NavList:

Dear Brad and George,

Brad, you wrote :

QUOTE

These altitudes are measured.

UNQUOTE

I fully agree with your position :

1 - YES, and until the contrary is proven, these published altitudes are observed, and

2 - Until the contrary is proven also, these published altitudes are NOT CORRECTED for either AUGMENTED SD or PARALLAX. If such were the case, they would be FAR TOO inconsistent with the published positions, and especially the published latitudes, at least in the 2 examples I have worked on. In other words, given published fixes in Latitude/ Longitude, if the associated published altitudes had been corrected for SD and Parallax we could immediately conclude with very little risk of error that the accuracy of such heights observations would have been in the range of +/- 40' !!! a possibility I prefer totally ruling out for the moment ... So, for the time being, these published altitudes are NOT CORRECTED for either Parallax or (Augmented) semi-diameter,

3 - Are these altitudes corrected for dip ? This is still an open question. From the very few examples of this page which I have processed - namely some of the altitudes tagged with the "double + on top of each other" printer sign - and for the time being, observed heights compared to computed fixes seem consistent with a height of eye somewhere around 23 FT, exactly in the range suggested by George. So, with currently computed results, I would favour HoE @ 23 FT.

Whether DIP remains CONSTANT for all the observations of this page is also a new question we must raise. If they are various HoE's throughout this page, I see no place for having coded this important information other than these mysterious "printer signs". Since we cannot get third party direct confirmation until now on their meanings, this is an additionnal "best guessing task" we have to work on.

4 - Are these altitudes corrected for refraction ? I would reply :

4.1 - "maybe" only in the case they would have already been corrected for dip, (i.e. HoE = 0 ) and

4.2 - "probably not" if the dip correction has not been performed on these printed heights.

Why "probably not" ? Simply because dip correction is to be performed first, a fact that was certainly well known to this highly scientific expedition.

Last item, the processing of the Sep 11, 1773 observations into observed Fixes which I recently published will be highly interesting to compare with the published WGS84 coordinates of the Harbour / Haven in which they were calling at that time. I did not take time yet to check this on Google Map, and on top of that - for lack of historical familiarity with these voyages which I am just discovering on some "practical standpoint" - I do not consider myself sufficiently knowledgeable to pinpoint the one or the various positions where they might likely have dropped their anchor in the bay - I am assuming it to be such - of Ohama Reno Harbor.

Best Regards

Antoine

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