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: [email protected] Chilton, Didcot, OX11 0QX, U.K. -- Scanned by iCritical. _______________________________________________ CF-metadata mailing list [email protected] http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
