Dear Alison,
Thank you for posting this old subject again. I re-read through the names, definitions and units and do not wish to modify them as they suit my needs and, I think, those of my community. As you noted, they are built upon the existing 'sea_ice_velocity' standard names, which makes it more easy to accept them, I guess.
As far as the other "open discussion" items you are answering at the end of this email, they were merely to bring the discussion forward and it is correct that some of the answers to my questions are now found in other threads, I do not think they should continue in this context.
As far as I am concerned, and unless someone comes with strong opinion against them, those names, definitions and units are suitable and I am happy if you close the subject.
Cheers,
Thomas Lavergne,
Research Scientist, met.no, Oslo, Norway
----- "alison pamment" <alison.pamment at stfc.ac.uk> wrote:
> Dear Thomas,
>
> Thank you for your email asking about the status of these names.
> Looking back at your last posting, I think we had essentially arrived
> at
> agreement on the following names, units and definitions:
>
> 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.
>
> However, you also raised a number of questions which could affect the
> final form of the names and I've tried to address these below.
>
> > My displacement is nothing more that a
> 'change_over_time_in_geoposition'. Is there actually a quantity X for
> 'geoposition' (which would be lat/lon)?> I saw that there was a
> reference to 'position' in
> magnitude_of_derivative_of_position_wrt_model_level_number where it is
> a
> 3D position.
> > Displacements can be 3D so 'displacement' is maybe nothing more than
> a
> 'change_over_time_in_position', with 'position' being a 3D point. I
> do
> not
> > know if we will win anything in recognizing the aliasing but it
> might
> help understanding how to build the standard_name as well as the time
> bounds
> > thing. Plus, I did not find the definition of what
> 'change_over_y_in_x' describes but it is clear that we cannot use it
> for
> displacement if the
> > 'change' is intended as being 'continuous', like dx/dy = constant
> for
> all y.
>
> There is no standard name for 'geoposition'. Indeed, recently there
> was
> a discussion thread entitled "standard name for position" (see
> http://mailman.cgd.ucar.edu/pipermail/cf-metadata/2009/003011.html
> and
> followups) in which this same issue arose. The general conclusion
> was
> that "position" or "geoposition" is already given by the values of
> variables with the standard names "latitude" and "longitude" and
> there
> was not a strong case for introducing a separate name for the
> combination of coordinates.
>
> 'Change_over_y_in_x' currently only appears in standard names for the
> case where y is time, but I dare say it could be defined for other
> situations if it became necessary. 'Change_over_time_in_X' is
> defined
> as 'change in a quantity X over a time-interval, which should be
> defined
> by the bounds of the time coordinate' and doesn't impose any
> condition
> about the change being continuous. However, if we were to change to
> your
> alternative suggestion of 'change_over_time_in_position', rather than
> 'displacement', I think your names actually become more difficult to
> express. For example, initially I considered that
> eastward_sea_ice_displacement might become
> change_over_time_in_sea_ice_longitude, but that would have units of
> degrees instead of metres which isn't what you are after. If we used
> something like change_over_time_in_sea_ice_eastward_position I think
> that it still isn't obviously a quantity in metres (although
> change_over_time_in_sea_ice_x|y_position might be OK for
> displacements
> on your map projection). I think the point is that we need to get
> across the idea of moving through a distance measured in units of
> length, rather than a change in coordinate with its associated units,
> and 'displacement' is a concise way of doing that.
>
> Names such as eastward_sea_ice_displacement are consistent with
> existing
> names like eastward_sea_ice_velocity and names proposed for the CMIP5
> model data, such as eastward_sea_ice_transport, which I think is
> another
> argument for sticking with the names we had agreed.
>
> Importantly, I don't think the choice of standard name for the
> displacements really affects your treatment of the time coordinate
> variable and time bounds, which is really a separate question. For
> this
> and all the other reasons, I think we should stick with the agreed
> names.
>
> >
> > I would however like to discuss here the time bounds thing and see
> if
> > someone can help me
> > understanding how/if I can apply the concept to my data model.
> >
> > I think we agree that :
> > 1) the time interval is crucial for the displacement variable to be
> > interpreted;
> > 2) that time bounds is the current implementation of an interval. If
> I
> > want CF compliance, I should
> > live with this fact.
> >
>
> I note that you have sought further discussion of the time coordinate
> and time_bounds question in the "Cell bounds associated with
> coordinate
> variable rather than data variable" thread, so I have not discussed
> it
> further here. I think discussing the issues separately is probably
> the
> best way to proceed.
>
> Best wishes,
> Alison
>
> ------
> 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 Wed Nov 18 2009 - 09:33:39 GMT
