NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Gary LaPook
Date: 2016 Dec 28, 02:35 -0800
From: John D. Howard
Date: 2016 Dec 27, 06:53 -0800
Gary,
I was in the US Air Force for 20 years and then flew for several US airlines fo another 20 years. I have flow over and navigated across Every ocean on earth and have flown to and landed onto Every continent on earth. I have never used a Mercator chart.
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That is because you flew and, in the U.S. aeronautical charts use the Lambert Conformal projection, with only a few exceptions. (see my other post today.) Marine charts are almost all Mercator projections so the ability to make a Mercator plotting sheet is very useful to sailors. You probably used GNCs which have a scale of five million to one which is perfectly adequate for plotting in flight celestial fixes given the lower accuracy possible with in flight sextant shots. Sailors work to a higher level of precision so need larger scale charts. The standard "Universal Plotting Sheet" has been around since 1931 and has a scale of 20 NM per inch, a scale of 1,458,240 to 1, more than three times larger than the GNC. The same GNC scale is used on the Polhemus computer, used by our Air Force for many years, which has graticules on the Lamberet Conformal projection covering all latitude bands. Plotting on the Polhemus is exactly like plotting on a GNC. See:
and
Naval Aviators used a different device for plotting celestial lines of position, the Mk 3 and the Mk6 plotting boards. These make it easy to construct a Mercator plotting sheet using the standard graphical methods. They followed the methods used by surface navigators.
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Take your own test and make a small area plotting chart using a Lambert conical projection and see how it compares to your Mercator chart.
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I've got a better idea, let's see you make a Lambert Conformal plotting sheet. What standard parallels will you use? What will be the convergence factor? What will be the apical angle? Go ahead, give it a try.
My point is that there is no easy way to make a Lambert Conformal plotting sheet, or chart, equivelent to the easy ways that you can make a Mercator plotting sheet using the graphical method. And even making a larger Mercator chart isn't very difficult using Meridional Parts, but you can't make a Lambert chart.
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Better yet take a piece of graph paper and use each line as one nauitical mile. Draw a horzonital line across the middle of the paper and call it 34 degrees north. Would you call that piece of graph paper a Mercator map ?
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NO.
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Would you call it Lambert ?
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No, because using graph paper does not make either of these projections.
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Not trying to be a wise a** but I think for small scale plotting sheets the main thing should be conformal. If I draw a line from my AP say 5 NM long that line on my plotting sheet should be the same length in any direction ( any azimuth ).
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That's fine until you have to figure out longitudes for plotting an assumed postion, or taking out the longitude of the resulting fix. To determine longitude you have to account for the convergence of the meridians and that is exactly what the Mercator small area plotting sheet accomplishes quite nicely and easily done using the graphical method.
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I agree that your Mercator plotting chart and Frank's plotting chart would be, for all practical reasons the same. But that begs the question: why would anyone go to all the trouble of using mecatorial parts to make a Mecator chart?
John H.
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Because the Mercator chart is the only one that is easy to construct and even a large latitude ranged chart is not difficult to make using Meridional Parts.
Now tell me again just how you are going to make a Lambert chart.
gl
