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Sight reduction, is the process of deriving from a sight 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.〔''The American Practical Navigator'' (2002)〕 Nowadays sight reduction uses the equation of the circle of equal altitude to calculate the altitude of the celestial body, and the azimuth Zn is obtained from Z by: With the observed altitude Ho, Hc and Zn are the parameters of the Marcq St Hilaire intercept for the line of position: With B the latitude (+ N / S), L the longitude (+ E / -W). LHA = GHA + L is the local hour angle, Dec and GHA are the declination and Greenwich hour angle of the star observed. And Hc is the calculated altitude. 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 Concise method (NASR) * Pub. 249 (formerly H.O. 249, Sight Reduction Tables for Air Navigation, A.P. 3270 in the UK) * Pub. 229 (formerly H.O. 229, Sight Reduction Tables for Marine Navigation * H.D. 486 (in the United Kingdom) * H.O. 214 (Tables of Computed Altitude and Azimuth) * H.O. 211 (Dead Reckoning Altitude and Azimuth Table, Third Edition, known as Ageton, and the Modified H.O. 211 Compact Sight Reduction Table, known as Ageton-Bayless) * H.O. 208 (Navigation Tables for Mariners and Aviators, Sixth Edition, known as Dreisonstok) * S-Table 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sight reduction」の詳細全文を読む スポンサード リンク
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