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 From looking at his chart and the Nautical Almanac it is clear that the
highest altitude he measured on sun was about 51 degrees which would
produce a three-tenths of a mile offset at 45 miles (Table 19) and maybe
one NM two degrees along the LOP, nothing to worry about.

For those curious about computing winds based on drift sights I have
attached an excerpt from Air Force Manual 51-40.

gl

Gary LaPook wrote:
> Chichester did use  Mercator charts, actually plotting sheets, for
> this flight. I have attached his charts. The first two are
> illustrations from his books. The third is figure 41 from Chichester's
> own explanation of the navigation of this flight published in the
> Observer's Book, which I have also attached. This chart is a
> simplified copy of the following charts, the actual chart that he used
> on his flight from Norfolk to Lord Howe. The last chart is his actual
> chart used for flying to Norfolk from New Zealand. I have also
> attached Hughes' explanation of the navigation.
>
> The actual charts may look confusing because of the arcs and the
> radial lines which was Chichester's method of determining the wind
> encountered in flight based on his observations of his angle of drift
> on two different headings. More modern practice is to plot the drift
> lines on an E-6B rather than on the chart itself to determine the
> winds. If you are just interested in the celnav aspects, you can
> ignore all of these lines as Chichester did in his simplified figure
> 41 in the Observer's Book.
>
> Your concern over plotting a curved LOP as a straight line on a
> Mercator chart is unwarranted. I have attached Table 19 from Bowditch
> which shows how much the curved LOP departs from a straight line. You
> can see that, except for very high altitudes, this factor can be
> ignored especially given the lower level of precision available with
> celnav in flight.
>
>
> You can download the complete American Practical Navigator for free
> from this website, choose in the "select" box.
>
> 
http://www.nga.mil/portal/site/maritime/?epi_menuItemID=35ad5b8aabcefa1a0fc133443927a759&epi_menuID=e106a3b5e50edce1fec24fd73927a759&epi_baseMenuID=e106a3b5e50edce1fec24fd73927a759Christian
> Scheele wrote:
>
> gl
>> Hello Gary,
>>
>> I hear you and I largely agree with you but please bear with me for a
>> moment while I try to work something out. If you're assuming that a
>> navigator is plotting on a Mercator chart ( I think Chichester was),
>> I wouldn't say that deliberate offset navigation is a modern
>> implementation of latitude sailing. Latitude sailing, using the noon
>> method, gives you a latitude line and this means a position line that
>> can be plotted on a Mercator chart as a straight line without the
>> need for any corrections. When you find a position line on the basis
>> of a sun shot at any other time other than noon, your plotted
>> position line, because it is plotted as an uncorrected straight line
>> on a Mercator chart, will always be "a bit" assumed unless..... you
>> are actually plotting a real great circle arc, i.e. a position line
>> corrected for the map projection. Could the astrograph do this? I
>> assume not, because the curves on its foils are rigid, unless it
>> actually came with maps to fit the curves, rather than curves to fit
>> the maps, but that would be impractical way to do something I would
>> imagine the astrograph was supposed to make easy in the first place.
>> I am assuming, therefore, that the astrograph was, from a precision
>> point of view, not a "perfect" plotter. I know that in world war 2
>> some navigators had an astrograph over their tables, but how many I
>> don't know, I don't know much about the military stuff. In any event,
>> Chichester didn't use an astrograph. I'm still assuming that
>> Chichester used a Mercator projection. Chichester was using sun, not
>> star shots. When using the stars, if you have a reasonably good idea
>> of where you are, say you are sure that you must be within about 2
>> degrees, depending on which region you are in, of a point on the the
>> position line which, again, is a small arc of a great circle track,
>> and assuming that the global position of the sun is far away enough,
>> then the aforementioned problem should not exist as you can then plot
>> your position more or less as a straight line - the great circle
>> radius is so big - just like Weems did in his books plotting star
>> altitude curves. I say 2 degrees because I am taking an extract of
>> Weems pages showing a latitude and great circle radius for which this
>> arc size to gradient of position line relationship holds. You can't
>> plot the "sun curves" in the same way as you would star curves,
>> because of the rapid change of declination. When I said Chichester
>> had an assumed  position line rather than a position line, I was
>> assuming that such a relationship which allows one to plot a great
>> circle arc as a straight line may not have been available to
>> Chichester, had his D.R. navigation not been so good, all the while
>> still assuming Chichester was plotting position lines as
>> "uncorrected" straight lines onto a Mercator map.... But I am not
>> entirely sure, maybe I should have phrased it as a question...here's
>> what I was thinking.......Let's say his D.R. had not been so good and
>> he had been on the unfavourable side of the position line, that is
>> away from the island, further than he had planned to be at the
>> precomputed time. Let us assume that this error had brought him
>> beyond 2 degrees (120 miles) or so of the predetermined point on the
>> position line.   Assuming this scenario, how serious would it have
>> been had he plotted his position line as a straight line on Mercator
>> chart? Imagine: The straight now really becomes a curve if, only a
>> very slight one. Could this produce a substantial error? Norfolk
>> island is 35sqkm. Lord Howe is 56sqkm. For this last stage of the
>> flight, D.R. is of course used again. Did he have the drift, wind
>> speed indicators that should have been around by the time of world
>> war 2? What about his compass? Did he have more than one? An
>> induction compass like Lindbergh - who did not use astro on his epic
>> flight - to check the magnetic one? Let's say some of these issues
>> come in and another error, albeit a much smaller one, is added on to
>> the existing one. Could a pilot miss the island due to the combined
>> sum of such errors? Very important in this regard is: How high was
>> Chichester flying, i.e. what what was the range of his visible
>> horizon? Perhaps I'm wrong and it wasn't that serious and any error
>> would have been "innocuous" due to the far visible horizon. In
>> referring to Chichester's navigational method, I was implicitly
>> including this speculation....
>> Again, I largely agree with you  ....but there's just that shred of
>> doubt.
>> I'll take a thorough look at your website as soon as possible.
>> Thanks, I appreciate the tip.
>>
>>
>> Christian Scheele
>>
>> ----- Original Message ----- From: "glapook@pacbell.net"
>> 
>> To: "NavList" 
>> Sent: Thursday, June 11, 2009 4:01 AM
>> Subject: [NavList 8612] Re: A-10 Sextant Manual
>>
>>
>>
>> Well, I don't know that you can call it "reckless" since Chichester's
>> deliberate offset navigation procedure became the standard navigation
>> method for finding islands and was taught to thousands of navy and air
>> corps navigators during the second world war. It was actually a just a
>> modern implementation of the centuries old method of latitude sailing,
>> approaching to one side of a destination, east or west, and then
>> following the latitude LOP to the destination.
>>
>> See the texts I have posted on my website at :
>>
>> http://www.geocities.com/fredienoonan/
>>
>> Go to "List of topics" then to "Single LOP landfall procedure."
>>
>> gl
>>
>>
>>
>> >>
>>
>>
>


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