Wave Height

[Ex-Calif]

Originally Posted by Rustic Charm

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 the gps is not a pressure instument.

It triangulates 3-dimenisional position from the satellite signals.

[StuM]

Originally Posted by Ex-Calif

No the gps is not a pressure instument.

It triangulates 3-dimenisional position from the satellite signals.

Depends - my handheld (GPS Map76CS) has a barometric pressure sensor (as well as a fluxgate compass). But even that doesn't give you anything good enough to measure sea state. It can vary by several meters over a few minutes when it is dead calm.

[Rustic Charm]

Originally Posted by Ex-Calif

No the gps is not a pressure instument.

It triangulates 3-dimenisional position from the satellite signals.

Ok, but you said it measures altitude? How can a standard, even aviation gps measure alitude? I thought you need an altimeter to measure altitude and to do that you need to input the always changing local environment conditions, usually obtained from the ATIS.

[StuM]

Originally Posted by Rustic Charm

Ok, but you said it measures altitude? How can a standard, even aviation gps measure alitude? I thought you need an altimeter to measure altitude and to do that you need to input the always changing local environment conditions, usually obtained from the ATIS.

The signals from satellites triangulate in 3-dimensional space. Basically, the "cocked hat of terrestrial navigation becomes a spheroid of error with the vertical error being about 1.5 times the horizontal.

[Rustic Charm]

This might be useful

[Rustic Charm]

Originally Posted by StuM

The signals from satellites triangulate in 3-dimensional space. Basically, the "cocked hat of terrestrial navigation becomes a spheroid of error with the vertical error being about 1.5 times the horizontal.

cool. Didn't know they could do that

[Ex-Calif]

Originally Posted by StuM

Depends - my handheld (GPS Map76CS) has a barometric pressure sensor (as well as a fluxgate compass). But even that doesn't give you anything good enough to measure sea state. It can vary by several meters over a few minutes when it is dead calm.

Is the baro information used for position or is it just a feature they included to provide a baro readout?

I can't imagine the gps using/needing baro for position?

<edit> Never mind... I Googled it. Interesting you can use gps altitude or barometric altitude (pressure altitude). When using baro altitidue you can calibrate altitude and baro from known sources. I suppose I'd have to have a unit and play with it to understand why I'd use baro altitude - Learn sumptin' every day I guess..

[Wotname]

Complete thread drift follows

If you give an aviation GPS an altitude input, it "creates" a pseudo satellite that is "fixed" in space directly above the antenna. That is, it assumes there is a satellite with the same lat and lon as the antenna and once you give it the third dimension (altitude), the pseudo satellite must be directly above. It can then use this fake satellite to help resolve RAIM etc.

Sorry - now back to wave heights (and swell heights)

For Ex-Calif - every IFR GPS I have ever seen has the mode C code wiring to the GPS for the above reason - this is (or was?) mandated in OZ and I assume elsewhere

[Lodesman]

My experience is that when looking down at the sea, especially from larger ships most people tend to underestimate wave height. As height of eye gets closer to the height of the crests, and certainly when looking up at the crests, most people tend to overestimate. For this reason I tend to take the "official reports" of wave height with a grain of salt. I particularly find NOAA reporting to be unreliable. I've talked with others, who were of the opinion that NOAA reports wave height from mean sea level to crest.

[valhalla360]

Originally Posted by StuM

The signals from satellites triangulate in 3-dimensional space. Basically, the "cocked hat of terrestrial navigation becomes a spheroid of error with the vertical error being about 1.5 times the horizontal.

I would have to run thru the math but my recolleciton is the vertical error is two or more times the horizontal. Of course it does differ based on the exact positioning of the satilites relative to the reciever. The lower they are towards the horizon, the worse they measure altitude.

While it's not a technical requirement, most GPS only spit out a position once per second. Mostly this is to do the exact opposite of finding wave height. In most use cases, smoothed data is prefered by the end user. If the numbers change faster than you can read them, it makes the data less useful (for most common purposes). If you have a short steep 3' chop, it's entirely possible you will read 0' wave height as you pick up successive wave crests at a 1 second interval.

Then you have the issue of the boat dampening the motion. If you have an old heavy mono, that just plow thru waves, you will get a different answer from a light multihull that follows the waves more closely. Even with the multihull, there will still be a dampening effect.

The you have the related issue of long vs short period waves. If you are talking about a big ocean swell, even the heavy mono will follow the waves pretty closely but as you move towards a short steep chop, even the light boat will push thru rather than over waves to a greater degree.

Bottom line, aircraft GPS are just fine with a 50-100' vertical error. Other than landing, there is little purpose in greater accuracy and even for landing, it's not that critical to be accurate to the nearest foot.

[ontherocks83]

Originally Posted by Cheechako

Well... I think eyeballing it is the norm. The height is from the trough to the top.... so looking afield it's not hard to estimate....

Originally Posted by builder dan

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.

Originally Posted by StuM

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)

Ok I may be a little slow but i'm missing something here. Is there a difference between swell, wave height, and significant wave height? Are there any more ways to clasify wave height? I always did it from trough to crest but have been corrected to reference mean sea level to crest and then re-corrected back to trough to crest. Which do the weather reports reference? Why does this make my head hurt?

[Sailormantx]

Originally Posted by ontherocks83

Ok I may be a little slow but i'm missing something here. Is there a difference between swell, wave height, and significant wave height? Are there any more ways to clasify wave height? I always did it from trough to crest but have been corrected to reference mean sea level to crest and then re-corrected back to trough to crest. Which do the weather reports reference? Why does this make my head hurt?

Significant wave height is an average of the higher waves. Wind waves are produced locally and swells may originate from thousands of miles away and are often from a different direction than the wind waves. In some areas the wind waves and swell height are separate measurements in the weather forecast.Their interaction can be interesting when they are both large and coming from different directions, that is why they are referred to as separate measurements.

[barnakiel]

Various terms for varied notions.

I call swell the long waves that come from elsewhere and may not be related to the current system in place.

The present waves I call them wind waves. (Yes, I know the other ones are wind waves too, just their wind is or was elsewhere; that's the point).

Significant wave height I use as they use it at NOAA. Check their site for definition.

b.

[Nord Sal]

I found this link (PDF file) a few years ago and it still up. It has a good description of significant wave height and also describes most probable height, average height, max height, etc.

www.mxak.org/weather/pdfs/waves.pdf

[jackdale]

Originally Posted by barnakiel

Various terms for varied notions.

I call swell the long waves that come from elsewhere and may not be related to the current system in place.

The present waves I call them wind waves. (Yes, I know the other ones are wind waves too, just their wind is or was elsewhere; that's the point).

Significant wave height I use as they use it at NOAA. Check their site for definition.

b.

The weatherfax has the definition: the average height of the highest 1/3 of the waves.

Northeast Pacific WX Briefing Package