![]() Sight Reduction Tables for Marine Navigation, Pub. Sight Reduction Table for Marine Navigation. This method’s disadvantages include: It requires an AP for table entry and plotting and requires dual entries of two different tables. Summary: The advantages of the NASR method are: It uses only one book and requires no interpolation, and the pages are organized by increasing latitude, facilitating multiple reductions from the same assumed latitude. The instructions recommend using this method for sights with altitudes less than 80, but the degree of error for altitudes above 80 would be acceptable for practical small-vessel navigation. The NASR method can be used for sights from any latitude and for bodies of any declination. Only occasional rounding in the usual manner is needed. All data is extracted directly, requiring no interpolation between table entries. These dual entries can be somewhat confusing but are easily mastered with a little practice. True azimuth is then determined by following the rules provided on each page of the sight-reduction tables. Azimuth angle is determined in two partsZ1 and Z2which are added together. The auxiliary table is used to adjust the computed altitude (Hc) for the difference between the actual values and the values used to enter the tables. When solving multiple sights from the same assumed latitude, all of the first table extractions can be made at the same time from the same page since the pages are arranged by increasing latitude, thus saving some page-turning time. The first entering arguments for the concise sight-reduction table are LHA and assumed latitude. These tables occupy 32 pages near the back of the Nautical Almanac and come with instructions and examples. ![]() The disadvantage is that it requires dual entries of a table appropriately titled Sight Reduction Table and dual entries of a second table entitled Auxiliary Table. ![]() Hence, an advantage of the NASR method is that it enables sights to be reduced using a tabular method with only one volume. The NASR method uses concise sight reduction tables that are included as part of the Nautical Almanac. The Nautical Almanac Sight Reduction Method. Other methods that use your DR position tend to be a little easier, if only from a plotting perspective, since there is no need for the additional plotting of an AP. Some methods require the use of an assumed position (AP) to compute an assumed local hour angle (LHA) and assumed latitude for getting into the table, while others use your dead reckoning (DR) position. An electronic calculator is usually required for the multiplication and division necessary to solve the trigonometric equations that are part of the Law of Cosines method. This means they require only addition and subtraction to reduce the sight. They utilize tables to solve the navigational triangle. ![]() In this overview, we’ll look at the differences among the various methods.Īll methods discussed below are table-based methods (with the exception of the Law of Cosines method). ![]() To the aspiring celestial navigator, the process of converting a sextant sight into a line of position (LOP) on a plotting sheet or chart may seem overwhelming and full of pitfalls.Ĭompounding the confusion for the student of celestial navigation is the fact that there are many different ways to do sight reduction. ![]()
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