# Wave height, how is it measured?

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**1**Senior Member

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Location: New England

Posts: 5,074

**Wave height, how is it measured?**

I wonder, is it from the bottom of the trough to the top of the wave?, or is it 1/2 of that?

I always think that if I could stand on water, at the bottom of a trough, and the water would be to come up to my knee/ 1 1/2', then that would be a 1-2 foot sea as many grossly overestimate wave height thus the term, "THT-seas" What is the right way to measure wave height?

I always think that if I could stand on water, at the bottom of a trough, and the water would be to come up to my knee/ 1 1/2', then that would be a 1-2 foot sea as many grossly overestimate wave height thus the term, "THT-seas" What is the right way to measure wave height?

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Location: Charleston, SC

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**Re: Wave height, how is it measured?**

I use trough to crest....but almost everyone overstates it once you get above 2. I think it's because once reach a height where it's higher than the freeboard it just seems bigger.

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**Re: Wave height, how is it measured?**

Pretty sure its measured as the vertical distance from the trough to the top.

I use to think it was half that too. But that was because of surfing. if the seas were "3ft" it was normally a head high wave "6ft".

But I guess thats because they get jacked up on the sandbar.

I use to think it was half that too. But that was because of surfing. if the seas were "3ft" it was normally a head high wave "6ft".

But I guess thats because they get jacked up on the sandbar.

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**5**Senior Member

Join Date: Nov 2003

Location: Matawan, New Jersey

Posts: 442

**RE: Wave height, how is it measured?**

Significant Wave Height

Is the average height (trough to crest) of the one-third highest waves valid for the indicated 12 hour period.

Significant wave height, Hs, is approximately equal to the average of the highest one-third of the waves. Hs is calculated using:

Hs = 4.0 * sqrt(m0)

where m0 is the variance of the wave displacement time series acquired during the wave acquisition period. However, since wave displacement time series are not returned from NDBC's wave measurement systems, variance is calculated using the nondirectional wave spectrum according to the following relationship:

m0 = sum(S(f)*df)

where the summation of spectral density, S(f), is over all frequency bands, f, of the nondirectional wave spectrum and df is the bandwidth of each band. NDBC wave analysis systems typically sum over the range from 0.03 to 0.40 Hz with frequency bandwidths of 0.01 Hz. Some newer systems sum up to 0.485 Hz with bandwidths that vary from 0.005 Hz at low frequencies to 0.02 Hz at high frequencies.

Dominant, or peak, wave period, Tp, is the period corresponding to the frequency band with the maximum value of spectral density in the nondirectional wave spectrum. It is the reciprocal of the peak frequency, fp:

Tp = 1/fp

Dominant period is representative of the higher waves encountered during the wave sampling period. Greater detail on the processing of NDBC wave data can be found in the 1996 technical document, Nondirectional and Directional Wave Data Analysis Procedures (a pdf file).

NDBC also provides estimates of the height and period of wind-seas and swell on each station page. Values for these quantities are calculated by applying the above process to the respective wind-sea and swell portions of the wave spectrum.

The algorithm used to estimate wave steepness is taken from work done by William Buckley, discussed in a paper that appeared in the Journal of Naval Engineers, September, 1988, titled " Extreme and Climatic Wave Spectra for Use in the Structural Design of Ships". The algorithm involves the relationship between significant wave height (Hs) and dominant wave period, or more precisely, its inverse, peak wave frequency (fp).

The algorithm follows:

val = exp(-3.3 * ln(fp))

if Hs > (1/250 * val)

steepness = 'very steep';

elseif Hs > (1/500 * val)

steepness = 'steep';

elseif Hs > (1/1000 * val)

steepness = 'average';

else

if Hswell >= Hwindwv

steepness = 'swell';

else

steepness = 'average';

where Hswell and Hwindwv are the respective significant heights of the swell and wind waves which are quantities that are also computed.

Bet you're sorry you asked

Is the average height (trough to crest) of the one-third highest waves valid for the indicated 12 hour period.

Significant wave height, Hs, is approximately equal to the average of the highest one-third of the waves. Hs is calculated using:

Hs = 4.0 * sqrt(m0)

where m0 is the variance of the wave displacement time series acquired during the wave acquisition period. However, since wave displacement time series are not returned from NDBC's wave measurement systems, variance is calculated using the nondirectional wave spectrum according to the following relationship:

m0 = sum(S(f)*df)

where the summation of spectral density, S(f), is over all frequency bands, f, of the nondirectional wave spectrum and df is the bandwidth of each band. NDBC wave analysis systems typically sum over the range from 0.03 to 0.40 Hz with frequency bandwidths of 0.01 Hz. Some newer systems sum up to 0.485 Hz with bandwidths that vary from 0.005 Hz at low frequencies to 0.02 Hz at high frequencies.

Dominant, or peak, wave period, Tp, is the period corresponding to the frequency band with the maximum value of spectral density in the nondirectional wave spectrum. It is the reciprocal of the peak frequency, fp:

Tp = 1/fp

Dominant period is representative of the higher waves encountered during the wave sampling period. Greater detail on the processing of NDBC wave data can be found in the 1996 technical document, Nondirectional and Directional Wave Data Analysis Procedures (a pdf file).

NDBC also provides estimates of the height and period of wind-seas and swell on each station page. Values for these quantities are calculated by applying the above process to the respective wind-sea and swell portions of the wave spectrum.

The algorithm used to estimate wave steepness is taken from work done by William Buckley, discussed in a paper that appeared in the Journal of Naval Engineers, September, 1988, titled " Extreme and Climatic Wave Spectra for Use in the Structural Design of Ships". The algorithm involves the relationship between significant wave height (Hs) and dominant wave period, or more precisely, its inverse, peak wave frequency (fp).

The algorithm follows:

val = exp(-3.3 * ln(fp))

if Hs > (1/250 * val)

steepness = 'very steep';

elseif Hs > (1/500 * val)

steepness = 'steep';

elseif Hs > (1/1000 * val)

steepness = 'average';

else

if Hswell >= Hwindwv

steepness = 'swell';

else

steepness = 'average';

where Hswell and Hwindwv are the respective significant heights of the swell and wind waves which are quantities that are also computed.

Bet you're sorry you asked

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Location: Tampa, Florida

Posts: 2,287

**Re: Wave height, how is it measured?**

I've always used the 1/2 of the distance from the trough to the crest. My thinking, as was my physics professor's, is that on calm seas the water level is stable. When you introduce wave mechanics into the situation the water level will drop from the stable level during the trough and rise during the crest in equal amounts. When I say 3 foot seas I mean 6 feet from trough to crest. Does anyone know how NOAA measures it? I guess I should look that up.

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**9**Senior Member

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Location: Tampa, Florida

Posts: 2,287

**Re: Wave height, how is it measured?**

Found my answer on NOAA's website (they measure from from the bottom of the trough to the crest):

SWH Swell height is the vertical distance (meters) between any swell crest and the succeeding swell wave trough.

WWH Wind Wave Height is the vertical distance (meters) between any wind wave crest and the succeeding wind wave trough (independent of swell waves).

SWH Swell height is the vertical distance (meters) between any swell crest and the succeeding swell wave trough.

WWH Wind Wave Height is the vertical distance (meters) between any wind wave crest and the succeeding wind wave trough (independent of swell waves).

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**Re: Wave height, how is it measured?**

Here is NOAA's definition of "significant wave height"

http://www.srh.noaa.gov/graphical/de...aveHeight.html

http://www.srh.noaa.gov/graphical/de...aveHeight.html

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**Re: Wave height, how is it measured?**

Gringo - 8/22/2007 11:34 AM

Wave height is usually measured by drunken rednecks on boats that are overpowered and travelling way too fast with too many fish onboard.

Wave height is usually measured by drunken rednecks on boats that are overpowered and travelling way too fast with too many fish onboard.

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Join Date: Jan 2005

Location: Asheville, NC

Posts: 6,493

**Re: Wave height, how is it measured?**

G8RDave - 8/22/2007 11:57 AM

Found my answer on NOAA's website (they measure from from the bottom of the trough to the crest):

SWH Swell height is the vertical distance (meters) between any swell crest and the succeeding swell wave trough.

WWH Wind Wave Height is the vertical distance (meters) between any wind wave crest and the succeeding wind wave trough (independent of swell waves).

Found my answer on NOAA's website (they measure from from the bottom of the trough to the crest):

SWH Swell height is the vertical distance (meters) between any swell crest and the succeeding swell wave trough.

WWH Wind Wave Height is the vertical distance (meters) between any wind wave crest and the succeeding wind wave trough (independent of swell waves).

HH

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**13**Senior Member

Join Date: Mar 2002

Location: Republic of West Florida - the ORIGINAL lone star state

Posts: 17,780

**RE: Wave height, how is it measured?**

Wave height, schmave height.

It's either rough (for the boat yer in), or it isn't.

Big Al

significant height, and able to wave

It's either rough (for the boat yer in), or it isn't.

Big Al

significant height, and able to wave

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**14**Senior Member

Join Date: Jun 2005

Location: Naples, FL

Posts: 3,914

**RE: Wave height, how is it measured?**

hlights2 - 8/22/2007 11:39 AM

Significant Wave Height

Is the average height (trough to crest) of the one-third highest waves valid for the indicated 12 hour period.

Significant wave height, Hs, is approximately equal to the average of the highest one-third of the waves. Hs is calculated using:

Hs = 4.0 * sqrt(m0)

where m0 is the variance of the wave displacement time series acquired during the wave acquisition period. However, since wave displacement time series are not returned from NDBC's wave measurement systems, variance is calculated using the nondirectional wave spectrum according to the following relationship:

m0 = sum(S(f)*df)

where the summation of spectral density, S(f), is over all frequency bands, f, of the nondirectional wave spectrum and df is the bandwidth of each band. NDBC wave analysis systems typically sum over the range from 0.03 to 0.40 Hz with frequency bandwidths of 0.01 Hz. Some newer systems sum up to 0.485 Hz with bandwidths that vary from 0.005 Hz at low frequencies to 0.02 Hz at high frequencies.

Dominant, or peak, wave period, Tp, is the period corresponding to the frequency band with the maximum value of spectral density in the nondirectional wave spectrum. It is the reciprocal of the peak frequency, fp:

Tp = 1/fp

Dominant period is representative of the higher waves encountered during the wave sampling period. Greater detail on the processing of NDBC wave data can be found in the 1996 technical document, Nondirectional and Directional Wave Data Analysis Procedures (a pdf file).

NDBC also provides estimates of the height and period of wind-seas and swell on each station page. Values for these quantities are calculated by applying the above process to the respective wind-sea and swell portions of the wave spectrum.

The algorithm used to estimate wave steepness is taken from work done by William Buckley, discussed in a paper that appeared in the Journal of Naval Engineers, September, 1988, titled " Extreme and Climatic Wave Spectra for Use in the Structural Design of Ships". The algorithm involves the relationship between significant wave height (Hs) and dominant wave period, or more precisely, its inverse, peak wave frequency (fp).

The algorithm follows:

val = exp(-3.3 * ln(fp))

if Hs > (1/250 * val)

steepness = 'very steep';

elseif Hs > (1/500 * val)

steepness = 'steep';

elseif Hs > (1/1000 * val)

steepness = 'average';

else

if Hswell >= Hwindwv

steepness = 'swell';

else

steepness = 'average';

where Hswell and Hwindwv are the respective significant heights of the swell and wind waves which are quantities that are also computed.

Bet you're sorry you asked

Significant Wave Height

Is the average height (trough to crest) of the one-third highest waves valid for the indicated 12 hour period.

Significant wave height, Hs, is approximately equal to the average of the highest one-third of the waves. Hs is calculated using:

Hs = 4.0 * sqrt(m0)

where m0 is the variance of the wave displacement time series acquired during the wave acquisition period. However, since wave displacement time series are not returned from NDBC's wave measurement systems, variance is calculated using the nondirectional wave spectrum according to the following relationship:

m0 = sum(S(f)*df)

where the summation of spectral density, S(f), is over all frequency bands, f, of the nondirectional wave spectrum and df is the bandwidth of each band. NDBC wave analysis systems typically sum over the range from 0.03 to 0.40 Hz with frequency bandwidths of 0.01 Hz. Some newer systems sum up to 0.485 Hz with bandwidths that vary from 0.005 Hz at low frequencies to 0.02 Hz at high frequencies.

Dominant, or peak, wave period, Tp, is the period corresponding to the frequency band with the maximum value of spectral density in the nondirectional wave spectrum. It is the reciprocal of the peak frequency, fp:

Tp = 1/fp

Dominant period is representative of the higher waves encountered during the wave sampling period. Greater detail on the processing of NDBC wave data can be found in the 1996 technical document, Nondirectional and Directional Wave Data Analysis Procedures (a pdf file).

NDBC also provides estimates of the height and period of wind-seas and swell on each station page. Values for these quantities are calculated by applying the above process to the respective wind-sea and swell portions of the wave spectrum.

The algorithm used to estimate wave steepness is taken from work done by William Buckley, discussed in a paper that appeared in the Journal of Naval Engineers, September, 1988, titled " Extreme and Climatic Wave Spectra for Use in the Structural Design of Ships". The algorithm involves the relationship between significant wave height (Hs) and dominant wave period, or more precisely, its inverse, peak wave frequency (fp).

The algorithm follows:

val = exp(-3.3 * ln(fp))

if Hs > (1/250 * val)

steepness = 'very steep';

elseif Hs > (1/500 * val)

steepness = 'steep';

elseif Hs > (1/1000 * val)

steepness = 'average';

else

if Hswell >= Hwindwv

steepness = 'swell';

else

steepness = 'average';

where Hswell and Hwindwv are the respective significant heights of the swell and wind waves which are quantities that are also computed.

Bet you're sorry you asked

mirage2521 - 8/22/2007 1:02 PM

Quote:
Correct!

Gringo - 8/22/2007 11:34 AM

Wave height is usually measured by drunken rednecks on boats that are overpowered and travelling way too fast with too many fish onboard.

Wave height is usually measured by drunken rednecks on boats that are overpowered and travelling way too fast with too many fish onboard.

Wrong...Here's how I do it: (And I never have more than thirty kingfish on board, thank you.)

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**16**Senior Member

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Location: Bermuda

Posts: 5,144

**Re: Wave height, how is it measured?**

Wave height calculation is variable. It has alot to do with the distance from the bow of the boat you are standing on to the stern multiplied by the distance you are from shore & divided by the amount of experience the skipper has in rough water then multiplied again by the time the skipper has to get to the nearest bar to tell the story.

The result is at least triple what NOAA says.

The result is at least triple what NOAA says.

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**17**Senior Member

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Location: Republic of West Florida - the ORIGINAL lone star state

Posts: 17,780

**Re: Wave height, how is it measured?**

SeaJay - 8/22/2007 12:36 PM

Many posters on this board when describing their last trio take the most accurate estimate of the wave size and add 3 feet to it.

Many posters on this board when describing their last trio take the most accurate estimate of the wave size and add 3 feet to it.

Big Al

keyboards, dontchaloveem?

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**19**Senior Member

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Location: Oakland, CA, USA

Posts: 1,450

**RE: Wave height, how is it measured?**

solarfry - 8/22/2007 10:52 AM

I keep it simple..

If it's higher than my bow it's a 3.5'.

It if comes over the bow its a 4'

If I can't see the top its a 12'

I keep it simple..

If it's higher than my bow it's a 3.5'.

It if comes over the bow its a 4'

If I can't see the top its a 12'

jky

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**20**Senior Member

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Location: Salem, MA

Posts: 1,107

**Re: Wave height, how is it measured?**

Bottom tier boats, 2' seas feel like 6' seas

Mid tier boats, 4' seas feel like 6' seas

Top tier boats, 6' seas =jack

Mid tier boats, 4' seas feel like 6' seas

Top tier boats, 6' seas =jack

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