[CF-metadata] Standard name(s) needed for satellite-based icedrift products

From: Thomas Lavergne <thomasl>
Date: Thu, 06 Nov 2008 10:07:42 +0000

Hi Alison and all,

Pamment, JA (Alison) wrote:
> Not all standard names proposals lead to a great deal of discussion. I
> think that your names are fairly straightforward and certainly nobody
> has objected to them. A very important part of including new names in
> the table is to make sure the quantities are clearly defined. Please
> see below for my suggested wording of the descriptions based on the
> short definitions you have already provided. Please let me know if you
> are happy with these or whether any clarifications are needed.
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 :-) )

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.
> 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."
> "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."

> 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.

>
> 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."
> 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
> 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."

>
> 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.


REWRITING THE DEFINITIONS :
eastward_sea_ice_displacement [m]
'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 in the file, for example
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.
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. 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.
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. 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, 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.
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. 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, 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.
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. 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.
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. 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.

Cheers,
Thomas
Received on Thu Nov 06 2008 - 03:07:42 GMT