Celestial Navigation in SH4

[6SJ7GT]

I uploaded the SH4 Nav package to the SH4 mods in the files section.

This one has updated Nautical almanacs (still the same information, just a different look as I generated them directly to pdf from python).

It has some other goodies, including an Excel sheet that plots a 3 star fix.

Cheers,
Mike 6sj7gt

[don1reed]

Great job, Mike

[don1reed]

Full disclosure, Mike. I mistakenly referred to Gnomonic chart as Conic in our last email.

An here is the reason DR doesn't work in the games:

from HO Pub No.9 The American Practical Navigator a.k.a. Bowditch, 1995




A Gnomonic chart provides the navigator a GREAT CIRCLE course. When laid out on a Mercator chart it becomes a curved course made up of many, small, Rhumb line courses. It provides the navigator the shortest distance course between two points...in case some of you have been wondering why you've been running out of fuel in operation Paukenschlag [sic] when crossing the Atlantic.

Soooo....every time you construct a long straight line course (>600 nm) on the F5 chart (which is made to look like a Mercator, but really isn't) you are going many extra miles out of your way to get between the two locations.
Proof: That straight line course on a Mercator chart, is now a curved course on a Gnomonic chart. (they are inversely proportional).



Simply put, the F5 chart is not a Mercator Projection chart and it makes DR navigation very difficult.
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[TorpX]

I just read your post don. I don't understand it at all.

I know a great circle route will not be a straight line on a mercator chart, and that this is vital in real world navigation. But I don't see how any of this applies to SH3/SH4. When I checked distances in SH4, I saw clearly that the meridians do not converge anywhere on the map. One degree of lat. or lon. is the same distance everywhere. For all practical purposes, the SH4 world is flat.
Perhaps I should say it's cylendrical, as you can go around the world east to west, but as I said above, every grid square is the same size as every other in the game.

Have you tried to verify your gnomonic idea?

Unless I am very much mistaken, your marked route from San Francisco to Yokohama, will add many extra miles to your cruise in SH4.

[don1reed]

We're on the same sheet of music, Torpx.

The F5 screen chart IS NOT, I say again, IS NOT a Mercator projection chart.
If it were, then the mathematical rules would apply for Distance = Speed x Time, and they don't.

A Real Mercator projection chart compensates for the longitudinal convergence by making the space between them longer, as they approach the poles.

I feel your pain.

[don1reed]

Have you tried to verify your gnomonic idea?

If you mean, Have I tried making a Great Circle course across the Atlantic?

Not in the game, but, I have used the "idea" in real life.

For Straight Line distances > 600 nm, it's a necessity at sea to make a Great Circle course. If you fashion a straight line course on a Mercator chart for long distance, you'd wind up going miles out of your way. This is why some players have run out of fuel crossing large bodies of water in the game.

You can prove this to yourself with a "desk globe".

Hold a piece of string between New York and England, pulled tight to the globe. That string represents a Great Circle course. Now, carefully, note where the string crosses meridians and parallels. Annotate those locations on a Mercator chart. You will now have a curved course to follow, which must be broken down into smaller, straight, Rhumb line courses. Even though the course now appears curved, it is the shortest distance between two points.

Rule of thumb:

If straight line course is N of Equator on a gnomonic chart or globe, it will curve toward the North on a Mercator chart.

[TorpX]

Originally Posted by don1reed

If you mean, Have I tried making a Great Circle course across the Atlantic?

Originally Posted by don1reed

Not in the game, but, I have used the "idea" in real life.

For Straight Line distances > 600 nm, it's a necessity at sea to make a Great Circle course. If you fashion a straight line course on a Mercator chart for long distance, you'd wind up going miles out of your way. This is why some players have run out of fuel crossing large bodies of water in the game.

I'm not disputing your expertise or anything your saying as it applies to real world navigation, I just don't see how it factors into the game. The SH4 world is a computer generated illusion, and everything I've seen in the game has led me to think that, they simply didn't consider it worthwhile to incorporate a sophisticated spherical model. They instead decided to use the simplest and easiest model, which is a "flat earth". Unless the model used was purposely designed to simulate what you describe, I don't see how it could occur.

[don1reed]

I know that, Torpx.

I'm saying:
1) Dead Reckoning doesn't work in SH3 or 4.
2) Celnav does work...as far as locating your position by three body fix.
3) RFixes only work when using two timed bearings on landmarks over short, line-of-sight distances.

I've quit trying to use DR in the games...too frustrating at my age.

cheers,

[TorpX]

Originally Posted by don1reed

1) Dead Reckoning doesn't work in SH3 or 4.
2) Celnav does work...as far as locating your position by three body fix.
3) RFixes only work when using two timed bearings on landmarks over short, line-of-sight distances.

I've quit trying to use DR in the games...too frustrating at my age.

cheers,

Yeah, I gave up on it too. DR was an exercise in frustration.

[fitzcarraldo]

Where can I download the files of this realnav? I downloaded the Stellarium program, but the rest of files, all links are death...

Many thanks!

Fitzcarraldo

[6SJ7GT]

Fitzcarraldo,

I uploaded the package to the SH4 gameplay mods on this site.

look for the SH4 Nav Package

Cheers,
Mike 6sj7gt

[fitzcarraldo]

Originally Posted by 6SJ7GT

Fitzcarraldo,

I uploaded the package to the SH4 gameplay mods on this site.

look for the SH4 Nav Package

Cheers,
Mike 6sj7gt

Many thanks, Mike! Downloading now

Regards.

Fitzcarraldo

[Daniel Prates]

Just a literature tip: LONGITUDE, by Dava Sobel. One of the best books I ever read about this subject. It covers the famous 'longitude problem' of centuries ago, the various attempts of solving it, and how a clock maker eventually nailed the prize. Of course, it is a history book, not a techical one.

[fitzcarraldo]

Originally Posted by Daniel Prates

Just a literature tip: LONGITUDE, by Dava Sobel. One of the best books I ever read about this subject. It covers the famous 'longitude problem' of centuries ago, the various attempts of solving it, and how a clock maker eventually nailed the prize. Of course, it is a history book, not a techical one.

Many thanks for this literature tip.

Only I need a sextant in my SH4...I dowloaded the pack for celestial navigation, but I don´t have the sextant. I read the thread about the sextant in the Obs. periscope, but I like a simple sextant-rule as that I have in SH3.

Where is that sextant, if it exist?

Best regards.

Fitzcarraldo

[Pisces]

Originally Posted by fitzcarraldo

Many thanks for this literature tip.

Only I need a sextant in my SH4...I dowloaded the pack for celestial navigation, but I don´t have the sextant. I read the thread about the sextant in the Obs. periscope, but I like a simple sextant-rule as that I have in SH3.

Where is that sextant, if it exist?

Best regards.

Fitzcarraldo

In this thread it is called "the program Stellarium"!