Re-Asessing Real Navigation...

vanjast · 08-02-14, 09:26 AM

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 · 08-02-14, 09:44 AM

Quote:

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 · 08-02-14, 09:52 AM

Quote:

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 · 08-02-14, 11:43 AM

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 · 08-02-14, 11:58 AM

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 · 08-04-14, 02:38 PM

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 · 08-05-14, 10:45 AM

Quote:

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 · 08-05-14, 12:48 PM

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 · 08-06-14, 04:12 PM

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 · 08-07-14, 01:33 AM

Quote:

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.

08-02-14, 11:58 AM	#5
vanjast Sea Lord Join Date: Jun 2006 Location: Somewhere else now Posts: 1,740 Downloads: 825 Uploads: 4	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 Last edited by vanjast; 08-02-14 at 12:13 PM.

08-06-14, 04:12 PM	#9
Karl Heinrich Soundman Join Date: Nov 2012 Posts: 146 Downloads: 60 Uploads: 2	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. __________________ “Die Südfrüchte runter vom Kartentisch. Auf Bananen kann ich nicht navigieren.” Last edited by Karl Heinrich; 08-06-14 at 04:28 PM.

08-02-14, 11:43 AM	#4
Tupolev Hauptman Join Date: Mar 2013 Location: FL410 Posts: 174 Downloads: 75 Uploads: 0	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

08-04-14, 02:38 PM	#6
vanjast Sea Lord Join Date: Jun 2006 Location: Somewhere else now Posts: 1,740 Downloads: 825 Uploads: 4	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.