Re-Asessing Real Navigation...

[vanjast]

Re-looking at the real nav stuff, as I found myself a bit lost at times

First stop is Dead Reckoning

Usually done by keeping a record of your speed and direction.
A quick test revealed that SH3 differs from the real world. This I think was mentioned by someone else, but here it is, for anyone else trying to decipher the Nav problems.

- SH3's world is 43,200 km (width) x 21,600 km (Height) - These are 2x the dimensions of the large map.

- 1 Knot = 1.8 km per hour. - The real world has 1 knot = 1.852 kph
Funny enough the distance covered seems to be irrespective of (not affected by) the sea conditions - This is still under testing.


Next on the list are the local and GMT times and anything else that come to mind.

[Aktungbby]

Originally Posted by vanjast

First stop is Dead Reckoning

http://www.subsim.com/radioroom/showthread.php?t=214436 "Very good grasshopper!" Originally Posted by Jimbuna

AktungBBY: Yup! time for a little DEAD RECKONING Here!

[vanjast]

No, not my posts..

About 2 or 3 years ago somebody posted something about the SH3 (or SH4) nautical mile, which isn't the same as real life.

I've done some measurments on a test mission going NWSE at 2 and 7 knots over 6 hours, in smooth and 'rough'ish' seas, and found I'm getting the same distance traveled on each speed setting.

@ 7 knots for 6 hours = 75.6km (75.6/6 = 12.6 -> 12.6/7 = 1.8)
@ 2 knot for 6 hours = 21.5km => 1.79


I've just re-edited so it makes better sense

RL:
1 Nautical Mile = 1852 m
1 minute of Arc = 1 Nautical mile
1 degree of arc (60 minutes of Arc) = 60 Nautical miles (111 km)

SH3:
1 Nautical Mile = 1800m (traveling at 1 Knot sub speed)
1 minute of Arc = 2000m
1 degree of Arc (60 minutes of Arc) = 120km

Wot a mess ?
As long as you keep the differences in mind you shouldn't have a navigation problem.

Funny enough it still seems that the stars, sun and moon 'rotate normally' as per SH3's grid.

[BigWalleye]

Originally Posted by vanjast

No, not my posts..

About 2 or 3 years ago somebody posted something about the SH3 (or SH4) nautical mile, which isn't the same as real life.

I done some measurments on a test mission going NWSE at 2 and 7 knots over 6 hours, in smooth and 'rough'ish' seas, and found I'm getting the same distance traveled on each speed setting.

@ 7 knots for 6 hours = 75.6km (75.6/6 = 12.6 -> 12.6/7 = 1.8)
@ 2 knot for 6 hours = 21.5km => 1.79

These measurements also coincide with the Real Life Longitude and Latitude grid.
1 knot = 1 nautical mile (1.8km in SH3's case) = 1 minute of arc around the globe - This angular measurement being consistent across the SH3 and Real Life maps.

So you found that sea state does not affect distance over the ground? So I can just pound my boat into the worst 15m/s wind waves at 15 kts and get there faster without wasting fuel?

Another defect of the SH3 model. But important to know. I had always cut speed in the worst weather. Apparently, there is no point.

[vanjast]

Originally Posted by BigWalleye

Another defect of the SH3 model. But important to know. I had always cut speed in the worst weather. Apparently, there is no point.

I'm not sure, as they might still have a fuel usage calculator that depends on weather and speed, which could be independent on the distance traveled - although this seems silly.

[Tupolev]

Wouldn't the distance discrepancies be location dependent? Since the SH3 map is basically a flattened globe, I would think at the equator the distances in SH3 vs RL would be equal. Near the poles SH3 distances would be vastly larger than RL. All points (lines) of latitude would give larger distance errors the further from equator.

I would think this makes deduced reckoning quite difficult in game.

T

[vanjast]

I still have to check this as I only done equator checks.

I'll come back later with a result

Edt: Exactly the same result at 80N - The world is square and flat

[vanjast]

Just a point to note:

Don't mix this lot up with your in-game optics/3D visuals... These are two separate items.

The visuals rely on the cameras.dat file and is a different story to the maps.

[Pisces]

Originally Posted by vanjast

...

I've done some measurments on a test mission going NWSE at 2 and 7 knots over 6 hours, in smooth and 'rough'ish' seas, and found I'm getting the same distance traveled on each speed setting.

@ 7 knots for 6 hours = 75.6km (75.6/6 = 12.6 -> 12.6/7 = 1.8)
@ 2 knot for 6 hours = 21.5km => 1.79

How did you set speed? By aligning the indicator on the speed dial to a specific mark? Or by using a special throttle setting (fraction of top MaxSpeed) in: SH3folder\data\Submarine\(subtype)\(subtypename).c fg
[EngineProperties]

The graphical scale on the dial might not be completely linear, and account for the suggestion that 1 SH3 nautical mile is close to 1.8km.

Personally, the 1.852 km/nm value has always seemed accurate to me. I'm sure I would have noticed the 2.8% difference at some point along the years.

[vanjast]

Speed controlled on the indicator dial.

I set the position as accurately as possible and wait for the number indicator to hit '7'. I suppose to do it scientifically I should average out a number of tries... but this is good enough
I tried 7 and 1 knot speeds and the SH3 Nautical Mile in both, came to 1.8Km.

[Karl Heinrich]

I've been doing the same research for my printable SH3 charts and came to the same conclusions - just as we know the "circumference" of SH3 World is 3125km larger than RL.

Shameless chart plug: http://www.subsim.com/radioroom/showthread.php?t=213987

The world is definitely equiretangular - programmed from the 120km long lat, which makes life easier for navigation in some ways. Also allows for easy RL comparisons with my coord method (which is less relevant with your Real Nav chart in-game of course)

Another shameless plug: http://www.subsim.com/radioroom/showthread.php?t=209834

Sea state does affect your speed, just watch the dial. If it moves between 6 and 7 kts I've found 6.5 kts is remarkably accurate for dead reckoning. However I suspect the level of time compression affects this. In still trying to break it to try and find out exactly how much it affects your speed more precisely.

[vanjast]

Originally Posted by Karl Heinrich

Sea state does affect your speed, just watch the dial. If it moves between 6 and 7 kts I've found 6.5 kts is remarkably accurate for dead reckoning. However I suspect the level of time compression affects this. In still trying to break it to try and find out exactly how much it affects your speed more precisely.

What I found (so far) is that the speed dial is representive of your horizontal speed (similar to Ground Speed in aircraft), instead of the 'Indicated/True' speed through the current sea conditions.

Say your indicated Log speed is 7 knots:
1) On a dead flat sea the horizontal speed would also be 7 knots.
2) As the sea conditions get worse, your ship has a vertical speed/movement component, which effectively reduces your horizontal speed, even though your indicated speed is still 7 knots.

SH3 seem to bypass this 2) reality and calculates your horizontal 1) speed for you, no matter which sea conditions. The wobbly Log in high seas.

It'll be nice to have this, wind resistance, currents, tides and seasonal conditions effecting navigation.

[CaliEs]

I need the formula for calculating real world latitude / longitude data from SH3 positioning data.


Example from the exam navigation mission "Navigation.mis":

sub starting position:
SH3 data:
InitLongOff = 6856974.000
InitLatOff = 5584324.500

Estimated real world data:
Long 09.43°E
Lat 55.15°N
(start location is Aro sund / Denmark)

How to calculate from 6856974.000 to get 09.43°?

[Pablo]

Originally Posted by CaliEs

I need the formula for calculating real world latitude / longitude data from SH3 positioning data.


Example from the exam navigation mission "Navigation.mis":

sub starting position:
SH3 data:
InitLongOff = 6856974.000
InitLatOff = 5584324.500

Estimated real world data:
Long 09.43°E
Lat 55.15°N
(start location is Aro sund / Denmark)

How to calculate from 6856974.000 to get 09.43°?

Hi!


Table 7 on page 50 of the GWX Manual that gives you this information. Basically,


GWX latitude = 120 km/degree; Real latitude = ~110 - 111 km/degree


GWX longitude = 120 km/degree, real Longitude varies with latitude, so that it's about 111 km/degree at the equator, ~85 km at 40 degrees latitude, and 0 km at the poles.


There's a precise equation somewhere, but I've forgotten where it is.


Hope this helps!

[Fader_Berg]

While working with patSH3r and the positioning system within it. I somehow (can't remember how today) came up with a pretty good estimate of the SH3 world globe median radius. The source code says that it's 6875493 meters (while it's ~6371000 IRL).