JN Chapter 4 - St Paul Sail & Power Squadron
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Transcript JN Chapter 4 - St Paul Sail & Power Squadron
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
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– allows
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the user
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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