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Author Topic: Navigating the LOP with the offset method.  (Read 198526 times)

Chris Owens

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Re: Navigating the LOP with the offset method.
« Reply #105 on: June 03, 2011, 03:59:27 PM »

OK bear with me "stupid warning If I knew I was running on a LOP like FN may or may not have plotted, could i not just use the compass to keep a bearing of ?South, South East? to head towards the Phoenix group.  If the wind was against me could I not just keep to a heading that I had in mind?

(this is based upon the idea that as a hill walker in fog/mist and lost if i just keep to a compass bearing that directs me towards something then i should get there?)

The problem is the difference between heading (the direction in which your plane or ship is pointed) and course - the track you are following over the ground.  This is a new concept for people who are used to using a compass while walking, or in a car, because on foot and in a car, the ground stays still (i.e. point south and walk in a straight line and you go south).  But in a ship or a plane, the water or air does not stay still.  Point a plane south and fly in a straight line and you go south x the wind velocity.   Unless you have a very good estimate of what the wind is doing (in a plane) or current (in a boat), you can't steer to a course, only to a heading
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Chris Johnson

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Re: Navigating the LOP with the offset method.
« Reply #106 on: June 03, 2011, 04:02:31 PM »

OK one final stupid question and thanks for your patience.

On foot i walk and if i stray to my left as is common i can use my compass to see this? Why not see the arrow in my compass on a plane move?
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Ric Gillespie

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Re: Navigating the LOP with the offset method.
« Reply #107 on: June 03, 2011, 06:35:03 PM »

The second quote given by Mr. van Asten is shown to be at the top of page 239, with what appears to be a footnote number of "8".  To see from whom that statement about Pan American's use of the technique came will probably require a visit to a library.  Copies of the book are said to be in the vicinity of Washington, DC, at the Library of Congress, the Smithsonian, NOAA (in Silver Spring, MD), and in the Nimitz Library at the U.S. Naval Academy.

As you correctly imply, we can't judge the credibility of the allegation unless we know the source.  Noonan makes no mention of using an offset and I don't recall any reference to Pan Am using offsets in various books (i.e. The Chosen Instrument, Bender/Altschul, 1982; Pan American's Pacific Pioneers, Krupnick, 2000; etc.)
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Daniel Paul Cotts

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Re: Navigating the LOP with the offset method.
« Reply #108 on: June 03, 2011, 10:29:38 PM »

OK one final stupid question and thanks for your patience.

On foot i walk and if i stray to my left as is common i can use my compass to see this? Why not see the arrow in my compass on a plane move?

Because the compass in the plane does not move. It tells us the direction the nose of the plane is pointed not the actual movement of the plane relative to the ground. In an extreme example the plane would appear from the ground to be flying with its nose rotated left or right from its actual movement.  See "crabbing" = To direct (an aircraft) partly into a crosswind to eliminate drift.

In the hiking example I see two possibilities. One takes place in clear conditions. A sight is taken on a landmark. The hiker follows the azimuth. Ground conditions may obscure the landmark at times. Each time the landmark is viewed a new sight is taken and the corrected new azimuth is followed. In the second case the landmark is not in view. The azimuth is taken from a map. The hiker follows that bearing. He may stray off course due to conditions such as heavy brush or fog. If there is any significant distance to traverse my guess is he would not reach his destination straight on. Once he has gone the requisite distance hopefully he will recognize a terrain feature or road that will lead him to the goal. Think of the straying off course as analogous to wind drift.
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Chris Owens

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Re: Navigating the LOP with the offset method.
« Reply #109 on: June 03, 2011, 10:41:50 PM »

OK one final stupid question and thanks for your patience.

On foot i walk and if i stray to my left as is common i can use my compass to see this? Why not see the arrow in my compass on a plane move?

There's a subtle but important difference between two different ways of "straying to your left."  

Imagine yourself on a football field.  Stand in the center of one goal and note that the other goal is directly South of you. Hold a compass down by your waist.  Now put a large bucket over your head so that you can look down and see the compass, but you can't look around and see any of the field at all. There are no painted lines and that the turf is very smooth and even so it gives you no visual cues.  All you can see is the compass.

 Start walking straight ahead, keeping the compass needle on 'S'.  You will eventually reach the middle of the other goal, or within a couple of feet.

Next, imagine that every 5 steps, I say "Stop", and instruct you to close your eyes while I pick you up, shake you around a little bit, and put you back down, at which point you can open your eyes again and resume walking. Let's say it takes 50 steps to cross the field, so I do this ten times.
  • If what I did while I picked you up was simply turn you, say, 10 degrees to the left, then when I put you back down and you open your eyes again, the compass needle won't be pointing at 'S' any more, and all you need to do is turn to the right until the compass needle points to 'S' again and resume walking.  You'll get to the other goal just fine.
  • On the other hand, what if what I did while I picked you up was not to turn you at all, but instead to carry you five feet to your left and put you back down. (remember, I'm shaking you so you can't really tell by the feel  if I'm turning your or displacing you). Now, when I put you back down and you open your eyes, the compass will still be pointing to 'S', so you'll just resume walking. But, after doing this ten times, by the time you reach the other end of the field you won't be at the goal at all, you'll be 50 feet to the left of it. Your compass won't have helped you at all.

Dealing with wind (in aircraft) or current (in boats) is like the second case, not the first.
« Last Edit: June 03, 2011, 10:44:20 PM by Chris Owens »
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Gary LaPook

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Re: Navigating the LOP with the offset method.
« Reply #110 on: June 04, 2011, 12:36:36 AM »

OK one final stupid question and thanks for your patience.

On foot i walk and if i stray to my left as is common i can use my compass to see this? Why not see the arrow in my compass on a plane move?

There's a subtle but important difference between two different ways of "straying to your left."  

Imagine yourself on a football field.  Stand in the center of one goal and note that the other goal is directly South of you. Hold a compass down by your waist.  Now put a large bucket over your head so that you can look down and see the compass, but you can't look around and see any of the field at all. There are no painted lines and that the turf is very smooth and even so it gives you no visual cues.  All you can see is the compass.

 Start walking straight ahead, keeping the compass needle on 'S'.  You will eventually reach the middle of the other goal, or within a couple of feet.

Next, imagine that every 5 steps, I say "Stop", and instruct you to close your eyes while I pick you up, shake you around a little bit, and put you back down, at which point you can open your eyes again and resume walking. Let's say it takes 50 steps to cross the field, so I do this ten times.
  • If what I did while I picked you up was simply turn you, say, 10 degrees to the left, then when I put you back down and you open your eyes again, the compass needle won't be pointing at 'S' any more, and all you need to do is turn to the right until the compass needle points to 'S' again and resume walking.  You'll get to the other goal just fine.
  • On the other hand, what if what I did while I picked you up was not to turn you at all, but instead to carry you five feet to your left and put you back down. (remember, I'm shaking you so you can't really tell by the feel  if I'm turning your or displacing you). Now, when I put you back down and you open your eyes, the compass will still be pointing to 'S', so you'll just resume walking. But, after doing this ten times, by the time you reach the other end of the field you won't be at the goal at all, you'll be 50 feet to the left of it. Your compass won't have helped you at all.

Dealing with wind (in aircraft) or current (in boats) is like the second case, not the first.
-------------------------------

Or,

While you are walking with your head in the bucket looking at your compass, the grounds crew is pulling the astroturf that you are walkin on off to the west. When you have walked your estimated hundred yards and you look straight ahead again, no goal posts.

gl
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Chris Johnson

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Re: Navigating the LOP with the offset method.
« Reply #111 on: June 04, 2011, 01:03:07 AM »

Thanks guys for humoring a land lubber  :D
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pilotart

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Re: Navigating the LOP with the offset method.
« Reply #112 on: June 04, 2011, 02:30:39 AM »

Another good analogy to illustrate the difference between heading and course is to visualize crossing a river in a boat.

You could just point the bow at the dock across the river and as the current carried you downstream, keep turning and pointing the bow at the dock and you will get there, but your course will be a French Curved arc and not a straight line.  (In radio navigation of an aircraft, this would be called "homing".)

Now if you want the boat to follow a straight course across that river, you would just aim your bow upstream to cancel the current's effect (or drift) and have a straight line across the river for your course.

The degree of correction depends on both the speed of the boat and speed of the current.  In computing the heading and ground speed for your aircraft, you draw a "Wind Triangle" and for a flight with a ten mile-an-hour wind; you would first draw a ten mile offset from your starting point in the direction of the wind.  Now you draw the third leg of your triangle from that point to a point one hour flight time (zero-wind) up your course line and you can measure on that line the heading you will need and the length of that line will give you your ground speed (or how far through the moving air you will need to fly to cover that distance).

In 1937 you would have used a simple 'slide-rule' type calculator and pencil to do that 'wind-triangle' for you.

Art Johnson ATP/AGI/CFII

Art Johnson
 
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Gary LaPook

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Re: Navigating the LOP with the offset method.
« Reply #113 on: June 04, 2011, 12:35:50 PM »

Well then , in document "Francis Chichester , 1922 Portuguese flight , Navigator , p.  238 ,  we read : "April 12 , 1922 . At 1815 navigator found plane to be on a LOP which , extended to the left , cut through the tiny rocky objective. Pilot changed course accordingly and objection was attained" . And on p. 239 : " Pan American World Airways later made island landfalls on transoceanic flights by using the same technique" .

Is that a book or an article?  Who wrote it?  Please provide a proper citation.
Francis Millet Rogers, “Precision astrolabe: Portuguese navigators and transoceanic aviation,” Academia Internacional da Cultura Portuguesa; [distributed in the U.S.A. by W. S. Sullwold, Taunton, Mass.], 1971, 397 pages

Google digitized it in 2008 from a copy at the library of the University of California -- Davis, but only snippets can be seen online with Google Books. 

From page 238,
Quote
Realizing that the late-afternoon LOP's were trending ever more toward a NS stance, navigator at 1715 recommended to pilot that he turn right and start to cut across the LOP's. At 1815 navigator found plane to be on an LOP which, extended to the left, cut through the tiny rocky objective. Pilot changed course accordingly and objective was attained.

The second quote given by Mr. van Asten is shown to be at the top of page 239, with what appears to be a footnote number of "8".  To see from whom that statement about Pan American's use of the technique came will probably require a visit to a library.  Copies of the book are said to be in the vicinity of Washington, DC, at the Library of Congress, the Smithsonian, NOAA (in Silver Spring, MD), and in the Nimitz Library at the U.S. Naval Academy.

-------------------------------------------------------------------

You don't have to visit a library since I have the book, see: https://sites.google.com/site/fredienoonan/resources/precision-astrolabe-rogers-1971

It is confusing trying to figure out that footnote. After flipping back and forth through the book and looking at the bibliography it appears that footnote 8 cites to Avigation, by Bradley Jones. The problem though is that Avigation was written in 1931, long before Pan Am starting flying the Pacific. It appears that the footnote was the basis for other statements in the paragraph about Coutinho and is not related to the statement about Pan Am. The author must have had some reason to state that Pan Am used the landfall method to find islands but, unfortunately, he doesn't document what his source was for that statement.

gl
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Ric Gillespie

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Re: Navigating the LOP with the offset method.
« Reply #114 on: June 04, 2011, 12:42:19 PM »

The author must have had some reason to state that Pan Am used the landfall method to find islands but, unfortunately, he doesn't document what his source was for that statement.

He obviously thought it was true but that doesn't make it true. His statement is therefore an undocumented secondary source.  Can't use it.
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Gary LaPook

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Re: Navigating the LOP with the offset method.
« Reply #115 on: June 04, 2011, 12:51:32 PM »

Well then , in document "Francis Chichester , 1922 Portuguese flight , Navigator , p.  238 ,  we read : "April 12 , 1922 . At 1815 navigator found plane to be on a LOP which , extended to the left , cut through the tiny rocky objective. Pilot changed course accordingly and objection was attained" . And on p. 239 : " Pan American World Airways later made island landfalls on transoceanic flights by using the same technique" .

Is that a book or an article?  Who wrote it?  Please provide a proper citation.
Francis Millet Rogers, “Precision astrolabe: Portuguese navigators and transoceanic aviation,” Academia Internacional da Cultura Portuguesa; [distributed in the U.S.A. by W. S. Sullwold, Taunton, Mass.], 1971, 397 pages

Google digitized it in 2008 from a copy at the library of the University of California -- Davis, but only snippets can be seen online with Google Books. 

From page 238,
Quote
Realizing that the late-afternoon LOP's were trending ever more toward a NS stance, navigator at 1715 recommended to pilot that he turn right and start to cut across the LOP's. At 1815 navigator found plane to be on an LOP which, extended to the left, cut through the tiny rocky objective. Pilot changed course accordingly and objective was attained.

The second quote given by Mr. van Asten is shown to be at the top of page 239, with what appears to be a footnote number of "8".  To see from whom that statement about Pan American's use of the technique came will probably require a visit to a library.  Copies of the book are said to be in the vicinity of Washington, DC, at the Library of Congress, the Smithsonian, NOAA (in Silver Spring, MD), and in the Nimitz Library at the U.S. Naval Academy.

-------------------------------------------------------------------

You don't have to visit a library since I have the book, see: https://sites.google.com/site/fredienoonan/resources/precision-astrolabe-rogers-1971

It is confusing trying to figure out that footnote. After flipping back and forth through the book and looking at the bibliography it appears that footnote 8 cites to Avigation, by Bradley Jones. The problem though is that Avigation was written in 1931, long before Pan Am starting flying the Pacific. It appears that the footnote was the basis for other statements in the paragraph about Coutinho and is not related to the statement about Pan Am. The author must have had some reason to state that Pan Am used the landfall method to find islands but, unfortunately, he doesn't document what his source was for that statement.

gl

-----------------------------------------------------------

"The author must have had some reason to state that Pan Am used the landfall method to find islands but, unfortunately, he doesn't document what his source was for that statement."

The author was a professional historian and a professor at Harvard and wrote at least twenty books so it seems unlikely that he would have written that statement without some authority for it. Unfortunately we can't ask him because he died in 1989.

See: http://www.nps.gov/history/history/online_books/npswapa/extContent/usmc/pcn-190-003125-00/sec5a.htm



gl
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Ric Gillespie

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Re: Navigating the LOP with the offset method.
« Reply #116 on: June 04, 2011, 01:08:25 PM »

The author was a professional historian and a professor at Harvard and wrote at least twenty books so it seems unlikely that he would have written that statement without some authority for it. Unfortunately we can't ask him because he died in 1989.

We run into reputable sources who are dead wrong all the time.  Harry Maude and Eric Bevington knew Gallagher personally and were distinguished Colonial Service officers intimately familiar with the Phoenix Islands Settlement Scheme (it was Maude's idea). Both were absolutely certain that no bones had ever been found on Gardner Island.
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Gary LaPook

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Re: Navigating the LOP with the offset method.
« Reply #117 on: June 04, 2011, 02:55:34 PM »

The author was a professional historian and a professor at Harvard and wrote at least twenty books so it seems unlikely that he would have written that statement without some authority for it. Unfortunately we can't ask him because he died in 1989.

We run into reputable sources who are dead wrong all the time.  Harry Maude and Eric Bevington knew Gallagher personally and were distinguished Colonial Service officers intimately familiar with the Phoenix Islands Settlement Scheme (it was Maude's idea). Both were absolutely certain that no bones had ever been found on Gardner Island.

--------------------------

He might have been wrong about this but I doubt that it just came to him in a dream one night. He must have heard it from somebody or read it somewhere and his sources might have gotten it wrong, or they might have gotten it right, but we can't tell from this book.

gl
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Ric Gillespie

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Re: Navigating the LOP with the offset method.
« Reply #118 on: June 04, 2011, 07:16:14 PM »

He must have heard it from somebody or read it somewhere and his sources might have gotten it wrong, or they might have gotten it right, but we can't tell from this book.

Agreed, but if Pan Am did use offsets to find the islands on their trans-Pacific route (Oahu, Midway, Wake) there should be mention of it somewhere.  They weren't flying in secret. 
This was scheduled passenger service and there is an extensive literature about it.
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h.a.c. van asten

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Re: Navigating the LOP with the offset method.
« Reply #119 on: June 05, 2011, 03:14:54 AM »

Mr.Campbell ,

By the sunset fix you can see that Noonan precompted for checking position by celnav which was normal operating practice. For sunrise , due the on first leg delay , Noonan had to recompute en route since sunrise o/b would originally have occurred at the Turn Off Point before going inbound for Howland. If having gone astray , Itasca would have been long before 1912 GMT be warned by radio , but on the contrary : before 1912 GMT there was not any sign of distress. That holds Noonan precisely knowing where he was when commencing the approach procedure. Do we have any indication about this statement in terms of air navigation ? Yes we have : without exact initial calibration of latitude , the sunrise point of time having been precomputed , we know that the precise LHA of sun was 90-03-50 , computed by H.O.208 , Tab. II by navigator himself. @ 175453 GMT sunrise time the GHA of sun was  [(175453) x 15] - 180 deg = 88-43-15. Thence, @ observed sunrise the longitude was :  88-43-15-W + 90-03-50-W = 178-47-05-W. The latitude was already contained in the precomputation by homework. Whatever pre-sunrise track was flown , great circle , rhumb line , any other initial point , From sunrise Noonan knew exactly where he was , given some decline from observation error. I do btw not comment for "make point" , if a computation is good you can safely follow I.Newton : Non fingo hypothesis.
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