Sight-Reduction Error Cited

A CW reader shares his correspondence with the National Geospatial-Intelligence Agency's H.O. 249, Sight Reduction Tavels for Air Navigation.

Message to the CW Mailbag:

Dear Mr. Pillsbury,

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.

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 there are likely to be similar errors 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 these errors.

Gary LaPook

Message to the National Geospatial-Intelligence Agency:

Dear Sir:

Quite by chance I noticed an error in the on line edition of HO 249 volume 2. I then looked for similar errors and found that there is a systematic error in both volumes two and three. For example, on page 5 of volume 2 covering latitude 0º in the first column covering declination of 15º at LHA 89 the altitude given is 00-58'; for LHA 90 the altitude is given as 00-00'; and for LHA 91 the altitude given is 00-58' which is the same value as given for LHA 89! This is mathematically impossible. The problem is that the altitude for LHA 91 should be negative 00-58'. The systematic error found all across the tables up to at least latitude 39º is that all of the last positive altitudes tabulated should all be negative altitudes! This is also true for the values given for contrary name declinations. For example on page
163 of volume 2 covering latitude 27º in the first column for 0º declination tabulated for LHA 91 an altitude of 00-53'; for LHA 90 an altitude of 00-00'; and again at LHA 89 an altitude of 00-53'. The altitude for LHA 91 should be negative 00-53'.

I compared these examples with my printed copies of these tables printed in 1970 and these errors do not appear in the printed version which correctly label these altitudes as negative altitudes.


Gary LaPook

Response from the National Geospatial-Intelligence Agency:

Mr. LaPook

Thank you for bringing this error to our attention. It seems that the Sight Reduction Table generation code improperly formatted entries beginning with -0, inserting 00 instead.

I am in the process of manually updating the electronic publications, which will be posted to our Web site in the very near future.

The errant publications were originally produced and posted to our Web site in 1999. Which mean that, in their 8+ years of service, you are the first and only user to notice the error!


Tim Doherty
Office of Global Navigation/Technology
National Geospatial-Intelligence Agency

Message to the National Geospatial-Intelligence Agency:

Dear Mr. Doherty,

Thank you for your reply. I know that it is very rare that a navigator
would use altitudes near zero so it is not surprising that the error
was not discovered sooner. I found it when I was testing a method for
computing Hc and was comparing my results with H.O 249 to ensure that
method I was testing was working. As part of the test protocol I
tested cases near zero and, at first, couldn't figure out why my
results didn't match, assuming there was a problem with my method.
Only then did I notice the discrepancy in H.O. 249.

This also caused me concern that there might be other errors in H.O.
249 that were not so apparent. Hopefully your review has eliminated
this concern.

Gary LaPook

Additional Message to the National Geospatial-Intelligence Agency:

Dear Mr. Doherty,

Last year I called your attention to some errors I had found in the
on-line version of H.O. 249 for Hc's near zero and I checked recently
and these appear now to have been corrected. However, when I was
reviewing the updated version I found some additional errors.

In volume 2 page 158 is completely wrong and appears to be a duplicate
of page 152 and page 133 is also completely wrong and appears to be a
duplicate of page 139! I suspect that there are many other similar
errors hiding in this volume that my cursory review didn't reveal.

Gary LaPook