Sight reduction

In navigation, sight reduction is the process of deriving from a sight, (in celestial navigation usually obtained using a sextant), the information needed for establishing a line of position.

Sight is defined as the observation of the altitude, and sometimes also the azimuth, of a celestial body for a line of position; or the data obtained by such observation.[1]

Nowadays sight reduction uses the equation of the circle of equal altitude to calculate the altitude of the celestial body,

${\displaystyle \sin(Hc)=\sin(Lat)\cdot \sin(Dec)+\cos(Lat)\cdot \cos(Dec)\cdot \cos(LHA)}$

and the azimuth ${\displaystyle Zn}$ is obtained from ${\displaystyle Z}$ by:

${\displaystyle \cos(Z)={\frac {\sin(Dec)-\sin(Hc)\cdot \sin(Lat)}{\cos(Hc)\cdot \cos(Lat)}}={\frac {\sin(Dec)}{\cos(Hc)\cdot \cos(Lat)}}-\tan(Hc)\cdot \tan(Lat)}$

With the observed altitude ${\displaystyle Ho}$, ${\displaystyle Hc}$ and ${\displaystyle Zn}$ are the parameters of the Marcq St Hilaire (Intercept method) intercept for the line of position:

Correction to the sextant altitude

Marcq St Hilaire intercept for the line of position

With ${\displaystyle Lat}$ the latitude (North - positive, South - negative), ${\displaystyle Lon}$ the longitude (East - positive, West - negative), ${\displaystyle LHA=GHA+Lon}$ is the local hour angle, ${\displaystyle Dec}$ and ${\displaystyle GHA}$ are the declination and Greenwich hour angle of the body observed, and ${\displaystyle Hc}$ the calculated altitude. ${\displaystyle Z}$ is the calculated azimuth of the body.

Basic procedures involved computer sight reduction or longhand tabular methods.

Tabular sight reduction

The methods included are:

• The Nautical Almanac Sight Reduction (NASR, originally known as Concise Tables for Sight Reduction or Davies, 1984, 22pg)
• Pub. 249 (formerly H.O. 249, Sight Reduction Tables for Air Navigation, A.P. 3270 in the UK, 1947–53, 1+2 volumes)[2]
• Pub. 229 (formerly H.O. 229, Sight Reduction Tables for Marine Navigation, H.D. 605/NP 401 in the UK, 1970, 6 volumes.[3]
• The variant of HO-229: Sight Reduction Tables for Small Boat Navigation, known as Schlereth, 1983, 1 volume)
• H.O. 214 (Tables of Computed Altitude and Azimuth, H.D. 486 in the UK, 1936–46, 9 vol.)
• H.O. 211 (Dead Reckoning Altitude and Azimuth Table, known as Ageton, 1931, 36pg. And 2 variants of H.O. 211: Compact Sight Reduction Table, also known as Ageton–Bayless, 1980, 9+ pg. S-Table, also known as Pepperday, 1992, 9+ pg.)
• H.O. 208 (Navigation Tables for Mariners and Aviators, known as Dreisonstok, 1928, 113pg.)

Longhand haversine sight reduction

This method is a practical procedure to reduce celestial sights with the needed accuracy, without using electronic tools such as calculator or a computer. And it could serve as a backup in case of malfunction of the positioning system aboard.

Doniol

The first approach of a compact and concise method was published by R. Doniol in 1955[4] and involved haversines. The altitude is derived from ${\displaystyle \sin(Hc)=n-a\cdot (m+n)}$, in which ${\displaystyle n=\cos(Lat-Dec)}$, ${\displaystyle m=\cos(Lat+Dec)}$, ${\displaystyle a=\operatorname {hav} (LHA)}$.

The calculation is:

n = cos(Lat − Dec)
m = cos(Lat + Dec)
a = hav(LHA)
Hc = arcsin(n − a ⋅ (m + n))

Ultra compact sight reduction

Haversine Sight Reduction algorithm

A practical and friendly method using only haversines was developed between 2014 and 2015,[5] and published in NavList.

A compact expression for the altitude was derived[6] using haversines, ${\displaystyle \operatorname {hav} ()}$, for all the terms of the equation: ${\displaystyle \operatorname {hav} (ZD)=\operatorname {hav} (Lat-Dec)+\left(1-\operatorname {hav} (Lat-Dec)-\operatorname {hav} (Lat+Dec)\right)\cdot \operatorname {hav} (LHA)}$

where ${\displaystyle ZD}$ is the zenith distance,

${\displaystyle Hc=(90^{\circ }-ZD)}$ is the calculated altitude.

The algorithm if absolute values are used is:

if same name for latitude and declination (both are North or South)
 n = hav(|Lat| − |Dec|)
 m = hav(|Lat| + |Dec|)
if contrary name (one is North the other is South)
 n = hav(|Lat| + |Dec|)
 m = hav(|Lat| − |Dec|)
q = n + m
a = hav(LHA)
hav(ZD) = n + a · (1 − q)
ZD = archav() -> inverse look-up at the haversine tables
Hc = 90° − ZD

For the azimuth a diagram[7] was developed for a faster solution without calculation, and with an accuracy of 1°.

Azimuth diagram by Hanno Ix

This diagram could be used also for star identification.[8]

An ambiguity in the value of azimuth may arise since in the diagram ${\displaystyle 0^{\circ }\leqslant Z\leqslant 90^{\circ }}$. ${\displaystyle Z}$ is E↔W as the name of the meridian angle, but the N↕S name is not determined. In most situations azimuth ambiguities are resolved simply by observation.

When there are reasons for doubt or for the purpose of checking the following formula[9] should be used:

${\displaystyle \operatorname {hav} (Z)={\frac {\operatorname {hav} (90^{\circ }\pm \vert Dec\vert )-\operatorname {hav} (\vert Lat\vert -Hc)}{1-\operatorname {hav} (\vert Lat\vert -Hc)-\operatorname {hav} (\vert Lat\vert +Hc)}}}$

The algorithm if absolute values are used is:

if same name for latitude and declination (both are North or South)
 a = hav(90° − |Dec|)
if contrary name (one is North the other is South)
 a = hav(90° + |Dec|)
m = hav(|Lat| + Hc)
n = hav(|Lat| − Hc)
q = n + m
hav(Z) = (a − n) / (1 − q)
Z = archav() -> inverse look-up at the haversine tables
if Latitude N:
 if LHA > 180°, Zn = Z
 if LHA < 180°, Zn = 360° − Z
if Latitude S:
 if LHA > 180°, Zn = 180° − Z
 if LHA < 180°, Zn = 180° + Z

This computation of the altitude and the azimuth needs a haversine table. For a precision of 1 minute of arc, a four figure table is enough.[10][11]

An example

Data:
 Lat = 34° 10.0′ N (+)
 Dec = 21° 11.0′ S (−)
 LHA = 57° 17.0′
Altitude Hc:
 a = 0.2298
 m = 0.0128
 n = 0.2157
 hav(ZD) = 0.3930
 ZD = archav(0.3930) = 77° 39′
 Hc = 90° - 77° 39′ = 12° 21′
Azimuth Zn:
 a = 0.6807
 m = 0.1560
 n = 0.0358
 hav(Z) = 0.7979
 Z = archav(0.7979) = 126.6°
 Because LHA < 180° and Latitude is North: Zn = 360° - Z = 233.4°

See also

• Navigation
• Celestial navigation
• Circle of equal altitude
• Intercept method

References

1. The American Practical Navigator (2002)
2. Pub. 249 Volume 1. Stars; Pub. 249 Volume 2. Latitudes 0° to 39°; Pub. 249 Volume 3. Latitudes 40° to 89°
3. Pub. 229 Volume 1. Latitudes 0° to 15°; Pub. 229 Volume 2. Latitudes 15° to 30°; Pub. 229 Volume 3. Latitudes 30° to 45°; Pub. 229 Volume 4. Latitudes 45° to 60°; Pub. 229 Volume 5. Latitudes 60° to 75°; Pub. 229 Volume 6. Latitudes 75° to 90°.
4. Table de point miniature (Hauteur et azimut), by R. Doniol, Navigation IFN Vol. III Nº 10, Avril 1955 Paper
5. Rudzinski, Greg (July 2015). Ix, Hanno. "Ultra compact sight reduction". Ocean Navigator. Portland, ME, USA: Navigator Publishing LLC (227): 42–43. ISSN 0886-0149. Retrieved 2015-11-07.
6. Altitude haversine formula by Hanno Ix http://fer3.com/arc/m2.aspx/Longhand-Sight-Reduction-HannoIx-nov-2014-g29121
7. Azimuth diagram by Hanno Ix. http://fer3.com/arc/m2.aspx/Gregs-article-havDoniol-Ocean-Navigator-HannoIx-jun-2015-g31689
8. Hc by Azimuth Diagram http://fer3.com/arc/m2.aspx/Hc-Azimuth-Diagram-finally-HannoIx-aug-2013-g24772
9. Azimuth haversine formula by Lars Bergman http://fer3.com/arc/m2.aspx/Longhand-Sight-Reduction-Bergman-nov-2014-g29441
10. http://fer3.com/arc/m2.aspx/Longhand-Sight-Reduction-HannoIx-nov-2014-g29172
11. Natural-Haversine 4-place Table; PDF; 51kB

External links

• Navigational Algorithms: resources for Longhand Haversine Sight Reduction
• NavList A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
• Celestial Tools for the USPS/CPS JN/N Student
• Graphical all-haversine Hc reduction
• Sight Reduction - free App for android