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Errors in H.O. 249 sight reduction tables

Mar 23, 2009

To the editor: Readers should be cautioned that there are many errors in the online version of H.O. 249 and also in the recently printed editions, both in the government version and in the commercial reprints. Any computed altitude within one degree of zero should be suspect as there was a programming error that generated these tables that did not place negative signs for the first entry below zero, printing them as positive values.

Although it is unlikely that a navigator would be using altitudes this low, the possibility is there and the error could approach 60 minutes of altitude producing up to a 60-nautical error in the line of position. Although such large errors are possible, they are less dangerous than smaller errors since large errors would probably be noticed by the navigator. Small errors, however, might go undetected. I pointed out this problem a year ago to the Office of Global Navigation and the National Geospatial-Intelligence Agency which generated these tables. They have taken action and corrected the online version of the tables, but the errors will persist for a long time in the printed tables on the shelves in chandleries.

But there are even greater errors to be found in these tables. While checking the online version to see that the previous errors had been cured, I found additional errors. I did not make a complete comparison of the new version with my older, error free, edition but the errors I did find suggests that similar errors are likely to be scattered throughout H.O. 249, possibly in all three volumes. Two glaring errors I found in volume 2 are that both pages 133 and 158 are completely wrong and appear to be duplicates of pages 139 and 152 respectively! These errors are more dangerous than the first group as all values on these pages are wrong and a navigator is more likely to be using these values than the values near zero associated with the first problem. Every navigator should check these pages to see if he has tables with errors.

—Gary LaPook learned to fly in 1970 and has been an airline transport pilot since 1978. In the 1970s he ferried new airplanes from the factory to aircraft dealers, including crossing the Atlantic using celestial navigation to find the Azores. He started sailing in the 1960s and always brings his sextant along.

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