 5 ? 1 7 ? 3 0 1 7 3 0 – 1 3 17 25 2 13 2 17 25.2 13 2 0 0 + 13 2 17 38 4 12 Find Ho 15 13 11FOR 9 YOUR 7 5 ANSWERS? 4 3 2 1 READY UL Sun sight is taken on 25 Jun. HE is 6.9 ft NH IE is 3.1´ on the arc hs is 56° 31.2´ LL Sun sight is taken on 10 May. HE is 5.5 ft Ds 345 yds IE is 0.5´ off the arc hs is 43° 50.0´ Use excerpts (Appendix B), pg 212 13 Find Ho UL Sun sight is taken on 25 Jun. HE is 6.9 ft NH IE is 3.1´ on the arc hs is 56° 31.2´ Use excerpts (Appendix B), pg 212 6 9 56 31 2 3 1 2 5 0 0 5 6 – 5 6 56 25 6 HE is 6.9ft From table correction is 2.7 for 8ft and correction is 2.4 for 6ft Change in correction is 0.3 in 2ft Your HE from 8ft is -1.1ft 1.1ft is 55% of 2ft 55% of 0.3 is 0.165 Correction is 2.7 - 0.2 = 2.5 16 5 0 0 16 5 – 16 5 56 09 1 14 Find Ho LL Sun sight is taken on 10 May. HE is 5.5 ft Ds 345 yds IE is 0.5´ off the arc hs is 43° 50.0´ Use excerpts (Appendix B), pg 212 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’ 5 5 43 50 0 0 5 18 3 0 5 18 3 - 17 8 43 32 2 15 0 15 0 0 0 + 15 0 43 47 2 15 ‘Celestial Tools’ 16 Using ‘Celestial Tools’ 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. The Sailings – calculate, using mid-latitude or Mercator sailing, or the method Sight Sight LoC/NASR Planner Reduction –and calculate and Fix Fix – allows the – completely twilight the user times reduce to for enter aaselected data sight from to date intercept theand upper location andpart azimuth of and used in VPP2, and distance initial and final positions, final position TVMDC MoBoard Length – will of–course Degree update perform the –several calculate values calculations offrom the thelength compass typically of a variables degree done of on (True, latitude a maneuvering Variation, and find by athe the sight Law azimuth, reduction of Cosines altitude, form method, (derived and magnitude allowing from data students of the extracted visible to check navigational fromthe the quality Nautical their at a Noon Favorite Sight Places – calculate – save the thetime latitudes, of Local longitudes, Apparent range Noon (LAN) of useful forbodies azimuths, aof selected Dist. by Brngs –to calculate the distance to atwo fixed (and related from initial position, and distance, and set and drift position, Magnetic, board, longitude e.g. in Closest various Point linear Compass) of units Approach for both between any athe spherical one is changed. moving earth for vessels ItaDR will specified also (or Distances Sight Yellow CTS/SOA Interpolation Averaging Pages –Deviation, a2 –distance calculate – will –course, analyze –and produce will the the do “Course a natural single the run values ofwhen to or horizon sights Steer” double of and calculator, and interpolation, Nautical calculate “Speed aobject Almanac geographical the offrom and Advance” average includes Increments watch range for a of selected sights Almanac) while time, still and as – “at areduce list the ordip beach”. the athen star sight chart. After using a It minimum will the also Law of of Cosines two the times and are of Nautical moonrise reduced and date type and of horizon, longitude, and and short calculates distance latitude forto locations from that aofsights noon will sight. be reused It will for also 60D=ST Arc <=> –Time calculate convert speed, an time, angular or distance value when itsfind equivalent two the time values and are vice known. versa. quantities) byelapsed taking two bearings onspheroid the object), object from a moving vessel. fix position, and time. Itconverter. will calculate, using great circle sailing, calculate between latitude, the anorth moving value orcalculator, of south. vessel variation The and for WGS84 a three stationary a also selected yearoutput course duplicates on the and data speed Bowditch provided of visibility time and course Corrections a and of sexagesimal-to-decimal and an sextant object speed table. altitude. affected by and current. distance bybased vertical angle calculators. moonset, saved, Almanac you the have Sight phase the Reduction offrom option theAutomatically Moon, of methods, establishing and the giving availability a this the fix using same sights intermediate acceptable and from final the calculate sight-taking longitude sessions. the observed loads zone time date forofLAN. into theselected SightSun-Moon Planner. initial course and final apparent position, wind. plus maximum latitude, final course, oncontact 2002 a chart. Table vessel, 7. distance true windorfrom fixes. listvalues of up to as ten. would be obtained with a manual reduction. and points on the great circle route. 17 Using ‘Celestial Tools’ 18 Using ‘Celestial Tools’ 30 June 2012 12-58-29 – + 1 Sun LL 0-14 12-58-15 1 0 65 21 2 4 0 4 0 3 30 June 2012 12-58-29 – 71 39 50 6 15-58-15 30 June 0-14 G g 1 Sun LL o 1 0 – 3 0 71 71 39 50 6 41 2 17 0 44 2 15 6 65 21 2 G g 17 0 44 2 15 6 00 + 15 6 71 56 8 19 Using ‘Celestial Tools’ 71 17 0 44 2 1 0 4 0 4 0 1 0 – June 30 3 0 2012 12-58-29 41 2 0-14 – o 12-58-15 71 15 +6 3 15-58-15 30 June 15 6 00 + 15 6 71 56 8 20 Quiz 1. Dip correction is the angular difference between the visible horizon and a true horizontal. a. True b. False 21 Quiz 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 22 Quiz 3. Find the dip correction: HE Correction 18 ft - 4.1' ____ 22 ft - 4.5' ____ 72 ft - 8.2' ____ 23 Quiz 4. For the following data, determine main sextant altitude corrections: Body Date ha Main Sun UL 21 June 17° 32.1' -18.8' Sun LL 29 Sept 43° 46.0' +15.0' Sun UL 29 Sept 43° 46.0' -16.8' Sun LL 10 Mar 61° 41.6' +15.7' 24 Next class BRING INFORMATION ON ANY SIGHTS YOU HAVE TAKEN. 25 Altitude Corrections End of Junior Navigation Chapter 4 26