Your readers should be cautioned that there are many errors in the on-line version of H.O 249, Sight Reduction Tables for Air Navigation and also in the recently printed editions, both in the government version and in in the commercial reprints. Any computed altitude within one degree of zero should be suspect as there was a programing 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 LOP. Although such large errors are possible they are less dangerous than smaller errors as such large errors would probably be noticed by the navigator while small errors might go undetected. I pointed out this problem a year ago to the Office of Global Navigation/Technology, National Geospatial-Intelligence Agency which had generated these tables. They have taken action and corrected the on-line version but the errors will persist for a long time in the printed tables on navigation station shelves.