Hi all -
I like Ken K's definitions; simple and to the point;? And I agree that
we need to be explicit
about positive direction (which term, by the way, doesn't google well).
I'm not sure I agree with Roy that 'zero is platform at rest' - could
you please explain
that? By 'at rest' do we mean that no forces are operating on the
platform? Would? a buoy
have roll and heave angles if its deck load were out of balance?? If its
deck isn't level, what
term would describe that? I suppose you're building on the fact that
these rotations are
relative to the internal axes of the platform, with no relation to the
'real world'.
It does seem like we're converging ... maybe.
Cheers - Nan
-
On 7/29/18 7:29 AM, Lowry, Roy K. wrote:
> > Dear All, > > > Giving it some thought over the weekend I realise
that where we lost > the plot in this discussion was when we encountered
'direction of > travel'. Jim succinctly described platform motion with
the phrase > 'nested co-ordinate systems'. What I failed to realise -
and I'm > guessing I'm not alone - is that the pitch, roll, heave etc.
family > of terms for platform motion refer SOLELY to the innermost >
co-ordinate reference in that nest and that the 'zero' for these >
measurements is 'platform at rest'. This innermost co-ordinate >
reference comprises three orthogonal axes that intersect at the >
platform's centre of gravity. Two of these are horizontal (Ken's >
longitudinal X-axis and transverse Y-axis) and the third vertical >
(Ken's vertical Z-axis). Others make no attempt to treat these >
parameters in the same way as zenith, and I now realise CF shouldn't >
be any different. > > > Having come to terms with this, Ken's definition
elements have a > beautiful simplicity that can be slotted into Alison's
compound > definitions. My only problem is the inclusion of nautical
terms like > 'bow' and 'stern', but these can easily be replaced by
generic > equivalents such as 'front' and 'back'. I would also make it
clearer > is that zero is platform at rest. > > > For example the
definition pair for yaw become: > > > platform_yaw_angle > > "yaw
_angle" is the amount of rotation from the rest position around > the
vertical Z-axis with positive values resulting in clockwise > motion
when viewed from above. The vertical Z axis, also known as the > "yaw
axis", is an imaginary line running vertically through the > platform's
centre of gravity. 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. > > > platform_yaw_rate > >
"platform_yaw_rate" is the change per unit time of "yaw_angle". "yaw >
_angle" is the amount of rotation from the rest position around the >
vertical Z-axis with positive values resulting in clockwise motion >
when viewed from above. The vertical Z axis, also known as the "yaw >
axis", is an imaginary line running vertically through the platform's >
centre of gravity. 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. > > > How does that work for people? > > >
Cheers, Roy. > > > -------------------------
> From: CF-metadata on behalf of Kenneth Kehoe <kkehoe at ou.edu> > Sent: 27 July 2018 16:49 > Subject: Re: [CF-metadata] Platform Heave
> > All, > > Sorry for joining this conversation late. This is an
important > discussion for my group and finding a resolution would be
very > helpful. For my purposes I only need a good definition, which
might > coincide with the nautical definitions. For example this
reference > <
https://www.wartsila.com/encyclopedia/term/ship-motions>
would > suffice for most of my needs except for the missing definition
of > positive direction. I've asked about defining a positive direction
in > the past using the "positive" attribute and it was decided to not >
expand that attribute. If we can define the positive direction in all >
the platform standard names that would be great, but it should be >
universally existent for all current and future platform > definitions.
> > I would prefer to not get into the details of how a value is
derived > in the definition as that is more of the cell_methods domain.
Also I > find that confusing as heave on an ocean going ship is not
always > measured as the difference between two GPS points but could be
an > integration of a speed or acceleration. This would result in two >
different measurements that depend on the method, but both are >
correct. > > I next attempted to come up with some definitions but I
ended up > going down a wormhole of different reference frames only to
realize > in the end my definitions will never match with the values
from the > vendor supplied data values because my definitions were
becoming too > specific. I can't find a definition of heave that takes
into account > tides, large waves (water or atmospheric), or orientation
of the > platform. They all seem to be relative to the platform position
some > x time ago. So here is my attempt at the definitions: > >
platform_heave = Heave is the linear motion along the vertical > Z-axis
(e.g. keel to top of mast) with positive values representing > upward
motion. > > platform_sway = Sway is the motion along the transverse
Y-axis (e.g. > port to starboard) with positive values towards the
right-hand side > the platform (starboard) when oriented towards leading
edge of the > platform. > > platform_surge = Surge is the motion along
the longitudinal X-axis > (e.g. stern to bow) with positive values
indicating motion towards > the leading edge of the platform (bow). > >
platform_roll_angle = Roll is a rotation around a longitudinal X-axis >
with positive values resulting in counter clockwise motion (e.g. >
right-hand side rising) when oriented towards leading edge of the >
platform. > > platform_pitch_angle = Pitch is a rotation around the
transverse > Y-axis with positive values resulting in counter clockwise
motion > (e.g. leading edge of the platform rising). > >
platform_yaw_angle = Yaw is a rotation around the vertical Z-axis > with
positive values resulting in clockwise motion of the forward > section
(bow) when viewed from above. > > platform_roll_rate = Roll rate is
rotation change per unit time > around a longitudinal X-axis with
positive values resulting in > counter clockwise motion (e.g. right-hand
side rising) when oriented > towards leading edge of the platform. > >
platform_pitch_rate = Pitch rate is rotation change per unit time >
around the transverse Y-axis with positive values resulting in > counter
clockwise motion (e.g. leading edge of the platform rising). > >
platform_yaw_rate = Yaw rate is rotation change per unit time around >
the vertical Z-axis with positive values resulting in clockwise > motion
of the forward section (bow) when viewed from above. > > I know these
differ from the current definitions, but I'm not > completely
understanding how the definitions are created. Is > platform_orientation
always prepended? Is a rate always defined with > the same as the angle
definition but with a final sentence explaining > it's actually a rate?
> > Thanks, > > Ken > > > > On 2018-7-25 09:50, Jim Biard wrote: >> >>
Alison, >> >> It's a lovely nested reference frames problem, isn't it?
Roll, >> pitch, and yaw are usually defined relative to a center of
motion >> (CM) reference frame defined using the (mean) direction of
motion >> and the up direction. In my (satellite-based) experience, the
Y >> axis unit vector is defined by the normalized cross-product of the
>> up unit vector with the direction of motion unit vector (Z x X). >>
The X axis unit vector is then defined by the cross-product of the >> Y
unit vector and the up unit vector (Y x Z). This means of forming >> the
CM reference frame decouples orientation from motion. The X >> axis is
not necessarily identical to the direction of motion. The >> vehicle
reference frame may have fixed offsets in x, y, z, roll, >> pitch, and
yaw relative to the CM reference frame, but in my >> limited experience
those offsets have been zero. >> >> Platforms that aren't moving are an
even more entertaining case, >> for sure! >> >> In the end, I'd tend
towards referring to a CM or geospatial >> reference frame with the Z
direction defined as "up" if I'm going >> to try and get detailed about
it, as opposed to 'mean orientation'. >> But I only have experience with
satellites (and a bit with >> airplanes). >> >> Grace and peace, >> >>
Jim >> >> >> On 7/25/18 9:37 AM, Alison Pamment - UKRI STFC wrote: >>>
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 >>> >>> >>> >>> -----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). 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: 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> >>>> 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> <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. >>>> Ship motions -
Wikipedia >>>> <https://en.wikipedia.org/wiki/Ship_motions> ... >>>>
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 Fri, May 25, 2018 at 7:28 >>>> AM, Lowry, Roy K. <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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* Nan Galbraith??????? Information Systems Specialist *
* Upper Ocean Processes Group??????????? Mail Stop 29 *
* Woods Hole Oceanographic Institution?? *
* Woods Hole, MA 02543??? (508) 289-2444 *
*******************************************************
Received on Mon Jul 30 2018 - 11:12:09 BST
