[CF-metadata] Platform Heave

Nan Galbraith ngalbraith at whoi.edu
Tue Jul 10 10:39:06 MDT 2018


Hi Alison, Steve, and all -

Since we have a little time to finalize this, could we also consider 
updating the definitions
of platform_pitch_angle, platform_roll_angle and platform_yaw_angle?

Currently, these all say 'Standard names for platform describe the 
motion and orientation
of the vehicle from which observations are made e.g. aeroplane, ship or 
satellite.'

John Helly pointed to the helpful Wikipedia page for ship motion,
https://en.wikipedia.org/wiki/Ship_motions. The suggestions below are 
merged from
different sections of that page, and might be a little ... long, but I'd 
also
like to append something like 'Platforms include but are not limited to 
satellites, aeroplanes,
ships, instruments, and buoys.'

Pitch
The up/down rotation of a platform about its transverse/Y axis. The 
transverse/Y
axis, lateral or pitch axis is an imaginary line running horizontally 
across the platform
and through its center of gravity. A pitch  motion is an up-or-down 
movement of the
bow and stern of the platform.

Roll
The tilting rotation of a platform about its longitudinal/X axis. The 
longitudinal/X axis,
or roll axis, is an imaginary line running horizontally through the 
length of the platform,
through its center of gravity, and parallel to the waterline. A roll 
motion is a side-to-side
or port-starboard tilting motion of the superstructure around this axis.

Yaw
The turning rotation of a platform about its vertical/Z axis. The 
vertical/Z axis, or yaw axis,
is an imaginary line running vertically through the platform and through 
its center of gravity.
A yaw motion is a side-to side movement of the bow and stern of the ship.

And we had something like this for heave:
platform_heave (m) = upwards vertical displacement

I suppose these could also be applied to platform_*_rates.

Regards -
Nan


On 7/4/18 4:47 AM, Alison Pamment - UKRI STFC wrote:

> Dear Steve,  > > Thank you for your message and apologies for not having processed 
 > your proposals as yet. I have been working on the CMIP names, but > 
they are reaching a conclusion and I will shortly be looking through > 
the many other proposals that have been waiting for attention. > > A 
quick look through the discussion of your names shows they are > pretty 
much agreed. You need take no further action at this time - I > will 
check that the names and definitions are clear and consistent > with 
existing names and get back to you on the list with any final > comments 
or questions. Version 56 of the standard name table will be > published 
later today - I think we can probably finalise your names > in time for 
version 57. > > Best wishes, Alison
> ________________________________
> From: Hamilton, Steve <sj.hamilton at fugro.com>
> Sent: 03 July 2018 09:12
>
>
> Please can you advise if this standard name has now been accepted and when it will be included in the CF Standard Names
>
> If there is something else to do please let me know
>
> Thanks
>
> Steve
>
>
> ________________________________
> From: Jim Biard <jbiard at cicsnc.org<mailto:jbiard at cicsnc.org>>
> Sent: 01 June 2018 22:56
>
>
> Nan,
> Thanks for pulling things back in. I very much like the idea of keeping technology or specific methods out of the definition if at all possible, so I like your proposal. I expect we should include platform in the definition, as well as an indication that this is dynamic (over time). How about these definitions?
> platform_heave (m) = upwards vertical displacement of a platform over a measurement time interval
> platform_heave_rate (m s-1) = upwards rate of change in vertical displacement of a platform over a measurement time interval
> They leave out some detail but capture the relative nature of the quantities.
> (In my mind, the primary detail being left out is the 'net zero' nature of the quantities, which gets back to defining the 'moving-mean' sea level reference point.)
> Grace and peace,
>
> Jim

> On 6/1/18 11:23 AM, Nan Galbraith wrote:
> Hi all -
>
> The latest version is confusing to me. The term 'a platform that is nominally at rest' does
> not apply to many platforms for which heave is calculated; the original version of that,
> 'a moving object above the vertical level of that object when stationary' was maybe a little
> more clear... if also a little wordy.
>
> And, the term  'vertical displacement determined by integrating vertical accelerations' may
> also not apply - I've been looking at the different ways heave is calculated, and there
> are a few: 'Heave can be computed from GPS RTK height measurements and from
> vertical accelerations measured by linear accelerometers'
>
> Why  not keep it simple: platform_heave (m) = upwards vertical displacement?  Do we need
> to be more specific than that?
>
> Thanks - Nan
>
>
> From: Lowry, Roy K.
> Sent: 30 May 2018 21:37
>
> An afterthought. Heave is conventionally positive upwards so to make this clear I would add the word 'upwards' thus:
>
> platform_heave (m) = upwards vertical displacement determined by integrating vertical accelerations of a platform that is nominally at rest.
>
> platform_heave_rate (m s-1) = upwards vertical velocity determined by integrating vertical accelerations of a platform that is nominally at rest.
>
> Cheers. Roy.
>
>
> ------------------------------------------------------------------------
>
> From: Lowry, Roy K. <rkl at bodc.ac.uk<mailto:rkl at bodc.ac.uk>
> Sent: 30 May 2018 21:02
>
> Thanks Jim,
>
> That work for me.
>
> Cheers, Roy.
>
> ------------------------------------------------------------------------
>
> From: Jim Biard <jbiard at cicsnc.org>
> Sent: 30 May 2018 18:39
>
> Roy,
>
> So, heave is integrated vertical acceleration? How about
>
> platform_heave (m) = vertical displacement determined by integrating vertical accelerations of a platform that is nominally at rest.
>
> platform_heave_rate (m s-1) = vertical velocity determined by integrating vertical accelerations of a platform that is nominally at rest.
>
> Jim
>
> On 5/27/18 5:38 AM, Lowry, Roy K. wrote:
>
>      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.
>
>      ------------------------------------------------------------------------
>
>      From:Jim Biard <jbiard at cicsnc.org><mailto:jbiard at cicsnc.org>
>      Sent: 26 May 2018 23:18
>
>      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.
>
>
>      On Sat, May 26, 2018 at 3:11 AM, Lowry, Roy K. 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.
>
>          ------------------------------------------------------------------------
>
>          From: John Helly <hellyj at ucsd.edu>
>          Sent: 26 May 2018 04:48
>
>          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><https://en.wikipedia.org/wiki/Ship_motions>
>
>              en.wikipedia.org <http://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
>
>
>
>                  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.
>
>                      ------------------------------------------------------------------------
>
>                      From:  Nan Galbraith <ngalbraith at whoi.edu>
>                      Sent: 25 May 2018 14:46
>                      
>
>                      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.
>           
>
>
>
-- 
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* Upper Ocean Processes Group            Mail Stop 29 *
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