Differences Between Ground, Apparent, and True Wind Direction

[s/v Jedi]

Originally Posted by cal40john

Well I had to read your first sentence about ten times before it started to parse out and make sense to me.

Yah, imagine how it is inside my head... I confuse myself most of the time

Ok, I reread the Maretron definition a few more times and I think they're saying the same thing we have been, but have made a very poor choice of words IMHO.
[...]
Ground wind is the speed and direction of the wind referenced to a compass rose, which is different than apparent and true wind which are referenced to the bow of the boat.

Well, I'm pretty sure that all they mean is that a ground wind from 90 degrees comes out of the east while a true or apparent wind from 90 degrees comes from starboard abeam, regardless the heading of the boat.

cheers,
Nick.

[TeddyDiver]

Don't you know they are farming ground wind nowadays
FAA and Air Force Ground Wind Farm Development

[Ex-Calif]

Originally Posted by s/v Jedi

If everybody starts to understand that the True from True and Apparent is different from the True from True and Magnetic, it'll help

As long as we do not start compensating for continental drift (!!), ground wind is the wind referenced to ground, as in solid rock above or below water. Ground wind is what is measured by a weather station that is mounted ashore. It is what is broadcast in weather forecasts.

Wind direction is always True, never Magnetic (at least it is True when organizations like NOAA talk about it).

cheers,
Nick.

It's like we staarted this discussion in the middle od the semsester.

True wind - meterologist measure this with an instruments mounted on land! (ok they can also launch ballons and calculate things but lets just say thenmeteorlogist has the "truth") Therfore as everyone saying "ground referenced." wind speeds are measured by meteorlogists in "true" compass bearings. This is because in a large area forecast the wind information being provided may span one or more areas of different "magnetic" variation. So true wind +-var = magnetic wind.

Apparent wind - the anmometer on the boat simply measures what it "feels." what it feels can be a result of the wind blowing or the boat moving caused by sailing or current or unless anchored a combination of both. It is what is "apparent" to the instrument. Unlike the meteorlogists anemometer the boat anemometer is not fixed. It moves with the boat. Also until recent interfacing with position instruments the wind instrument does not know where it is. Therefore the anemometer on the boat is displaying magnetic apparent wind.

Boat true wind - becuase the instruments on the boat are not fixed in place the boat cannot measure true wind. True wind on a boat is calculated. Also until recently the wind instrument did not know where it was located and because the input was magnetic apparent wind and until gps we only had log speed, we ended up with magentic apparent wind corected for boat speed through the water. In boating we also called this "true" wind. But because then boat is moving it isn't the meteorlogists true wind. Dont get me wrong - sailors call this true wind but it should be called - "magnetic wind calculated from apparent wind caused by movement, true wind and currents."

So know we have gps and we can separate the boat speed through water from boat speed caused by current. If we add or subtract the boat speed through the water from sog we can see what element of boat speed is caused by current, what element is caused by "sailing" and what element of the measure is "unaccountd for" the unaccounted for becomes a component of the true wind because the boatnisnnever travelling "at true wind velocity"

In the simplest example a boat headed 180 travels at 10 knots boat speed and 15 knots sog. In this example local magnetic variation is assumed to be 0. The anemometer is reading 5 knots apparent at 180. For now lets argue the current is flowing 180. Boaters calculated magnetic "true" wind is 180 @ 15 knots. The metrologist on shore is measuring and reporting 180 @ 20kts.

If the boat were turned 180 and could sail directly into the wind at the same boat speed through the water we would meqsure 10 kts boat speed, 5 kts sog. The anemometer is reading 180 @ 25kts. The meteorlogist is still measuring 180@20.

So there is a persistent 5 knot error between the meteoroligist and the boat true wind. Of course. The difference is the current and the fact that one anemometer is fixed and the other is moving. And finally if there was +10 degvar the boat result would be a further 10deg off because the boat instrument doesn't know variation so the result is really mag referenced.

Now that we easily know position with gps we can use sog, boat speed and position to find out the effect of current. We could also apply magnetic variation and refeerence back to the meterologists true wind. The difference of course is we start with mag referenced apparent and use sog, boat speed, local variation to calculate backwards. There is another minor factor in regards to how your boat apparent speed is referenced and that is if the particular compass on your boat has any built in error. This is called deviation and is determined through a compass calibration exercise.

So big deal - what does all this mean?

True wind - nice to know for passage planning. If you know a 5 day forecast of true wind you can calculate headings across multiple areas of magnetic variation, if yu know set (dispacement caused by sailing and adverse current) and your boats sailing angles you can decide if you make harbor, or get set downwind and need tontack at some point.

Boat true wind (adjusted for variation and deviation) - when you are on the passage it would be nice to know if the forecast true wind matches the true wind where you currently are. That allows me to update my passage plan and decide will I make it from here? But it includes confounded information about wind and current.

Magnetic Apparent wind - this is the most important for sailing the boat. Is the boat going through the water as thenperformance specs say it should and I am I sailing efficiently. With apparent wind and boat speed we can find out if we are efficiently sailing the boat?

Boat speed and sog - with these pieces of information you can find out your wind and current components of speed. Current information is very useful. If you are racing you would like to find areas of favorable or least disadvantageous currents. On passage making you can compare and explain position and set based on predicted currents, from passage "almanacs" and set based on boat performance.

Boat ground speed - I this is a useful definition I guess. And I guess it is either trying to define "current caused" wind speed or create a new definition for meteorologists true wind speed. If the former, great. If the later we are wrong to try and create a definition for something that already exists.

Once you understand the various speed definitiions you can start working the vector math. It is pointless to discuss this until the definitions are agreed to.

And finally, if your boat integrates wind information via a computer, you need to know what yu are looking at when the display says "15kt true.". Is that true wind or boat true wind? Is your fancy plotter integrating boat speed, sog, variation and deviation? Or something else.

It doesn't matter so much what you call it unless you are trying to communicate to someone else. It does really matter to undrstand what components of wind are included in whatever value you are using.

[Cheechako]

True wind was always "true wind" to me. "whatever the wind was doing regardless of the existance of my boat or movement".

Apparant wind was always the "resultant wind on my boat, however my boat was moving".
Does it really need to be more complicated than that?

[s/v Jedi]

Here you go: can display true wind and apparent wind, but not ground wind:



displays ground wind:



Funny that it says it's showing TWS

No matter what anybody wants to call it, this is how the instrument guys call it. Only few of them are smart enough to realize that water referenced wind can differ from ground referenced wind. So, if they only talk about True and Apparent, you have to assume they don't know water referenced. Only regatta sailors understand why water referenced wind makes sense sometimes.

cheers,
Nick.

[cal40john]

Well as long as we get everyone to agree to your definitions and tell the GPS companies to stop using the the term ground wind we're set.

your term boat ground speed sounds like SOG, did you just mean ground wind?

To me ground wind = meteorlogical earth referenced wind, what NOAA reports as true wind.

Your "boat true wind" term works for me for what you see when the boat is stationary with respect to the water. Now go tell the GPS companies to stop using the term ground wind.

John

Originally Posted by Ex-Calif

It's like we staarted this discussion in the middle od the semsester.

True wind - meterologist measure this with an instruments mounted on land! (ok they can also launch ballons and calculate things but lets just say thenmeteorlogist has the "truth") Therfore as everyone saying "ground referenced." wind speeds are measured by meteorlogists in "true" compass bearings. This is because in a large area forecast the wind information being provided may span one or more areas of different "magnetic" variation. So true wind +-var = magnetic wind.

Apparent wind - the anmometer on the boat simply measures what it "feels." what it feels can be a result of the wind blowing or the boat moving caused by sailing or current or unless anchored a combination of both. It is what is "apparent" to the instrument. Unlike the meteorlogists anemometer the boat anemometer is not fixed. It moves with the boat. Also until recent interfacing with position instruments the wind instrument does not know where it is. Therefore the anemometer on the boat is displaying magnetic apparent wind.

Boat true wind - becuase the instruments on the boat are not fixed in place the boat cannot measure true wind. True wind on a boat is calculated. Also until recently the wind instrument did not know where it was located and because the input was magnetic apparent wind and until gps we only had log speed, we ended up with magentic apparent wind corected for boat speed through the water. In boating we also called this "true" wind. But because then boat is moving it isn't the meteorlogists true wind. Dont get me wrong - sailors call this true wind but it should be called - "magnetic wind calculated from apparent wind caused by movement, true wind and currents."

So know we have gps and we can separate the boat speed through water from boat speed caused by current. If we add or subtract the boat speed through the water from sog we can see what element of boat speed is caused by current, what element is caused by "sailing" and what element of the measure is "unaccountd for" the unaccounted for becomes a component of the true wind because the boatnisnnever travelling "at true wind velocity"

In the simplest example a boat headed 180 travels at 10 knots boat speed and 15 knots sog. In this example local magnetic variation is assumed to be 0. The anemometer is reading 5 knots apparent at 180. For now lets argue the current is flowing 180. Boaters calculated magnetic "true" wind is 180 @ 15 knots. The metrologist on shore is measuring and reporting 180 @ 20kts.

If the boat were turned 180 and could sail directly into the wind at the same boat speed through the water we would meqsure 10 kts boat speed, 5 kts sog. The anemometer is reading 180 @ 25kts. The meteorlogist is still measuring 180@20.

So there is a persistent 5 knot error between the meteoroligist and the boat true wind. Of course. The difference is the current and the fact that one anemometer is fixed and the other is moving. And finally if there was +10 degvar the boat result would be a further 10deg off because the boat instrument doesn't know variation so the result is really mag referenced.

Now that we easily know position with gps we can use sog, boat speed and position to find out the effect of current. We could also apply magnetic variation and refeerence back to the meterologists true wind. The difference of course is we start with mag referenced apparent and use sog, boat speed, local variation to calculate backwards. There is another minor factor in regards to how your boat apparent speed is referenced and that is if the particular compass on your boat has any built in error. This is called deviation and is determined through a compass calibration exercise.

So big deal - what does all this mean?

True wind - nice to know for passage planning. If you know a 5 day forecast of true wind you can calculate headings across multiple areas of magnetic variation, if yu know set (dispacement caused by sailing and adverse current) and your boats sailing angles you can decide if you make harbor, or get set downwind and need tontack at some point.

Boat true wind (adjusted for variation and deviation) - when you are on the passage it would be nice to know if the forecast true wind matches the true wind where you currently are. That allows me to update my passage plan and decide will I make it from here? But it includes confounded information about wind and current.

Magnetic Apparent wind - this is the most important for sailing the boat. Is the boat going through the water as thenperformance specs say it should and I am I sailing efficiently. With apparent wind and boat speed we can find out if we are efficiently sailing the boat?

Boat speed and sog - with these pieces of information you can find out your wind and current components of speed. Current information is very useful. If you are racing you would like to find areas of favorable or least disadvantageous currents. On passage making you can compare and explain position and set based on predicted currents, from passage "almanacs" and set based on boat performance.

Boat ground speed - I this is a useful definition I guess. And I guess it is either trying to define "current caused" wind speed or create a new definition for meteorologists true wind speed. If the former, great. If the later we are wrong to try and create a definition for something that already exists.

Once you understand the various speed definitiions you can start working the vector math. It is pointless to discuss this until the definitions are agreed to.

And finally, if your boat integrates wind information via a computer, you need to know what yu are looking at when the display says "15kt true.". Is that true wind or boat true wind? Is your fancy plotter integrating boat speed, sog, variation and deviation? Or something else.

It doesn't matter so much what you call it unless you are trying to communicate to someone else. It does really matter to undrstand what components of wind are included in whatever value you are using.

[Dockhead]

It would really not be so complicated at all, if we could get rid of the confusion -- which seems to affect a few people here -- between "true" as used in the phrase "true wind" and "true" as used in the phrase "true north". They have NOTHING to do with each other. There is no such thing as "magnetic apparent wind" -- this is a mishmash. Apparent wind is what is apparent to your boat -- it has nothing to do with the compass rose.

Really, forget true and magnetic north.

And another proposal: I suggest that it's not really important where "ground wind" came from, nor is it important how meteorologists use the term "true wind" (nor is it any contradiction, actually).

Why? Because the instrument makers have a certain definition of Apparent, True, and Ground and in my opinion they are not idiots. In my opinion this is an extremely logical and useful way to look at the wind. And if you decide to look at it that way yourself, then your instruments will be giving you the information in the right form.

Someone was confused by Maretron -- saying that Ground Wind is measured with reference to the compass, while Apparent and True with reference to the boat's bow. I don't know why this seems confusing to anyone. That's the way Maretron gives this information, that's all. You could perfectly well calculate True or even Apparent Wind direction with reference to the compass (and magnetic or true according to your preference!) but why would you do it? Apparent and True Wind are sailing winds -- this information is useful with reference to your bow (particularly Apparent -- a compass calculated Apparent Wind would be entirely useless information, I think). Apparent Wind is what you are sailing in right now -- how your trim your sails -- how much further you can point up. True Wind is relevant to planning a change of course. You also want this in reference to your bow, generally.

Now Ground Wind -- maybe we had better called it Meteorological Wind -- is the weather. We don't sail in it at all. But we might want to know it in order to understand what will happen when the tide changes. Or in regard to some longer range weather forecast. So this wind -- since it's not "here and now", is better understood in reference to the compass. True North, I guess - since that's the way the weather is reported, but there's no reason why you couldn't calculate it to Magnetic North if you preferred for some reason.

OK, so, is that really so complicated?

[TeddyDiver]

Originally Posted by Dockhead

OK, so, is that really so complicated?

No it isn't that complicated. According your description your GPS knows the tides and ocean currents all over the seas every possible time to the distant future.. ?? The whole mess is to blame some nerd messing up with terms..
Anyone care to compare the readings of their True and Ground?

[Dockhead]

Originally Posted by TeddyDiver

No it isn't that complicated. According your description your GPS knows the tides and ocean currents all over the seas every possible time to the distant future.. ?? The whole mess is to blame some nerd messing up with terms..
Anyone care to compare the readings of their True and Ground?

Sorry, you're still confused.

Nobody needs any data about tides and currents to calculate any of these winds. To calculate Ground Wind, we just need:

1. Apparent wind speed and direction, as measured directly by our wind instrument.
2. Compass heading as measured by your heading sensor, so the above data can be related to the compass rather than your bow
3. GPS - measured speed in relation to the ground -- SOG
4. GPS - measured course in relation to the ground -- COG.

Then your system can calculate the speed and direction of the wind in relation to the ground. Hence Ground Wind. Got it?


To calculate True Wind, we just need:

1. Apparent wind speed and direction, as measured directly by our wind instrument.
2. Speed through the water as measured by the log impeller.

That's it. Then the system can calculate what the wind speed and direction would be IF our boat was not moving in relation to the water. So we get wind speed and direction -- related to the bow of the boat -- and with the water as a reference point, without regard for whether the water is moving or not.


If you want to know tide or current speed and set (direction), well -- you can't. At least, you can't calculate it accurately if there is any leeway going on. If there's no leeway, then you can calculate the current speed and set by comparing SOG and COG with Heading and STW. But leeway messes it up -- why? Because Heading is NOT equal to course through the water. We don't have any instrument which measures course through the water, unfortunately. But that's kind of off the subject.



The GPS data knows where the ground is, not the water. That's why the GPS measured speed -- SOG --speed over ground -- and GPS measured course over ground -- COG -- is used to calculate Ground Wind, speed and direction. Got it?

Your regular log -- speed impeller -- is what is used to calculated True Wind. That's because it knows where the water is, in relation to your boat -- it knows your speed through the water -- STW.

[Ex-Calif]

The problem is that the "boaties" have been using "true wind" incorrectly forever. Probably becuase "back then" they said, "If I take apparent and correct for boat speed I have "true wind." And becuase currents arer small and I can't measure them on the boat, I will ignore current."

The meteorologists "own" the term "true wind." it is measured wind referenced to true north. If the gps folks are inventing somethng called ground speed that is defined the same way as meterological true wind then they are unnecessarily confusing things and I agree that once again a couple of geeks are confusing things.

The only new functionality a gps brings is to be able to correct for currents and immediately find location (for mag var). As a result we can now calculate true wind speed correctly. We could not before.

So true wind speed should have a definition cosnsitent with the meteorologist.
Apparent wind has always been a useful sailing definitiion and it seems we agree what it is.
The only thing we dont have is "drift" or "current" wind

Nick - your photos are classic! It matters not what the instrument reads and displays (tws in this example) one needs to know what is included in the displayed number presented.

And for the sailors who say, "I know what tws is for a boat" - fine. Just remember that your tws has no reference to the meteorlogical report.

I believe my explanation allows one to tie back and forth between met reports and the reading on the boat that can only "start" as "apparent wind referenced to boat magnetic" because that is all one can measurre on the boat.

You can only "forget about" true compass headings on the boat as long as you are not trying to concile with a met report.

[Ex-Calif]

Originally Posted by Dockhead

Sorry, you're still confused.

Nobody needs any data about tides and currents to calculate any of these winds. To calculate Ground Wind, we just need:

1. Apparent wind speed and direction, as measured directly by our wind instrument.
2. Compass heading as measured by your heading sensor, so the above data can be related to the compass rather than your bow
3. GPS - measured speed in relation to the ground -- SOG
4. GPS - measured course in relation to the ground -- COG.

Then your system can calculate the speed and direction of the wind in relation to the ground. Hence Ground Wind. Got it?

To calculate True Wind, we just need:

1. Apparent wind speed and direction, as measured directly by our wind instrument.
2. Speed through the water as measured by the log impeller.

That's it. Then the system can calculate what the wind speed and direction would be IF our boat was not moving in relation to the water. So we get wind speed and direction -- related to the bow of the boat -- and with the water as a reference point, without regard for whether the water is moving or not.

If you want to know tide or current speed and set (direction), well -- you can't. At least, you can't calculate it accurately if there is any leeway going on. If there's no leeway, then you can calculate the current speed and set by comparing SOG and COG with Heading and STW. But leeway messes it up -- why? Because Heading is NOT equal to course through the water. We don't have any instrument which measures course through the water, unfortunately. But that's kind of off the subject.

The GPS data knows where the ground is, not the water. That's why the GPS measured speed -- SOG --speed over ground -- and GPS measured course over ground -- COG -- is used to calculate Ground Wind, speed and direction. Got it?

Your regular log -- speed impeller -- is what is used to calculated True Wind. That's because it knows where the water is, in relation to your boat -- it knows your speed through the water -- STW.

I lay no claim to being 100% correct.

The on.y way to proceed is if we agree we are solving for tws as defined by the met report.

If you are using the "yachties" definition of tws you must adjust for mag variation and deviations to get mets tws. You are correct that total boat displacement is all needed to calculate tws. However, you will not know if that boat displacement is a result of boat propulsion, leeway or current. Coupled with boat speed and gos sog one can derive leeway+current.

Knowing leeway+current is definitely valuable information when passage planning and execution.

[Dockhead]

Originally Posted by Ex-Calif

I lay no claim to being 100% correct.

The on.y way to proceed is if we agree we are solving for tws as defined by the met report..

Well, I don't think we are solving for that. Meteorologists sit on land and we sit in water. Their point of reference is one thing and ours is another. Hence True Wind is different for us, than it is for them. Their True Wind is our Ground Wind --

that is, if we accept the instrument makers' definitions. There's no law against talking like a weatherman, if you so choose.

But another argument for the idea that our True Wind is not their True Wind is that we express True Wind in reference to our bow, not the compass. The weather reports, of course, talk about compass directions. Like our Ground Wind.

Originally Posted by Ex-Calif

If you are using the "yachties" definition of tws you must adjust for mag variation and deviations to get mets tws. You are correct that total boat displacement is all needed to calculate tws. .

What does compass corrections have to do with it?? That is not at all the difference between meteorologists' True Wind, and ours. The difference is that we reference True Wind to water, not to ground. So if the water is moving, there will be a difference between our point of reference, and theirs.

[twistedtree]

Unfortunately, I think Maretron means what they wrote. This is exactly what left me so befuddled after talking to them at Ft Lauderdale.

I think Ground Wind and True Wind are like Family Values and Patriotism - everyone defines them to meet their own needs.

[noelex 77]

Originally Posted by TeddyDiver

.
Anyone care to compare the readings of their True and Ground?

As a cruising boat switching between true and ground (if that is how we define them) a couple of years ago, I can comment. The big practical difference is "ground wind" is always accurate. It does not matter if there is a bit a growth or weed on the impellor, it does not matter if I am on port or starboard tack, upwind or down, current or no current.. I can look at the "ground" wind and see if the windspeed, or direction is changing, combined with apparent wind it provides me with the information I need.

IMHO the inherent accuracy and consistency of a ground wind display is more important than the subtle difference between ground and true wind which many sailors do not understand anyway.

[Noname]

Got a pilot's license in 1974 and been sailing for 35 years and never heard of "ground wind". I've heard of surface wind, upper air winds, winds aloft, true wind, apparent wind and headwind and tailwind; but not ground wind.