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.hamil...@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 <jbi...@cicsnc.org<mailto:jbi...@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. <r...@bodc.ac.uk<mailto:r...@bodc.ac.uk>
Sent: 30 May 2018 21:02
Thanks Jim,
That work for me.
Cheers, Roy.
------------------------------------------------------------------------
From: Jim Biard <jbi...@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 <jbi...@cicsnc.org><mailto:jbi...@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 <hel...@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.
<r...@bodc.ac.uk<mailto:r...@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 <ngalbra...@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.
--
*******************************************************
* Nan Galbraith Information Systems Specialist *
* Upper Ocean Processes Group Mail Stop 29 *
* Woods Hole Oceanographic Institution *
* Woods Hole, MA 02543 (508) 289-2444 *
*******************************************************
_______________________________________________
CF-metadata mailing list
CF-metadata@cgd.ucar.edu
http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
