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Old 25-10-2014, 20:27   #16
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Re: Wave Height

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Originally Posted by MarkJ View Post
In the bar I much prefer to exaggerate that be in the real thing
Well you know what ruins a good war story is an eye witness...

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Yep, but my experience was that the resolution was way too big and the response too slow either that or I was really sailing in 200 foot waves with a period of 2 minutes

Others may have better GPS units!

The precision air pressure sensor I mentioned before was near instantaneous and had a resolution of around 2 foot.
Yeah - I figured the rsponse rate would be slow. I don't think the gps is slow, and does have resolution to the "foot" but the display is probably dampened for legibility.

OTOH - How big the waves are is kinda useless to know when you are in them. You deal with what you got.

How big they are predicted to be is kinda important to know before you go.

Now how "big" they were when you get to the bar, that's super important -
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Old 25-10-2014, 20:37   #17
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Re: Wave Height

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Originally Posted by Ex-Calif View Post
Well you know what ruins a good war story is an eye witness...



Yeah - I figured the rsponse rate would be slow. I don't think the gps is slow, and does have resolution to the "foot" but the display is probably dampened for legibility.
...........



Have a look how much the "altitude" changes say over 5 or 10 minutes while the GPS is sitting still on a table or whatever. Certainly changes one's attitude to GPS altitude - at least that is my experience.

YMMV
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Old 25-10-2014, 21:06   #18
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Re: Wave Height

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Have a look how much the "altitude" changes say over 5 or 10 minutes while the GPS is sitting still on a table or whatever. Certainly changes one's attitude to GPS altitude - at least that is my experience.

YMMV
yes - Totally true but in any particular "minute" one should be able to take a max and min. This is like circular error.

The gps may be off center by 3 meters but a 1 meter shift (short term) should be a one meter shift.

I get this is not doable, just interesting. It's about as accurate as most skipper's guesses I would say - LOL...
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Old 25-10-2014, 21:47   #19
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Re: Wave Height

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Most people measure with exaggeration.

Never let the truth get in the way of a good story


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Old 26-10-2014, 06:40   #20
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Re: Wave Height

For any fully blown up wx system the waves will correspond to the wind force. Give or take any cross seas running, shallowing, etc. Look up your wind instrument, consider the water body type, stretch, depth, etc. and you get a sound approximation of what the waves are.

There are tables on the web that give the figures.

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Old 26-10-2014, 14:31   #21
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Re: Wave Height

Some wave buoys measure the waves by processing GPS signal. See for example http://www.datawell.nl/Portals/0/Doc...g4_b-06-06.pdf

I have had opportunities to "calibrate" my estimates with reference to such a buoy. In "longcrested" (mono-directional) waves, estimating the height is easy. In "shortcrested" (multidirectional) waves, it's much harder because you don't see so many well-defined waves. I have been sailing in a gale when I found that my estimate was lower than the official "hindcast" (estimating the seastate with full knowledge of the weather).

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Old 26-10-2014, 15:08   #22
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Re: Wave Height

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IPads with GPS have a gyroscope in them. No one knows what to do with it.... But if you can write and ap the gyroscope should be able to work out a wave height, shouldnt it?
Considering I ran two and a half miles in my neighborhood the other day and my running app on my iphone said I climbed 1171 feet when my neighborhood is only 40-50 feet above sea level, I'd not count too much on the GPS in an ipad or iphone for elevation measurements. Heck, the highest point on all of Long Island (where I live) is only 400 feet!
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Old 26-10-2014, 15:29   #23
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Re: Wave Height

Maybe you jiggle when you jog?
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Old 26-10-2014, 16:44   #24
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Re: Wave Height

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Maybe you jiggle when you jog?
Well, that's a given but then again, that would be a LOT of jiggling!!
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Old 27-10-2014, 02:52   #25
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Re: Wave Height

When I was working on a trawler over here we used to send in weather reports and swell sizes for the marine forecast,and swell size was always taken from mean sea level,so if the swell was 4 metres from crest to trough it was called 2 metres.So a four metre swell was 8 metres from trough to crest.Was time to go home when it was this big though.
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Old 27-10-2014, 03:25   #26
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Re: Wave Height

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When I was working on a trawler over here we used to send in weather reports and swell sizes for the marine forecast,and swell size was always taken from mean sea level,so if the swell was 4 metres from crest to trough it was called 2 metres.So a four metre swell was 8 metres from trough to crest.Was time to go home when it was this big though.
Interesting - I've always thought that waves height was from trough to crest and assumed swell heights would be the same.

So what is the norm for both wave and swell height???
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Old 27-10-2014, 03:39   #27
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Re: Wave Height

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I have an aviation GPS but never taken it on the boat. Aviation gps, obviously, gives altitude.

Anyone tried one on a boat and seen anything in terms of altitude?
But, don't you have to put the local atis in, or sea pressure for it to be accurate? Then what is it going to show you?
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Old 27-10-2014, 03:46   #28
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Re: Wave Height

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But, don't you have to put the local atis in, or sea pressure for it to be accurate? Then what is it going to show you?
No, not really, the idea is that you measure the differential between the crest and trough, not any absolute height so it wouldn't matter anyhow.

My limited experience is that an aviation GPS altitude is too inaccurate for meaningful reading.

YMMV.
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Old 27-10-2014, 03:46   #29
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Re: Wave Height

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Interesting - I've always thought that waves height was from trough to crest and assumed swell heights would be the same.

So what is the norm for both wave and swell height???
From the BOM:
Marine Weather Definitions

"Wave height (trough to crest) for both sea and swell refers to 'significant wave height' which represents the average height of the highest one-third of the waves. Some waves will be higher and some lower than the significant wave height. The probable maximum wave height can be up to twice the significant wave height."

and:
Bureau - Glossary of Terms
"Wave Height: Generally taken as the height difference between the wave crest and the preceding trough."

(red highlights added)
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Old 27-10-2014, 03:57   #30
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Re: Wave Height

Well, I never exaggerate wave heights. When talking to my wife back at home then I under rate them, but if I'm re telling a story to work collegues I measure them by the height of the mast and double it for a good reaction.

So far, I've found the Bass Strait forecasts and observations to be pretty accurate on wave heights, though their disclaimer is always that the heights can be %40 either way.
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