Dear Nan,
I think your suggestion for changing the order of the components of the definitions is a good one. I also agree that making the yaw angle sign convention totally unambiguous is an improvement.
However, your yaw definition leaves open the Devil's Advocate argument that if the platform rotates around its vertical axis then its orientation changes by the amount that the platform has rotated. So how can the yaw be measured relative to something that has also changed? We both know how it works for ships with the inertia of the gyrocompass, but how does one put that into words so that it covers all the things that we want to label 'platform'?
That is what I was attempting to do by considering yaw as a high-frequency variance in where the platform is pointing and platform orientation as its smoothed equivalent. Could using the word 'smoothed' be our get-out clause here? It covers both inertial measurement and averaging. If 'In yaw motion, the platform rotates
horizontally relative to its orientation' were replaced by 'In yaw motion, the platform rotates
horizontally relative to its smoothed orientation' then my objection abates.
That leaves us with the vexed question about what to do about the current Standard Name platform_orientation. You would like it to cover both undamped compass and gyrocompass orientations. I can see the argument for this within CF philosophy which would dictate a cell method be used to distinguish smoothed and unsmoothed orientations. However, if we do that then the reference to the platform_orientation Standard Name has to be removed from the yaw_angle definition as you can't define against something that is ambiguous.
That would change the yaw_angle definition to:
Platform_yaw_angle is the rotation of a platform in the horizontal plane about its vertical/Z axis. This axis, also known as the "yaw axis", runs vertically through the center of gravity of the platform. In yaw motion, the platform rotates horizontally relative to its smoothed orientation Yaw angle is measured positive clockwise from that smoothed orientation. Standard names for "platform" describe the motion and orientation of the vehicle from which observations are made. Platforms include, but are not limited to, satellites, aeroplanes, ships, instruments and buoys.
and the existing platform_orientation Standard Name can be left asis to cover both smoothed and unsmoothed values.
Cheers, Roy.
I have now retired 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 Nan Galbraith <ngalbraith at whoi.edu>
Sent: 26 July 2018 14:34
To: cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] Platform Heave
Hi all -
Thank you, Alison and all, this is really good. I haven't looked through
all the
current versions of the definitions yet but I hope to do that shortly.
I think the proposed yaw definition may be a bit too detailed; getting into
details like the removal of high frequency variations seems unnecessary
(and
maybe counter productive). Could we consider something simpler? (see below,
please)
Second, would it be possible to turn all these definitions around so
that the
definition of the concept is at the start, and the term platform comes
at the
end? That would make it much more efficient when people are looking for
the standard name at cf-standard-name-table.html.
And last, could we specify that the yaw angle value (though not the motion
itself) is clockwise? A phrase like 'usual sign convention' might lead
to confusion
interpreting the values, or require extra attributes to clarify.
If that's all acceptable, platform_yaw_angle would be:
"platform_yaw_angle is the rotation of a platform in the horizontal
plane about its
vertical/Z axis. This axis, also known as the "yaw axis", runs
vertically through the
center of gravity of the platform. In yaw motion, the platform rotates
horizontally
relative to its orientation, which has the standard name
platform_orientation. Yaw
angle is measured positive clockwise from platform_orientation. Standard
names
for "platform" describe the motion and orientation of the vehicle from
which observations
are made. Platforms include, but are not limited to, satellites,
aeroplanes, ships,
instruments and buoys."
Thanks again -
Nan
On 7/25/18 12:34 PM, Alison Pamment - UKRI STFC wrote:
> Dear Roy and Jim,
>
>
> Thanks again both for your help.
>
>
> Both your replies are saying that referring to direction of motion for measuring yaw is a bad idea, and in any case it doesn't apply to stationary platforms (which presumably have some means of determining their own orientation relative to concepts such as 'up', 'north', etc.) You are both advising against saying 'mean orientation' and I agree that it's not really a well-defined concept.
>
>
> I like Roy's suggested text which refers to the platform's own axis to define yaw. So the full definition of platform_yaw_angle would be:
>
> 'Standard names for "platform" describe the motion and orientation of the vehicle from which observations are made. Platforms include, but are not limited to, satellites, aeroplanes, ships, instruments and buoys. "Yaw" means rotation of the platform in the horizontal plane about its vertical/Z axis. The vertical/Z axis, also known as the "yaw axis", is an imaginary line running vertically through the platform and through its center of gravity. In yaw motion, the platform rotates clockwise or counter clockwise in the horizontal, relative to its orientation, which has the standard name platform_orientation. Platform yaw angle is the angle at a given instant between the platform's longitudinal/X axis and the position of that axis with high frequency variations (e.g. the effect of surface waves on a ship) removed. Zero yaw angle means the longitudinal axis is aligned with the platform_orientation. The usual sign convention is that yaw angle is measured positive when the front or
> leading
> edge of the platform is rotated clockwise from the platform_orientation.'
>
>
> Okay?
>
>
> Like Roy, I had wondered whether 'platform_orientation' should really be an instantaneous quantity or something with high frequency variability removed. If it is the latter (which I think was probably the original intention of the standard name) then we should amend the definition as follows:
>
> 'Standard names for "platform" describe the motion and orientation of the vehicle from which observations are made. Platforms include, but are not limited to, satellites, aeroplanes, ships, instruments and buoys. The platform orientation is the direction in which the "front" or longitudinal axis of the platform is pointing with high frequency variations (e.g. the effect of surface waves on a ship) removed. (This is not necessarily the same as the direction in which the platform is travelling, called platform_course).'
>
>
> Okay?
>
>
> As an additional point, I note that besides the names already discussed in this thread, there are a further 11 existing platform names. I will include the new text for 'platform' in their definitions as part of the August standard names update.
>
>
> Best wishes,
>
> Alison
>
> ________________________________
> From: Lowry, Roy K. <rkl at bodc.ac.uk>
> Sent: 25 July 2018 16:35:27
> To: Pamment, Alison (STFC,RAL,RALSP); cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] Platform Heave
>
>
> Hi again,
>
>
> This is an area where it is easy to get tied up in knots because there are multiple reference frames. If we talk ships then there is the platform_orientation (or heading) which is measured using a gyro-compass - a stabilised instrument that eliminates high-frequency variations in where the bow is actually pointing and provides the zero reference point for yaw.
>
>
> The concept of 'travel' relates to another reference frame external to the platform - say a GPS CRS - but yaw only has relevance to the platform's internal reference frame. So you are right that bringing 'direction of travel' into a definition of yaw is a bad thing even though it's reasonably common practice to do so.
>
>
> Mean orientation is also possibly best avoided as the platform_orientation isn't necessarily determined by averaging instantaneous longitudinal axis orientations. It could be - and often is - measured by something that has greater inertia than the platform.
>
>
> So how about using :
>
>
> Platform yaw angle is the angle at a given instant between the platform's longitudinal/X axis and the position of that axis with high frequency variations (e.g. the effect of surface waves on a ship) removed. Zero yaw angle means the longitudinal axis is aligned with the platform_orientation. The usual sign convention is that yaw angle is measured positive when the front or leading edge of the platform is rotated clockwise from the platform_orientation.
>
>
> This raises the question as to whether the platform_orientation definition should have the clarification 'with high-frequency variability removed' added. This would be an explicit statement of what - to me at least - is commonly understood meaning of 'heading'.
>
>
> Does that help?
>
>
> Cheers, Roy.
>
>
> I have now retired 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 Alison Pamment - UKRI STFC <alison.pamment at stfc.ac.uk>
> Sent: 25 July 2018 14:37
> To: cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] Platform Heave
>
> Hi Roy and Jim,
>
> Thanks for your quick comments on the definitions. I have just been looking again at the suggested text for yaw_angle:
> 'Platform yaw angle is the angle between the platform's longitudinal/X axis and the direction of travel. Zero yaw angle means the longitudinal axis is aligned with the direction of travel, or a reference direction if the platform is stationary. The usual sign convention is that yaw angle is measured positive when the front or leading edge of the platform is rotated clockwise from its orientation (which has the standard name platform_orientation).'
>
> The problem is how to describe the reference direction which the angle is calculated relative to. I started out by talking about 'direction of travel' and later referred to 'platform_orientation'. The definition of platform_orientation says 'The platform orientation is the direction in which the "front" or longitudinal axis of the platform is pointing (not necessarily the same as the direction in which it is travelling, called platform_course).' I've realised my new definition doesn't really make sense if direction of travel and orientation aren't the same (and clearly they can be different). Also, if 'orientation' is the instantaneous direction of the longitudinal axis, then presumably it includes yaw angle, so it isn't the right reference for measuring yaw.
>
> I've revised the text as follows:
> 'Platform yaw angle is the angle between the platform's longitudinal/X axis and the platform's mean orientation (i.e. its orientation not including high frequency variations due to swaying and rocking motions, for example, ship motions caused by the passing of sea surface waves). Zero yaw angle means the longitudinal axis is aligned with the mean orientation. The usual sign convention is that yaw angle is measured positive when the front or leading edge of the platform is rotated clockwise from its mean orientation (which has the standard name platform_orientation).
>
> Does it sound okay to refer to a 'mean orientation' in this way? I'm having trouble thinking of a better wording!
>
> Best wishes,
> Alison
>
> ------
> Alison Pamment Tel: +44 1235 778065
> NCAS/Centre for Environmental Data Archival Email: alison.pamment at stfc.ac.uk
> STFC Rutherford Appleton Laboratory
> R25, 2.22
> Harwell Oxford, Didcot, OX11 0QX, U.K.
>
> -----Original Message-----
> From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu> On Behalf Of Alison Pamment - UKRI STFC
> Sent: 25 July 2018 13:12
> To: Hamilton, Steve <sj.hamilton at fugro.com>; cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] Platform Heave
>
> Dear Steve, Nan, et al,
>
> Thank you for proposing new standard names for platform_heave and improved definitions for existing names for platform pitch, roll and yaw. Thank you also to all those who submitted comments about these names.
>
> Regarding Steve's proposals for new names, the discussion seems to have reached consensus on the quantities themselves.
>
> Until now, our usual explanatory sentence for 'platform' has said 'Standard names for platform describe the motion and orientation of the vehicle from which observations are made e.g. aeroplane, ship or satellite.' Nan has suggested extending the list of possible platforms, which seems fair enough, so we would now have 'Standard names for platform describe the motion and orientation of the vehicle from which observations are made. Platforms include, but are not limited to, satellites, aeroplanes, ships, instruments, and buoys.' I've added this into the definitions of Steve's names, leading to:
>
> platform_heave (m)
> 'Standard names for "platform" describe the motion and orientation of the vehicle from which observations are made. Platforms include, but are not limited to, satellites, aeroplanes, ships, instruments, and buoys. "Heave" means the vertical displacement of a platform (positive upwards) over a measurement time interval.'
>
> platform_heave_rate (m s-1)
> 'Standard names for "platform" describe the motion and orientation of the vehicle from which observations are made. Platforms include, but are not limited to, satellites, aeroplanes, ships, instruments, and buoys "Heave" means the vertical displacement of a platform (positive upwards) over a measurement time interval. "Heave rate" means the rate of change of vertical displacement of the platform over a measurement time interval.'
>
> These two names are accepted for publication in the standard name table and will be added in the next update, planned for 6th August.
>
> We have six existing platform pitch, roll and yaw names:
> platform_pitch_angle (degree)
> platform_pitch_rate (degree s-1)
> platform_roll_angle (degree)
> platform_roll_rate (degree s-1)
> platform_yaw_angle (degree)
> platform_yaw_rate (degree s-1)
>
> Nan has suggested the following definitions, based on https://en.wikipedia.org/wiki/Ship_motions. (A quick search of other online sources yields definitions consistent with these).
Ship motions - Wikipedia<
https://en.wikipedia.org/wiki/Ship_motions>
en.wikipedia.org
Ship motions are defined by the six degrees of freedom that a ship, boat or any other craft can experience.
> Ship motions - Wikipedia<https://en.wikipedia.org/wiki/Ship_motions>
> en.wikipedia.org
> Ship motions are defined by the six degrees of freedom that a ship, boat or any other craft can experience.
>
>
>
> 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.
>
> These are useful and concise definitions. I suggest that we don't refer anywhere to 'ship', 'bow' or 'stern', since we want the definitions to apply to all possible platforms. I'm thinking also that 'port' and 'starboard' may apply to ships and aeroplanes, but perhaps not to a satellite, so are probably best avoided. Similarly, 'waterline' only applies to maritime platforms. I suggest the following amendments to make the definitions as generic as possible:
>
> Pitch
> "Pitch" means rotation of the platform in the vertical plane about its transverse/Y axis. The transverse/Y axis, also known as the "lateral axis" or "pitch axis", is an imaginary line running horizontally across the platform and through its center of gravity. In pitch motion, the leading edge of the platform moves vertically upwards while the rear moves vertically downwards, and vice versa.
>
> Roll
> "Roll" means rotation of the platform in the vertical plane about its longitudinal/X axis. The longitudinal/X axis, also known as the "roll axis", is an imaginary line running horizontally through the length of the platform and through its center of gravity. In roll motion, the platform tilts such that one side moves vertically upwards while the other moves vertically downwards, and vice versa.
>
> Yaw
> "Yaw" means rotation of the platform in the horizontal plane about its vertical/Z axis. The vertical/Z axis, also known as the "yaw axis", is an imaginary line running vertically through the platform and through its center of gravity. In yaw motion, the platform rotates clockwise or counter clockwise in the horizontal, relative to its orientation, which has the standard name platform_orientation.
>
> Are these okay?
>
> For names such as platform_view_angle and platform_zenith_angle we also describe how the angle itself is measured. We should do the same for pitch, roll and yaw angles while we are in the process of updating the definitions. I have come up with the following:
>
> Pitch angle
> Platform pitch angle is the angle between the local horizontal and the platform's longitudinal/X axis. Zero pitch angle means the longitudinal axis is horizontal. The usual sign convention is that pitch angle is measured positive when the front or leading edge of the platform is elevated above the horizontal, negative when it is below the horizontal.
>
> Roll angle
> Platform roll angle is the angle between the local horizontal and the platform's lateral/Y axis. Zero roll angle means the lateral axis is horizontal. The usual sign convention is that roll angle is measured positive when the right hand edge of the platform (when viewing towards the orientation direction or "front" of the platform) is elevated above the horizontal, negative when it is below the horizontal.
>
> Yaw angle
> Platform yaw angle is the angle between the platform's longitudinal/X axis and the direction of travel. Zero yaw angle means the longitudinal axis is aligned with the direction of travel, or a reference direction if the platform is stationary. The usual sign convention is that yaw angle is measured positive when the front or leading edge of the platform is rotated clockwise from its orientation (which has the standard name platform_orientation).
>
> Just so we can see a couple of examples of pulling all this together, I've written out the full revised definitions of platform platform_pitch_angle and platform_pitch_rate below.
>
> platform_pitch_angle (degree)
> 'Standard names for "platform" describe the motion and orientation of the vehicle from which observations are made. Platforms include, but are not limited to, satellites, aeroplanes, ships, instruments and buoys. "Pitch" means rotation of the platform in the vertical plane about its transverse/Y axis. The transverse/Y axis, also known as the "lateral axis" or "pitch axis", is an imaginary line running horizontally across the platform and through its center of gravity. In pitch motion, the leading edge of the platform moves vertically upwards while the rear moves vertically downwards, and vice versa. Platform pitch angle is the angle between the local horizontal and the platform's longitudinal/X axis. Zero pitch angle means the longitudinal axis is horizontal. The usual sign convention is that pitch angle is measured positive when the front or leading edge of the platform is elevated above the horizontal, negative when it is below the horizontal.'
>
> platform_pitch_rate (degree s-1)
> 'Standard names for "platform" describe the motion and orientation of the vehicle from which observations are made. Platforms include, but are not limited to, satellites, aeroplanes, ships, instruments and buoys. "Pitch" means rotation of the platform in the vertical plane about its transverse/Y axis. The transverse/Y axis, also known as the "lateral axis" or "pitch axis", is an imaginary line running horizontally across the platform and through its center of gravity. In pitch motion, the leading edge of the platform moves vertically upwards while the rear moves vertically downwards, and vice versa. The quantity with standard name platform_pitch_rate is the change per unit time in the quantity with standard name platform_pitch_angle.'
>
> The roll and yaw definitions would be constructed similarly.
>
> The pitch/roll/yaw names are still under discussion. I'd welcome further comments on these.
>
> Best wishes,
> Alison
>
> ------
> Alison Pamment Tel: +44 1235 778065
> NCAS/Centre for Environmental Data Archival Email: alison.pamment at stfc.ac.uk
> STFC Rutherford Appleton Laboratory
> R25, 2.22
> Harwell Oxford, Didcot, OX11 0QX, U.K.
>
> -----Original Message-----
> From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu> On Behalf Of Hamilton, Steve
> Sent: 11 July 2018 10:52
> To: cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] Platform Heave
>
> Hi Nan,
>
> I agree expanding on the existing standard name descriptions does make sense and standardising for _rate and _angle
>
> What you suggest below seems acceptable
>
> Thanks
>
> Steve
>
> -----Original Message-----
> From: CF-metadata <cf-metadata-bounces at cgd.ucar.edu> On Behalf Of Nan Galbraith
> Sent: 10 July 2018 17:39
> To: cf-metadata at cgd.ucar.edu
> Subject: Re: [CF-metadata] Platform Heave
>
> 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.
>>
--
*******************************************************
* 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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