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From:           	"George Huxtable" 

> Ken Muldrew wrote-
>
> On 19 Oct 2010 at 16:23, George Huxtable wrote:
>
> Ken is correct. Land explorers (in America anyway, where we have good
> documentation), such as Thompson, would calculate his altitudes,
> rather than measure them. Perhaps Ken can tell us whether Thompson
> just accepted the first resulting value, or whether he ever did any
> reiteration.

I've never seen any iterations from Thompson, Peter Fidler, or Philip Turnor.

> Of course, land explorers travelled much less far, in the course of a
> day, 15 miles or so, than ships did, which could travel 10 times as
> far.

The Fur Trade explorers were often traveling by canoe so 50-70 miles in a day
wasn't unusual. This was often in an Easterly or Westerly direction so
Thompson's pocket watch (an ordinary verge) was always out by several (and
sometimes several tens of) minutes when he took a time sight. Still, he always
had the shore to judge distance by, so his job was a lot easier than keeping an
account at sea.

> I don't know of any texts which have considered this question of a
> need for reiteration when altitudes have been calculated, and wonder
> whether travellers were even aware of its potential for causing error.
> Can anyone point to such a publication? Can Ken tell us whether Wales
> or Turnor addressed it? More to the point, it would be nice to have
> access to such a text to confirm whether or not I have analysed the
> problem correctly.

When Wales and Turnor were instructing others, they don't seem to have
mentioned the possiblity of iterating the clearing process when using calculated
altitudes. I don't think I've ever seen any mention of an iterative clearing process
in historical texts and logbooks.

Ken Muldrew.

   

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