Hi Jim,
If you can make clear suggestions that make the existing Standard Names more widely applicable without the need to rework legacy data that have been labelled with these Standard Names then that is obviously beneficial. However, please do this by quoting concrete alternative definitions so that we can all understand where you are proposing we go from where we currently are.
BTW if you look at my last reply to Nan then you'll see that the wisdom of coupling the platform_orientation Standard Name into the definition of yaw has already been questioned.
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 Jim Biard <jbiard at cicsnc.org>
Sent: 26 July 2018 20:05
To: cf-metadata at cgd.ucar.edu
Subject: Re: [CF-metadata] Platform Heave
Roy,
I'm just trying to lay out the scope of the challenge to see if we can avoid defining terms in ways that require us to have ship_yaw as a different term than satellite_yaw, for example. I appreciate that the marine community has a long history (longer than the aerospace communities) with these terms. All the same, it seems to me that we should be able to define basic terms such as pitch, roll, and yaw (and maybe surge, sway, and heave as well) in terms that are usable by all. As an example, using platform_orientation in the definition of platform_yaw renders the term quasi-meaningless for platforms that require more than one quantity to define the transform between the platform body reference system and its motion reference system. Using platform_course in these definitions has a similar limiting effect for platforms that require more and/or different terms to define their motion reference frames.
Maybe it's not possible to find a happy medium, but I'm hoping to do so. I'll suggest some standard name definitions of my own shortly.
Grace and peace,
Jim
On 7/26/18 2:46 PM, Lowry, Roy K. wrote:
Dear Jim,
I think the problem is that the 'platform' standard names have been developed by a community versed in the ship/aircraft/buoy and to a lesser extent submarine/glider use cases (i.e. those with which you are least familiar) and we're now talking as if they should be totally generic.
The one thing I understand in your e-mail (most of it is gobbledy-gook to me as an observational oceanographer) is that there are different kinds of platforms with different degrees of freedom and their own terminology. For example, pitch, roll and yaw are not (hopefully!) concepts applicable to a fixed oil rig.
These data streams have been returned by our primary platforms (research vessels) for decades - e.g. heading that CF decided to name platform_orientation - has been in every research vessel navigation file that I've handled from the late 1980s to around 2005. There is usually also speed through the water, latitiude, longitude, speed made good (speed over the seabed), course made good, velocity north over seabed and velocity east over seabed. Some also include pitch, roll and yaw. You cannot suddenly decide that the data of decades should be transformed into an idealised generic co-ordinate reference system for CF.
My view is that realistically an all-encompassing, totally generic solution should be confined to the 'too hard' basket and that we should define the standard names based on use cases of known data streams. Consideration then would then need to be given to data streams outside the original scope as to whether the existing Standard Names are appropriate or new Standard Names are required.
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><mailto:cf-metadata-bounces at cgd.ucar.edu> on behalf of Jim Biard <jbiard at cicsnc.org><mailto:jbiard at cicsnc.org>
Sent: 26 July 2018 18:25
To: cf-metadata at cgd.ucar.edu<mailto:cf-metadata at cgd.ucar.edu>
Subject: Re: [CF-metadata] Platform Heave
Alison, Roy,
I don't mean to be difficult, but I think that for greatest generality the "motion and orientation" of a platform pretty much has to be relative to a reference frame that depends only on the translational motion (if any) of the platform. When I look more closely at the definitions we have now, I also see that platform_course and platform_orientation are insufficient for the satellite case, and don't quite cover the airborne case. The full set of elements needed to define a point on a platform in its internal frame in terms of an earth-based reference frame are:
* Oim = Vector that describes the baseline offset of the origin of the platform's internal reference frame relative to the platform's motion frame.
* Mim = Matrix that describes the baseline rotation of the platform's internal reference frame relative to the platform's motion frame.
* Mrpy = Matrix that describes non-baseline roll, pitch, and yaw of the platform relative to the platform's motion frame.
* Ossh = Vector that describes non-baseline surge, sway, and heave of the platform relative to the platform's motion frame.
* Ome = Vector that describes the offset of the origin of the platform's motion frame relative to an earth-based reference frame.
* Mme = Matrix that describes the rotation of the platform's motion frame relative to an earth-based reference frame.
The motion frame is fixed to the baseline platform center of mass. So the full equation is:
Pe = Ome + Mme(Ossh + Mrpy(Oim + Mim(Pi)))
where Pi is a point on the platform in its internal reference frame and Pe is a point on the platform in an earth-based reference frame. There's a total of 18 terms (3 per vector or matrix) that may be needed to define the platform-to-earth transformation. Oim and Mim are almost always static, so that leaves up to 12 dynamic terms needed to define the others.
Here's a stab at how these vary for different kinds of platforms.
Satellites:
Mim sometimes includes a 180 degree baseline roll angle and a possible 90 degree baseline yaw. Ossh is usually merged with Ome. Mme is defined using the satellite's velocity vector in the earth reference frame to produce a reference frame that has its Z unit vector pointing away from the earth, its X unit vector tangent to the orbit, and its Y unit vector transverse to the orbit at any given moment. This gives us a minimum of 9 dynamic terms (roll, pitch, yaw, Vx, Vy, Vz, X, Y, Z) and a Mim matrix that can't be defined by a single platform_orientation angle.
Airplanes (& submarines?):
I don't have as much experience with this one. In my limited experience: Mim might require up to three angles. Ome is defined using X/Y/altitude. Mme is defined using the heading angle to produce a reference frame that has its Z unit vector pointing away from the earth's surface, its Y and X unit vectors parallel to the earth's surface, and its X unit vector pointing in the direction of the platform_course at any given moment. This gives us a minimum of 10 dynamic terms (roll, pitch, yaw, surge, sway, heave, X, Y, altitude, course), and Mim may not be definable using a single platform_orientation angle.
Ships (& land vehicles (& drifters?)):
I have no experience with ships. Based on what I've read in here and read online: Ome is defined using X/Y/(sea level). Mme is defined as it is in the airplane case. This gives us a minimum of 9 dynamic terms (roll, pitch, yaw, surge, sway, heave, X, Y, platform_course), and Mim is defined using the single platform_orientation angle.
Buoys:
No experience here either. But it appears to be: Ome is defined using a fixed X/Y/(sea level). Mme is defined (I guess?) with East, North, and Up as its X, Y, and Z unit vectors. This gives us a minimum of 6 dynamic terms (roll, pitch, yaw, surge, sway, heave), and Mim is defined using the single platform_orientation angle. At least that's my guess.
All that to point out that different kinds of platforms use different terms and have different degrees of freedom. We need to cover all these cases if we want to be both precise and general.
Grace and peace,
Jim
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><mailto:rkl at bodc.ac.uk>
Sent: 25 July 2018 16:35:27
To: Pamment, Alison (STFC,RAL,RALSP); cf-metadata at cgd.ucar.edu<mailto: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><mailto:cf-metadata-bounces at cgd.ucar.edu> on behalf of Alison Pamment - UKRI STFC <alison.pamment at stfc.ac.uk><mailto:alison.pamment at stfc.ac.uk>
Sent: 25 July 2018 14:37
To: cf-metadata at cgd.ucar.edu<mailto: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<mailto: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><mailto: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><mailto:sj.hamilton at fugro.com>; cf-metadata at cgd.ucar.edu<mailto: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><
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<mailto: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><mailto: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<mailto: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><mailto: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<mailto: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><mailto: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><mailto: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><mailto: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><mailto: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><mailto: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.
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From: John Helly <hellyj at ucsd.edu><mailto: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><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><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><mailto: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.
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From: Nan Galbraith <ngalbraith at whoi.edu><mailto: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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[CICS-NC] <http://www.cicsnc.org/> Visit us on
Facebook <http://www.facebook.com/cicsnc> Jim Biard
Research Scholar
Cooperative Institute for Climate and Satellites NC <http://cicsnc.org/>
North Carolina State University <http://ncsu.edu/>
NOAA National Centers for Environmental Information <http://ncdc.noaa.gov/>
formerly NOAA's National Climatic Data Center
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e: jbiard at cicsnc.org<mailto:jbiard at cicsnc.org>
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Connect with us on Facebook for climate<https://www.facebook.com/NOAANCEIclimate> and ocean and geophysics<https://www.facebook.com/NOAANCEIoceangeo> information, and follow us on Twitter at _at_NOAANCEIclimate<https://twitter.com/NOAANCEIclimate> and @NOAANCEIocngeo<https://twitter.com/NOAANCEIocngeo>.
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