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 Tue Nov 17 2009 - 03:09:51 GMT
