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I used photocopies of the original maps of
Cook's expedition published by Hakluyt Soc.
by Cambridge University Press
in 1955 (Available at Purdue library).
There are 68 maps and shore views,
and the collection is claimed to be complete.
Only the following maps have the right scale and
coordinates on the sides, with longitude from Greenwitch,
to determine positions
of certain points with 1' accuracy.

I choose a conspicious point on a map,
usually a cape, take its coordinates from the map,
and then try to identify the same point on
Terraserver photos. The results are rounded to 1 minute.
First error is in latitude, second in longitude.

First voyage (no chronometers were available):

Chart 9.  Cook's coords:      16d40'S 151d29'W
          Terraserver:        16d41'S 151d32'W Er:-1', -3'
Chart 13. East Cape: 37d43'S 179d0'E,
          Terraserver:        37d41'  178d33'  Er:+2', +27'
          Table Cape:         39d07'  178d25'
          Terraserver:        39d12'  178d00'  Er:-5', +25'
Chart 18. Cape Teerawitte:    42d21'S 175d35'E
          Terraserver:        41d18'  174d42'  Er:+3', +53'
          Cape Campbel:       41d41'  175d13'
          Terraserver:        41d47'  174d20'  Er:-6', +53'
Chart 24. Cape York:          10d42'S 141d42'E
          Terraserver:        10d43'  142d36'  Er:-1', -54'


Second and third voyages (both chronometers and Lunars used):

Chart 35. Friendly Islands:   21d16'S 174d44'E
          Terraserver:        21d17'  174d55'  Er:-1', -11'
Chart 55. N point Hawaii isl: 20d17'N 156d00'W
          Terraserver:        20d16'  155d52'  Er:+1',  +8'
          Karakakooa bay:     19d28'N 156d00'
          Terraserver:        19d28'N 155d56'  Er:-0',  +4'
Chart 57. Aivatcvhka bay:     52d54'N 157d33'E
          Terraserver:        52d56'  158d28'  Er:-2',  -45'
          Another point there:52d51'  157d48'

Second and third voyages (both chronometers and Lunars used):

Chart 35. Friendly Islands:   21d16'S 174d44'E
          Terraserver:        21d17'  174d55'  Er:-1', -11'
Chart 55. N point Hawaii isl: 20d17'N 156d00'W
          Terraserver:        20d16'  155d52'  Er:+1',  +8'
          Karakakooa bay:     19d28'N 156d00'
          Terraserver:        19d28'N 155d56'  Er:-0',  +4'
Chart 57. Aivatcvhka bay:     52d54'N 157d33'E
          Terraserver:        52d56'  158d28'  Er:-2',  -45'
          Another point there:52d51'  157d48'
          Terraserver:        52d53'  158d42'  Er:-2',  -54'

The average absolute error in latitude is 2.3'.
The average absolute error in longitude is 33' (36' in
the first voyage).
This corresponds to a Lunar distance measurement
accuracy of about 1', approximately.

I may conclude that the measurement accuracy was
about 1', and extra 1' in latitudes comes from
the horizon (or perhaps they used wood quadrants
instead of metal sextants for
altitudes, as Norrie advises:-)

The log of astronomical observations shows that enormous
number of measurements was made for each Lunar,
with 3 or 4 sextants and different observers.
It seems that to determine longitudes of important points
on the shore they made around 100 measurements altogether
for each such point.

It is interesting to find out how much of these
errors is due to the almanac errors.

Alex.


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