Thanks for everyone’s input, the below seems acceptable for now
Regards
Steve
From: CF-metadata <[email protected]> On Behalf Of
Lowry, Roy K.
Sent: 30 May 2018 21:37
To: Jim Biard <[email protected]>; [email protected]
Subject: Re: [CF-metadata] Platform Heave
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.
I have now retired but will continue to be active through an Emeritus
Fellowship using this e-mail address.
------------------------------------------------------------------------
From: CF-metadata <[email protected]
<mailto:[email protected]>> on behalf of Lowry, Roy K.
<[email protected] <mailto:[email protected]>>
Sent: 30 May 2018 21:02
To: Jim Biard; [email protected] <mailto:[email protected]>
Subject: Re: [CF-metadata] Platform Heave
Thanks Jim,
That work for me.
Cheers, Roy.
------------------------------------------------------------------------
From: CF-metadata <[email protected]
<mailto:[email protected]>> on behalf of Jim Biard
<[email protected] <mailto:[email protected]>>
Sent: 30 May 2018 18:39
To: [email protected] <mailto:[email protected]>
Subject: Re: [CF-metadata] Platform Heave
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: CF-metadata <[email protected]>
<mailto:[email protected]> on behalf of Jim Biard
<[email protected]> <mailto:[email protected]>
Sent: 26 May 2018 23:18
To: [email protected] <mailto:[email protected]>
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.
On Sat, May 26, 2018 at 3:11 AM, Lowry, Roy K. <[email protected]
<mailto:[email protected]>> 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: CF-metadata <[email protected]
<mailto:[email protected]>> on behalf of John
Helly <[email protected] <mailto:[email protected]>>
Sent: 26 May 2018 04:48
To: Jim Biard; [email protected]
<mailto:[email protected]>
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
On Fri, May 25, 2018 at 7:28 AM, Lowry, Roy K.
<[email protected] <mailto:[email protected]>> 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: CF-metadata
<[email protected]
<mailto:[email protected]>> on
behalf of Nan Galbraith <[email protected]
<mailto:[email protected]>>
Sent: 25 May 2018 14:46
To: [email protected]
<mailto:[email protected]>
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
<[email protected]
<mailto:[email protected]>> on
behalf of
> Hamilton, Steve <[email protected]
<mailto:[email protected]>>
> *Sent:* 25 May 2018 08:51
> *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 <[email protected]
<mailto:[email protected]>>
> *Sent:* 21 May 2018 19:18
> *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
> <[email protected]
<mailto:[email protected]>
<mailto:[email protected]>> 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
