Hi Jim,
Does
"Heave" is a term used to describe the vertical displacement of a moving object above the vertical level of that object when stationary.
help by getting rid of the semantically-loaded word 'height'? If not, what would?
I think the confusion is because you are thinking of heave in terms of position within a reference frame. To think of it as the vertical displacement between a real platform and a massless platform is misleading- such considerations are part of the derivation of wave height from high frequency heave measurements, which isn't relevant to a discussion of the raw measurement. It's also worth bearing in mind that whilst the debate has focused on platforms floating on the sea surface, the concept of heave could in theory be applied to objects in the atmosphere.
In practice, heave is measured by accelerometers that are usually combined with tilt sensors that give pitch, roll and yaw. Hence, it is totally decoupled from any reference outside the platform.
To answer your last muse, to get heave from a high frequency height relative to datum time series the method would need to determine the height of the object when 'stationary'. In the case of objects on the sea, 'stationary' is considered to be a flat calm sea (i.e. no waves), which can be approximated by averaging the raw time series. So, heave could be approximated by differencing the raw and averaged data. However, I can't think why anybody would want to do that.
Cheers, Roy.
I am retiring on 31/05/2018 but will continue to be active through an Emeritus Fellowship using this e-mail address.
________________________________
From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu> on behalf of Jim Biard <jbiard at cicsnc.org>
Sent: 26 May 2018 23:18
To: cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] Platform Heave
My biggest concern is that the standard name definition makes it clear in some fashion or other that this is a measure of deviations from some lower frequency (or low-pass filtered) measure of vertical position. (As are sway and surge in relation to their corresponding horizontal coordinates.) As was pointed out, heave is used in certain communities, so it's reasonable to provide a standard name, but it seems rather imprecise as it has been described so far.
If I have understood the explanations correctly, a time series of platform height relative to a fixed datum that has sufficient precision and frequency would fully represent the heave along with the more slowly varying effects of tide, waves, etc. So is heave, as usually used, the first-order instantaneous difference between the height of an actual platform and the height of a massless ideal platform that would maintain a fixed offset relative to the sea surface? And, just out of curiosity, how would a time series of instantaneous measures of height relative to a fixed datum be separated in practice into heave and "non-heave" height?
Getting back on track, it seems to me that the definition ought to somehow assist the reader in understanding how heave relates to other measures of height.
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On Sat, May 26, 2018 at 3:11 AM, Lowry, Roy K. <rkl at bodc.ac.uk<mailto:rkl at bodc.ac.uk>> wrote:
Dear Jim and John,
Heave is indeed a height relative to a datum, that datum being the calm sea surface, which is a local short interval mean sea level that isn't linked into any global reference system. Indeed the 'datum' moves relative to the rest of the world - but not the platform - as tide rises and falls so many would prefer to call it an 'instrument zero' rather than a 'datum'.
Heave is therefore a very different measurement to any sea level parameter and is the raw measurement recorded at high (Hz to kHz) frequency as a time series by floating wave instruments such as waveriders and shipborne wave recorders. It therefore cannot be sensibly described by the same or similar Standard Name as a measurement of height above a globally referenced datum like long-term mean sea level or geoid. Whilst the Standard Name could be 'platform_height_above_calm_sea_surface' or 'platform_height_above_stationary_position' I would argue that 'heave' is a term from the same domain vocabulary as 'pitch', 'roll' and 'yaw' and therefore should be used.
John is right to point out that the heave measurement is affected by the nature of the platform with a 250,000 tonne supertanker moving up and down much less than a rowing boat in a given wave climate, especially a wind sea. That was what was behind the SBWR corrections based on platform dimensions set up by Laurie Draper and Tom Tucker back in the 1980s.
Cheers, Roy.
I am retiring on 31/05/2018 but will continue to be active through an Emeritus Fellowship using this e-mail address.
________________________________
From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu<mailto:cf-metadata-bounces at cgd.ucar.edu>> on behalf of John Helly <hellyj at ucsd.edu<mailto:hellyj at ucsd.edu>>
Sent: 26 May 2018 04:48
To: Jim Biard; cf-metadata at cgd.ucar.edu<mailto:cf-metadata at cgd.ucar.edu>
Subject: Re: [CF-metadata] Platform Heave
Can't let go of this yet.
If you think about the inverse problem of deriving the sea surface elevation from the heave you would have to account for the latency of ship motion relative to the sea-surface. A wave passing under a ship induces motions that are not instantaneous either in attack or decay.
J.
On 5/25/18 20:42, John Helly wrote:
I believe it's a synonym within the oceanographic community for the vertical motion of an ocean-going platform.
https://en.wikipedia.org/wiki/Ship_motions
Ship motions - Wikipedia<
https://en.wikipedia.org/wiki/Ship_motions>
en.wikipedia.org<
http://en.wikipedia.org>
Ship motions are defined by the six degrees of freedom that a ship, boat or any other craft can experience.
Could just be jargon but it strike me as more complex: nonetheless a vertical position relative to a datum, but the buoyancy, stability and momentum of the platform are implied as part of the dynamics. It seems that the datum is not a geophysical one alone but confounded with the 'normal' waterline for a platform so it may be relative to the water level in which the platform is embedded. That's a tough one. Two different platforms on the same sea surface would have different 'heave', for example.
J.
On 5/25/18 19:54, Jim Biard wrote:
Hi.
I get and endorse the need for pitch, roll, and yaw, but I remain perplexed about heave. How is a time series of 'heave' different from a time series of height relative to some vertical datum? I've yet to see a proposed definition that convinces me that this is a uniquely different quantity.
Grace and peace,
Jim
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On Fri, May 25, 2018 at 7:28 AM, Lowry, Roy K. <rkl at bodc.ac.uk<mailto:rkl at bodc.ac.uk>> wrote:
Dear All,
I agree with Nan that definitions of pitch roll and yaw would improve the existing Standard Name definitions. I also agree with using the existing orientation Standard Names for ADCPs and that the 'platform' definition wording could make this clearer. However, such an enhancements should be submitted as a separate proposal and not be considered as part of Steve's proposal.
Cheers, Roy.
I am retiring on 31/05/2018 but will continue to be active through an Emeritus Fellowship using this e-mail address.
________________________________
From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu<mailto:cf-metadata-bounces at cgd.ucar.edu>> on behalf of Nan Galbraith <ngalbraith at whoi.edu<mailto:ngalbraith at whoi.edu>>
Sent: 25 May 2018 14:46
To: cf-metadata at cgd.ucar.edu<mailto:cf-metadata at cgd.ucar.edu>
Subject: Re: [CF-metadata] Platform Heave
I'd really like to see pitch, roll and yaw defined in the CF standard
name table; currently
the definitions only say 'Standard names for platform describe the
motion and orientation
of the vehicle from which observations are made e.g. aeroplane, ship or
satellite.'
Also, not to get too far into the weeds, but many of the platform terms
are important
for instruments like ADCPs, so I'd just like to confirm that these
definitions - and
the names themselves - can be used to describe instruments, not just
vehicles
'e.g. aeroplane, ship or satellite'. We already use pitch roll and
yaw for these
instruments on surface moorings, and I hope (and assume) this is legal.
Thanks - Nan Galbraith
On 5/25/18 8:53 AM, Lowry, Roy K. wrote:
>
>
> Dear Steve,
>
>
> One of the reasons I was interested in your definitions was your
> perspective on the datum (i.e. zero value) for heave. The datum
> 'mean_sea_level' is well used in CF, but with the definition 'time
> mean of sea surface elevation at a given location over an arbitrary
> period sufficient to eliminate the tidal signals.' This is obviously
> not appropriate for platform heave which doesn't take any account of
> the state of the tide and so I would exclude 'mean_sea_level' from the
> Standard Name.
>
>
> I think my preference would be to keep the term 'heave' as we already
> have 'pitch', 'yaw' and 'roll', giving:
>
>
> platform_heave (m)
>
>
> Standard names for platform describe the motion and orientation of the
> vehicle from which observations are made e.g. aeroplane, ship or
> satellite. "Heave" is a term used to describe the vertical
> displacement of the platform above its position when not moving.
>
>
> tendency_of_platform_heave (m s-1)
>
>
> Standard names for platform describe the motion and orientation of the
> vehicle from which observations are made e.g. aeroplane, ship or
> satellite. "Tendency_of_X" means derivative of X with respect to time.
> "Heave" is a term used to describe the vertical displacement of the
> platform above its position when not moving.
>
>
> What do you think?
>
>
> Cheers, Roy.
>
>
> I am retiring on 31/05/2018 but will continue to be active through an
> Emeritus Fellowship using this e-mail address.
>
>
>
> ------------------------------------------------------------------------
> *From:* CF-metadata <cf-metadata-bounces at cgd.ucar.edu<mailto:cf-metadata-bounces at cgd.ucar.edu>> on behalf of
> Hamilton, Steve <sj.hamilton at fugro.com<mailto:sj.hamilton at fugro.com>>
> *Sent:* 25 May 2018 08:51
> *To:* cf-metadata at cgd.ucar.edu<mailto:cf-metadata at cgd.ucar.edu>
> *Subject:* Re: [CF-metadata] Platform Heave
>
> All,
>
> Thanks for all the comments, I have tried to capture as below -
>
> *Parameter Name***
>
>
>
> *Standard Name*
>
>
>
> *Definition*
>
>
>
> *Canonical Units*
>
> Platform Heave
>
>
>
> Platform_Height_above_mean_sea_Level
>
>
>
> Standard names for platform describe the motion and
> orientation of the vehicle from which observations are made e.g.
> aeroplane, ship or satellite. Height above mean sea Level is the
> linear vertical (up/down) distance of the platform in respect to the
> mean sea level.
>
>
>
> m
>
> Platform Heave Rate
>
>
>
> Tendency_of_Platform_Height_above_mean_sea_Level
>
>
>
> Standard names for platform describe the motion and
> orientation of the vehicle from which observations are made e.g.
> aeroplane, ship or satellite. "tendency_of_X" means derivative of X
> with respect to time. Height above mean sea Level is the linear
> vertical (up/down) distance of the platform in respect to the mean sea
> level.
>
>
>
> m s-1
>
> Please let me know if you have further comments
>
> Thanks
>
> Steve
>
> *From:*Steven Emmerson <emmerson at ucar.edu<mailto:emmerson at ucar.edu>>
> *Sent:* 21 May 2018 19:18
> *To:* Hamilton, Steve <sj.hamilton at fugro.com<mailto:sj.hamilton at fugro.com>>
> *Cc:* cf-metadata at cgd.ucar.edu<mailto:cf-metadata at cgd.ucar.edu>
> *Subject:* Re: [CF-metadata] Platform Heave
>
> Whatever name you come up with, the canonical unit of the heave rate
> shouldn't be "ms-1", but rather one of the following:
>
> m s-1
>
> m/s
>
> m.s-1
>
> I favor "m/s".
>
>
> Regards,
>
> Steve Emmerson
>
> On Mon, May 21, 2018 at 6:32 AM, Hamilton, Steve
> <sj.hamilton at fugro.com<mailto:sj.hamilton at fugro.com> <mailto:sj.hamilton at fugro.com>> wrote:
>
> Hi
>
> I am trying to find the CF name for heave of a vessel or
> platform. platform_roll_angle and platform_pitch_angle already
> exist but nothing on heave
>
> Would be the following be acceptable
>
> Platform_heave (m)
>
> Platform_heave_rate (ms-1)
>
> Standard names for platform describe the motion and orientation of
> the vehicle from which observations are made e.g. aeroplane, ship
> or satellite.
>
> Kind Regards,
>
> Steve
>
>
--
*******************************************************
* Nan Galbraith Information Systems Specialist *
* Upper Ocean Processes Group Mail Stop 29 *
* Woods Hole Oceanographic Institution *
* Woods Hole, MA 02543 (508) 289-2444 *
*******************************************************
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