Dear Thomas,
> I think I am mostly happy with those definitions and am absolutely ok
> to wait until next release
> (January 09) before having those names in the standard table. It will
> give me the opportunity to 1)
> work on those definitions with the CF list and 2) work on those
> definitions with my satellite
> ice-drift colleagues which (I guess) are not participating to this
> list? (that's a call :-) )
Thanks for looking at the definitions. I am happy to iterate them on
the list and if you could also get some input from your own scientific
community that would be great. Please see below for my comments on the
definitions.
>
> As each of your definition is more or less based on the same scheme, I
> will try to analyse/comment
> the first definition and then refer to it for the others. At the end
of
> the mail, I will rewrite
> definitions which include my proposals for the sake of clarity.
> Of course these are only proposals and should be discussed and
> disagreed upon. I especially hope we
> can discuss the best way to describe the time interval, as 'bounds'
> might not be the easiest
> solution to implement in my case.
>
> > eastward_sea_ice_displacement [m]
> > 'Displacement' means the change in geospatial position of an object
> that
> > has moved over time.
> I agree.
> > The time interval during which the motion took place must be
> specified
> > using a bounds variable for the time coordinate variable.
> I don't agree. For 2 reasons :
> a) is it the role of the table of standard names to define how a CF
> file should be formatted? I
> thought it was the role of the main CF-1.x document?
> b) shouldn't we be less affirmative like:
> "The time interval during which the motion took place must be
specified
> in the file, for example
> using a bounds variable for the time coordinate variable."
> This would be a more flexible definition which could be adapted
when/if
> CF decides on an other
> standard way of representing time in the future.
To take your two points in order:
a) The purpose of including the text about the bounds variable is to
provide a 'handy hint' for data providers and users on how to use these
standard names in conjunction with other CF attributes to obtain a
complete description of the data. This is not an uncommon thing to do
in the standard name table because often the standard name on its own
does not completely describe the data. For example, the definition of
the standard name
change_over_time_in_atmospheric_water_content_due_to_advection reads:
"The specification of a physical process by the phrase due_to_process
means that the quantity named is a single term in a sum of terms which
together compose the general quantity named by omitting the phrase.
'change_over_time_in_X' means change in a quantity X over a
time-interval, which should be defined by the bounds of the time
coordinate. 'Content' indicates a quantity per unit area. 'Water' means
water in all phases."
If you were trying to work with real data 'change_over_time' would be
difficult to interpret unless you also knew the time interval involved.
If I have understood your names correctly I think the same would be true
for 'displacement' so there is a need to include the text about time
bounds.
b) At present I think the only way in the CF conventions of describing
the start and end of a time interval is to use a bounds variable for the
time axis. (I hope someone will correct me if I am wrong). It is one
of the fundamental philosophies of CF development that the conventions
are only modified in response to current needs, rather than trying to
foresee all possible future requirements. (The same is true of standard
names - we don't add them unless somebody has an actual need for them).
If the CF conventions on specifying time intervals were to be modified
in future we would need also to modify that part of all the standard
name definitions which explains how to specify the time interval.
I note that you say that 'bounds' may not be the easiest solution for
you to implement. Please could you explain a bit more about why this
is? If your start and end times are not clearly defined then perhaps we
could cope with that using the cell_methods attribute applied to a
standard_name of time, rather than to the time coordinate variable
itself. I realise that my last sentence may seem rather cryptic -
please see section 7.3 of the conventions, particularly the text
immediately following example 7.5 for a fuller explanation.
I think we could soften my original text by inserting 'if possible', and
taking into account your later comment about the use of 'during', I now
suggest: "If possible, the time interval over which the motion took
place should be specified using a bounds variable for the time
coordinate variable." Does that sound better?
> > A displacement is a vector quantity. Sea ice displacement is
> > defined as a two-dimensional vector, with no vertical component.
> I mostly agree. But would like to be more specific:
> "A displacement can be represented as a vector. Such a vector should
> however not be interpreted as
> describing a rectilinear, constant speed motion but merely as an
> indication that the start point of
> the vector is found at the tip of the vector after the time interval
> associated with the
> displacement variable. A displacement does not prescribe a
trajectory.
> Sea ice displacement can be
> defined as a two-dimensional vector, with no vertical component."
That sounds good to me.
> > "Eastward" indicates a vector component which is positive when
> directed
> > eastward (negative westward).
> OK
> > An eastward displacement is the distance across the earth's surface
> calculated
> > from the change in a moving object's longitude
> OK
> > during the interval given in the time bounds variable.
> Not OK. For the same reasons as above. A "time bounds variable" might
> not be the best and only way
> to give the information. Besides the 'during' might be interpreted as
> 'describing a motion
> occurring' (its trajectory) which is not what we want. I agree more
> with the sentence you used for
> the x_displacement. I thus propose:
> "[from the change in a moving object's longitude] between the start
and
> end of the time interval
> associated
> with the displacement variable."
I have already explained why I think we need to refer to the bounds
variable at some point in the definition. However, in this instance I
think your wording works just as well as mine and I take your point
about the use of 'during'. I'm happy to use your wording here.
>
> > northward_sea_ice_displacement [m]
> See above for introducing sentences.
> > "Northward" indicates a vector component which is positive when
> directed
> > northward (negative southward). A northward displacement is the
> > distance across the earth's surface calculated from the change in a
> > moving object's latitude
> OK
> > during the interval given in the time bounds variable.
> Not ok, same as above.
My comments are the same as for eastward_sea_ice_displacement.
>
> >
> > sea_ice_x_displacement [m]
> see above for introducing sentences.
> > "x" indicates a vector component along the grid x-axis, when this is
> not
> > true longitude, positive with increasing x.
> I do not really get this 'when this is not true longitude'. I see what
> it means (that the 'x'
> component is actually the 'eastward' component on a
> lat/lon-earth-mapping-whose-north-is-upward-and-east-is-towards-right)
> but am not sure we need to
> keep this. Does it mean I should not use a sea_ice_x_displacement when
> I am on the above cited earth
> mapping? That would not be fair. I would write:
> "'x' indicates a vector component along the grid x-axis positive with
> increasing x."
The 'when this is not true longitude' phrase appears in all standard
name definitions referring to x_components. See, for example, the
definition of the standard name projection_x_coordinate: " 'x' indicates
a vector component along the grid x-axis, when this is not true
longitude, positive with increasing x. Projection coordinates are
distances in the x- and y-directions on a plane onto which the surface
of the Earth has been projected according to a map projection. The
relationship between the projection coordinates and latitude and
longitude is described by the grid_mapping." Corresponding names and
definitions exist for y_components.
In CF, if the horizontal coordinates of the data are true longitude and
latitude (i.e., lines of longitude converge at the earth's geographical
poles and lines of latitude are orthogonal to that) then it is
appropriate to use eastward|northward standard names to refer to
components of motion. If, however, your data are on a rotated pole grid
or a planar map projection you should use x|y standard names. Does this
answer your question? Section 5.6 of the conventions document
"Horizontal Coordinate Reference Systems, Grid Mappings, and
Projections" gives more information.
I would prefer that we keep this sentence as it is in view of the
distinction that CF makes between (lon,lat) and (x,y) coordinates. Also
for consistency with other standard name definitions. Do you agree?
> > An x displacement is
> > calculated from the difference in the moving object's grid x
> coordinate
> OK
> > between the start and end of the interval given in the time bounds
> > variable.
> not OK, same as above.
>
> > sea_ice_y_displacement [m]
> see above for introducing sentences.
> > "y" indicates a vector component along the grid x-axis,
> should be y-axis here (as already noted by Philip Cameron-Smith)
> > when this is not true longitude,
> should not it be 'latitude' here? but I would anyway propose to remove
> this sentence altogether
As above, I would prefer to keep the sentence. You and Philip are both
correct - I meant y-axis and latitude. Copy-and-paste isn't always a
good thing!
> > positive with increasing y. A y displacement is
> > calculated from the difference in the moving object's grid y
> coordinate
> OK.
> > between the start and end of the interval given in the time bounds
> > variable.
> not OK, same as above.
> >
> > sea_ice_displacement
> > 'Displacement' means the change in geospatial position of an object
> that
> > has moved over time. The time interval during which the motion took
> > place must be specified using a bounds variable for the time
> coordinate
> > variable. The displacement is the distance across the earth's
surface
> > calculated from the change in a moving object's (lon,lat) position
> > between the start and end of the interval given in the time bounds
> > variable. A displacement is a vector quantity. Sea ice
displacement
> is
> > defined as a two-dimensional vector, with no vertical component.
> There might have been a mis-understanding here. This might be the
proof
> that the standard name is
> not good enough :-). I wanted to use 'sea_ice_displacement' to express
> the length of the vector
> (unit [m]), just like sea_ice_velocity expresses the magnitude of the
> speed (unit [m/s]). So it
> would read:
> sea_ice_displacement [m]
> [The introcuding sentences]
> In that case, "displacement" is also the distance across the earth's
> surface calculated from the
> change in a moving object's (lon,lat) position between the start and
> end of the time interval associated
> with the displacement variable."
>
OK. I think I had understood the name correctly, but perhaps I hadn't
conveyed it well enough! I am happy with your wording.
> >
> > direction_of_sea_ice_displacement [degrees]
> [introducing sentences].
> > "direction_of_X" means direction of a vector, a bearing. The
> 'direction
> > of displacement' is the angle between due north and the displacement
> > vector.
>
>
So now the full definitions would read as follows:
eastward_sea_ice_displacement [m]
'Displacement' means the change in geospatial position of an object that
has moved over time. If possible, the time interval over which the
motion took place should be specified using a bounds variable for the
time coordinate variable. A displacement can be represented as a
vector. Such a vector should however not be interpreted as describing a
rectilinear, constant speed motion but merely as an indication that the
start point of the vector is found at the tip of the vector after the
time interval associated with the displacement variable. A displacement
does not prescribe a trajectory. Sea ice displacement can be defined as
a two-dimensional vector, with no vertical component. "Eastward"
indicates a vector component which is positive when directed eastward
(negative westward). An eastward displacement is the distance across the
earth's surface calculated from the change in a moving object's
longitude between the start and end of the time interval associated with
the displacement variable.
northward_sea_ice_displacement [m]
'Displacement' means the change in geospatial position of an object that
has moved over time. If possible, the time interval over which the
motion took place should be specified using a bounds variable for the
time coordinate variable. A displacement can be represented as a
vector. Such a vector should however not be interpreted as describing a
rectilinear, constant speed motion but merely as an indication that the
start point of the vector is found at the tip of the vector after the
time interval associated with the displacement variable. A displacement
does not prescribe a trajectory. Sea ice displacement can be defined as
a two-dimensional vector, with no vertical component. "Northward"
indicates a vector component which is positive when directed northward
(negative southward). A northward displacement is the distance across
the earth's surface calculated from the change in a moving object's
latitude between the start and end of the time interval associated with
the displacement variable.
sea_ice_x_displacement [m]
'Displacement' means the change in geospatial position of an object that
has moved over time. If possible, the time interval over which the
motion took place should be specified using a bounds variable for the
time coordinate variable. A displacement can be represented as a
vector. Such a vector should however not be interpreted as describing a
rectilinear, constant speed motion but merely as an indication that the
start point of the vector is found at the tip of the vector after the
time interval associated with the displacement variable. A displacement
does not prescribe a trajectory. Sea ice displacement can be defined as
a two-dimensional vector, with no vertical component. "x" indicates a
vector component along the grid x-axis, when this is not true longitude,
positive with increasing x. An x displacement is calculated from the
difference in the moving object's grid x coordinate between the start
and end of the time interval associated with the displacement variable.
sea_ice_y_displacement [m]
'Displacement' means the change in geospatial position of an object that
has moved over time. If possible, the time interval over which the
motion took place should be specified using a bounds variable for the
time coordinate variable. A displacement can be represented as a
vector. Such a vector should however not be interpreted as describing a
rectilinear, constant speed motion but merely as an indication that the
start point of the vector is found at the tip of the vector after the
time interval associated with the displacement variable. A displacement
does not prescribe a trajectory. Sea ice displacement can be defined as
a two-dimensional vector, with no vertical component. "y" indicates a
vector component along the grid y-axis, when this is not true latitude,
positive with increasing y. A y displacement is calculated from the
difference in the moving object's grid y coordinate between the start
and end of the time interval associated with the displacement variable.
sea_ice_displacement [m]
'Displacement' means the change in geospatial position of an object that
has moved over time. If possible, the time interval over which the
motion took place should be specified using a bounds variable for the
time coordinate variable. A displacement can be represented as a
vector. Such a vector should however not be interpreted as describing a
rectilinear, constant speed motion but merely as an indication that the
start point of the vector is found at the tip of the vector after the
time interval associated with the displacement variable. A displacement
does not prescribe a trajectory. Sea ice displacement can be defined as
a two-dimensional vector, with no vertical component. In that case,
"displacement" is also the distance across the earth's surface
calculated from the change in a moving object's geospatial position
between the start and end of the time interval associated with the
displacement variable.
direction_of_sea_ice_displacement [degrees]
'Displacement' means the change in geospatial position of an object that
has moved over time. If possible, the time interval over which the
motion took place should be specified using a bounds variable for the
time coordinate variable. A displacement can be represented as a
vector. Such a vector should however not be interpreted as describing a
rectilinear, constant speed motion but merely as an indication that the
start point of the vector is found at the tip of the vector after the
time interval associated with the displacement variable. A displacement
does not prescribe a trajectory. Sea ice displacement can be defined as
a two-dimensional vector, with no vertical component. "direction_of_X"
means direction of a vector, a bearing. The
'direction of displacement' is the angle between due north and the
displacement vector.
Best wishes,
Alison
==> Please note new email address: alison.pamment at stfc.ac.uk <==
------
J Alison Pamment Tel: +44 1235 778065
NCAS/British Atmospheric Data Centre Fax: +44 1235 446314
Rutherford Appleton Laboratory Email: alison.pamment at stfc.ac.uk
Chilton, Didcot, OX11 0QX, U.K.
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Received on Fri Nov 07 2008 - 06:51:33 GMT
