[CF-metadata] axis attribute

Sebastien Villaume sebastien.villaume at ecmwf.int
Wed Apr 19 14:26:20 MDT 2017


Hi David,

I am still missing an attribute attached to "latitude" and "longitude" to bind them with their respective axis/spatial dimension.
The "link" is still implicit if nothing else than what you wrote is added. I also feel that the dimension and axis attributes are somehow redundant.

I also find your long_name attribute misleading because all the values in latitude(j,i) goes on the Y axis and all the values in longitude(j,i) goes on the X axis! i and j are only the indices of the horizontal grid. Your long_name suggests something else.

Is it ok if I add in your example an "axis_mapping" attribute to lat and lon:

dimensions:
    i = 100 ;
    j = 200 ;
variables:
    char x:
        x:dimension = "i" ;
        x:axis = "X" ;
        x:standard_name = "???" ;
    char y: 
        y:dimension = "j" ;
        y:axis = "Y" ;
        y:standard_name = "???" ;
    double longitude(j, i) ;
        longitude:units = "degrees_east" ;
        longitude:axis_mapping = x ;
    double latitude(j, i) ;
        latitude:units = "degrees_north" ;
        latitude:axis_mapping = y  ;
    double tas(j, i) ;
        tas:standard_name = "air_temperature" ;
        tas:units = "K" ;
        tas:coordinates = "longitude latitude" ;
        tas:axes = "x y" ;  // new data variable attribute which points to dimension variables



/Sébastien

----- Original Message -----
From: "David Hassell" <david.hassell at ncas.ac.uk>
To: "Sebastien Villaume" <sebastien.villaume at ecmwf.int>
Cc: "Jim Biard" <jbiard at cicsnc.org>, "CF Metadata" <cf-metadata at cgd.ucar.edu>
Sent: Tuesday, 18 April, 2017 13:09:18
Subject: Re: [CF-metadata] axis attribute

Hello Sébastien,

If we were to go down a route like this (and I'm not sure myself about it),
I think a new generic "add metadata to a dimension" mechanism might be
better. This could be used to provide plotting hints, regridding hints (a
more compelling use case than plotting, for me) or anything else about a
dimension. The idea being that we have a place to store information
relating to a dimension which does not directly describe the other
variables which span that dimension.

A new "dimension variable" would have to say which dimension it relates to,
and may optionally have "axis", "long_name" and "comment" to describe the
dimension (any other attributes being considered non-standard). The
dimension variables would be referenced by data variables, rather than
coordinate variables:

dimensions:
    i = 100 ;
    j = 200 ;
variables:
    char x:                     // dimension metadata variable (of
arbitrary type becuase it has no data)
        x.dimension = "i" ;         // mandatory dimension property which
says which dimension is being described
        x:axis = "X" ;              // optional
        x.long_name = "provide plotting/regridding hint that i should go on
X axis" ;        // optional
    char y:                     // dimension metadata variable (of
arbitrary type)
        y.dimension = "j" ;
        y:axis = "Y" ;
        y.long_name = "provide plotting/regridding hint that j should go on
Y axis" ;
    double longitude(j, i) ;
        longitude:units = "degrees_east" ;
    double latitude(j, i) ;
        latitude:units = "degrees_north" ;
    double temperature(j, i) ;
        tas:standard_name = "air_temperature" ;
        tas:units = "K" ;
        tas:coordinates = "longitude latitide" ;
        tas:axes = "x y" ;  // new data variable attribute which points to
dimension variables


Thanks, David


On 18 April 2017 at 11:08, Sebastien Villaume <sebastien.villaume at ecmwf.int>
wrote:

> Dear Jonathan and Jim,
>
> thank you for your further comments.
>
> @Jonathan: I don't want to define an axis without coordinates or put the
> "axis" attribute on more than one object, I only propose to decouple the
> axis definition from the coordinate definition. I don't see why I can't
> define a dummy object, it is exactly what is done to define a crs: a
> variable without dimension, without data, only attributes:
>
>   int crs ;
>     crs:grid_mapping_name = "latitude_longitude" ;
>     crs:semi_major_axis = 6371000.0 ;
>     crs:inverse_flattening = 0 ;
>
>
> following the same principle, I could define:
>
>   int axis_x ;
>     axis_x:axis = X;
>     axis_x:standard_name = "axis" ;
>     axis_x:long_name = "axis for the longitude coordinate"
>     axis_x:units = "1" ;
>
> and then have my longitude:
>
> float longitude(j,i);
>     longitude:standard_name = "longitude" ;
>     longitude:units = "degree_east" ;
>     longitude:long_name = "longitude" ;
>     longitude:axis_mapping = "X" ;
>
> In that specific example, only axis_x has the "axis" attribute. I added a
> "axis_mapping" attribute to the longitude variable to indicate that it
> should be understood as the coordinate of the axis given in "axis_mapping".
>
> I favour this solution over creating a dummy coordinate variable with
> gridbox indices. It seems artificial to do so and usually creating
> something artificial to comply to a rule is the sign of a design problem or
> a lack of functionality to describe the use case.
>
> @Jim: I agree with your last message, It would be beneficial to have an
> explicit mechanism to identify axes, coordinates, and their relationships
> when the use case does not conform to the common  case dimension = axis =
> coordinate .
>
> What is the CF mechanism to extend the convention or to propose semantic
> changes, rewrites of some portions of chapters, etc. ?
>
> Best wishes,
> /Sébastien
>
> ----- Original Message -----
> From: "Jim Biard" <jbiard at cicsnc.org>
> To: cf-metadata at cgd.ucar.edu
> Sent: Friday, 14 April, 2017 23:11:44
> Subject: Re: [CF-metadata] axis attribute
>
> Jonathan,
>
> We've got no great options available. The axis attribute ought to be an
> indicator of an independent coordinate axis. Because of possible backward
> compatibility issues, I think we need to demote the axis attribute to being
> a plotting hint. I'm at a loss for a word to use for a new attribute, but
> overloading axis with the different meanings seems too confusing.
>
> The existing convention for identifying 1D coordinate variables (variable
> name = dimension name) works well for identifying independent axes as far
> as that goes, so what are we missing with a fuzzy or demoted axis
> attribute? It seems to me that we lack a clean way to identify the
> x/y/z/t/etc relationships between the different independent axes, and we
> lack a way to clearly identify a 1-D variable as an independent axis if it
> doesn't conform to the variable name = dimension name convention.
>
>
> I'm not sure what's the cleanest way to go about it, but I'll suggest a
> name for an attribute that would fill some portion of these purposes. How
> about basis (or bases)?
>
> Grace and peace,
>
> Jim
>
> On 4/11/17 10:36 AM, Jonathan Gregory wrote:
>
>
>
> Dear David and Sebastien
>
> In a given model the level numbers are meaningful indeed, but there's no
> universal convention for them. Maybe some atmosphere models number them
> from
> the TOA downwards, for instance. Nothing would prohibit that, and the
> standard_name of model_level_number doesn't prescribe any convention for
> it.
> There is likewise no universal convention for how to number the gridboxes
> in
> x and y, but in any given model there is a convention for it. Thus I think
> it's the same. The standard name identifies a conventional concept, but
> not a
> convention for the numbers themselves.
>
> As for the axis attribute itself, maybe we could distinguish the two
> meanings
> of "axis" and avoid backwards compatibility by removing the restriction in
> chapter 5 that it is not permissible for a data variable to have both a
> coordinate variable and an auxiliary coordinate variable having an axis
> attribute with any given value. That is, at present you can't have both a
> 1D
> x-coordinate variable with axis="X" and a 2D longitude auxiliary coordinate
> variable with axis="X". But we could allow them both, with different
> meanings.
> We could say that the axis attribute of 1D coordinate variables labels the
> index dimensions of the data variable, while the axis attribute of multi-
> dimensional auxiliary coordinate variables labels them as spatiotemporal
> dimensions, as a hint for plotting.
>
> Best wishes
>
> Jonathan
>
> ----- Forwarded message from David Hassell <david.hassell at ncas.ac.uk>
> -----
>
>
>
> Date: Tue, 11 Apr 2017 08:41:34 +0100
> From: David Hassell <david.hassell at ncas.ac.uk> To: Jonathan Gregory <
> j.m.gregory at reading.ac.uk> CC: CF Metadata <cf-metadata at cgd.ucar.edu>
> Subject: Re: [CF-metadata] axis attribute
>
> Hello all,
>
> I am still uncomfortable with creating a coordinate variable with arbitrary
> values. The analogy with model_level_number is not quite there, I think, as
> the model_level_number values are not arbitrary. For example, a value of 6
> means (in one model I know) that this is the first level above the boundary
> layer. That value and meaning is relevant however the data may have been
> subspaced/sliced.
>
> Perhaps an auxiliary coordinate variable could be used with missing data in
> all of its values, an axis attribute (of "X" or "Y") and *no* standard
> name? In the absence of a coordinate variable for that dimension, the axis
> attribute of the auxiliary coordinate variable would give meaning to the
> dimension.
>
> This is still not ideal, though ...
>
> All the best,
>
> David
>
>
>
> On 10 April 2017 at 18:25, Jonathan Gregory <j.m.gregory at reading.ac.uk>
> wrote:
>
>
>
> Dear Sébastien
>
> Yes, you're right, we can't have an axis without coordinates. This is
> because
> CF is a netCDF convention, and there's no way to attach attributes to a
> dimension other than by creating a coordinate variable. Similarly we don't
> have data variables with just dimensions but no data. We could have
> conventions
> for these concepts in CF-netCDF but it hasn't been proposed.
>
> However it seems pretty good to me to do as you suggest and create a
> coordinate variable with the gridbox indices in it, for which (as
> discussed)
> we could define a standard name. This is analogous to model_level_number,
> which is already a standard name and could be a vertical coordinate
> variable.
> It can do what you want, and I agree it makes sense to label these
> coordinate
> variables as X and Y. That indicates that they are the horizontal
> dimensions.
> It's probably less important which is X and which Y. That's a plotting
> issue.
>
> I think the confusion probably arises because we didn't define what we mean
> by "axis". We used the word as an "obvious" concept, but there is an
> ambiguity
> about whether it corresponds to a dimension of a data variable, or a
> physical
> variable (not necessarily spatiotemporal) which could be an independent
> variable on which the data depends. Latitude might be an axis in the second
> sense even if it's not an axis in the first sense. I prefer the first
> sense,
> which is the one the axis attribute originally had.
>
> Best wishes
>
> Jonathan
>
>
> ----- Forwarded message from Sebastien Villaume <
> sebastien.villaume at ecmwf.int > -----
>
>
>
> Date: Fri, 7 Apr 2017 09:10:09 +0000
> From: Sebastien Villaume <sebastien.villaume at ecmwf.int> To: David Hassell
> <david.hassell at ncas.ac.uk> CC: CF Metadata <cf-metadata at cgd.ucar.edu> ,
> Jonathan Gregory <j.m.gregory at reading.ac.uk> Subject: Re: [CF-metadata]
> axis attribute
> X-Mailer: Zimbra 8.6.0_GA_1200 (ZimbraWebClient - FF50
> (Linux)/8.6.0_GA_1200)
>
>
>
> Dear David,
>
> I see your point and you are probably right that a plotting routine will
> probably sort this out.
>
>
>
> However this is not what I am after, I am more interested in metadata
> discovery and indexing.
>
>
>
> I need to discover what I have in a file without plotting it, without
> having a human looking at it to confirm what it is and that it has been
> plotted correctly.
>
>
>
> I also would like to use these metadata informations to perform actions
> like merging netCDF files, slicing, cropping, aggregating, interpolating,
> comparing data in different grids and representations, etc.
>
>
>
> I understand that implicit is fine and that explicit is not required for
> some applications. I have no issue with this.
>
>
>
> My personal point of view is that explicit is better than implicit: I
> tend to prefer "mandatory" over "optional".
>
>
>
> Being implicit means that the assumptions made need to be valid 100% of
> the time to avoid accidents or corner cases.
>
>
>
> I would like to be explicit so I need all the proper mechanisms
> (variables, semantics, etc.) in place so I can use them.
>
>
>
> Right now it feels that I am missing some functionality.
>
> Let me copy below few bits of the terminology section in the CF 1.7
> draft document (very similar to 1.6). Please read it keeping in mind what
> is really an axis, a coordinate, a spatio-temporal dimension and an an
> array dimension. Each time you read "coordinate2, "dimension" or
> "dimensional", ask yourself what is implied and if it is not ambiguous:
>
>
>
> ------------------------
> variables
> ------------------------
> auxiliary coordinate variable
>     Any netCDF variable that contains coordinate data, but is not a
> coordinate variable (in the sense of that term defined by the NUG and used
> by this standard - see below). Unlike coordinate variables, there is no
> relationship between the name of an auxiliary coordinate variable and the
> name(s) of its dimension(s).
>
>
>
> coordinate variable
>     We use this term precisely as it is defined in section 2.3.1 of the
> NUG . It is a one-dimensional variable with the same name as its dimension
> [e.g., time(time) ], and it is defined as a numeric data type with values
> that are ordered monotonically. Missing values are not allowed in
> coordinate variables.
>
>
>
> grid mapping variable
>     A variable used as a container for attributes that define a specific
> grid mapping. The type of the variable is arbitrary since it contains no
> data.
>
>
>
> multidimensional coordinate variable
>     An auxiliary coordinate variable that is multidimensional.
>
> scalar coordinate variable
>     A scalar variable (i.e. one with no dimensions) that contains
> coordinate data. Depending on context, it may be functionally equivalent
> either to a size-one coordinate variable (Section 5.7, "Scalar Coordinate
> Variables") or to a size-one auxiliary coordinate variable (Section 6.1,
> "Labels" and Section 9.2, "Collections, instances, and elements").
>
>
>
> ------------------------
> dimensions
> ------------------------
> latitude dimension
>     A dimension of a netCDF variable that has an associated latitude
> coordinate variable.
>
>
>
> longitude dimension
>     A dimension of a netCDF variable that has an associated longitude
> coordinate variable.
>
>
>
> spatiotemporal dimension
>     A dimension of a netCDF variable that is used to identify a location
> in time and/or space.
>
>
>
> time dimension
>     A dimension of a netCDF variable that has an associated time
> coordinate variable.
>
>
>
> vertical dimension
>     A dimension of a netCDF variable that has an associated vertical
> coordinate variable.
>
>
>
> ------------------------
>
> So according to this terminology, I have in my file, 2 auxiliary
> coordinates variables, but no "real" coordinates variables (according to
> the NUG) so my auxiliary coordinates are auxiliary to what?
>
>
>
> What is a "multidimensional coordinate"? if dimension means
> spatio-temporal dimension it is a non sense because a coordinate can only
> reference 1 spatio-temporal dimension, if it is meant to be
> array-dimensions it is not clear...
>
>
>
> What are my 2D array latitude and longitude then? are they latitude and
> longitude dimension defined in the terminology? not really.... because
> there are no such things as latitude and longitude dimension: you can
> define latitude and longitude coordinates, associated with 2 axis that
> themselves define 2 spatial dimensions... but the coordinates can be
> defined in whatever n-D array.
>
>
>
> I like the definition of "grid mapping variable", I could use a similar
> variable to be a container for attributes for my "axis variable" with no
> data!
>
>
>
> I know that in the day-to-day life and discussions we don't make the
> effort to be precise (I don't) and that it is easy to overload the meaning
> of things but I think that the CF document needs to be very precise, non
> ambiguous and can not mix axes, coordinates, spatio-temporal and array
> dimensions.
>
>
>
> /Sébastien
>
> ----- Original Message -----
> From: "David Hassell" <david.hassell at ncas.ac.uk> To: "Sebastien Villaume"
> <sebastien.villaume at ecmwf.int> Cc: "CF Metadata" <cf-metadata at cgd.ucar.edu>
> , "Jonathan Gregory" <
> j.m.gregory at reading.ac.uk >
>
>
>
> Sent: Friday, 7 April, 2017 08:37:20
> Subject: Re: [CF-metadata] axis attribute
>
> Dear Sébastien,
>
> Please bear with me when I ask to right back to the beginning! I am not
> sure what the benefit is in labelling the dimensions as X or Y. In the
> original tripolar case we have:
>
> dimensions:
>     i = 96 ;
>     j = 73 ;
> variables:
>     float latitude(j, i) ;
>         latitude:units = "degrees_north" ;
>     float longitude(j, i) ;
>         longitude:units = "degrees_east" ;
>     float sit(j, i) ;
>         sit:units = "m" ;
>         sit:standard_name = "sea_ice_thickness" ;
>         sit:coordinates = "latitude longitude" ;
>
> There is nothing stopping anything from seeing that this is 2-d array of
> size i*j, and there is nothing stopping software subpacing the data by i
> and j indices.
>
> I don't think a plotting routine would benefit from knowing that the i
> dimension was "X", because there are no 1-d coordinates it can use along
> that dimension.
>
> Many thanks and all the best,
>
> David
>
> On 6 April 2017 at 22:45, Sebastien Villaume <
> sebastien.villaume at ecmwf.int >
>
>
>
> wrote:
>
>
>
> Dear Mark and Jonathan,
>
> thank you for your comments.
>
> @Mark:
> the short answer: you can put in principle whatever you want in that
> variable because in this case it is a dummy variable only there to
> hold the
>
>
>
>
>
> axis attribute. But please read the long explanation!
>
> the long, boring explanation:
> As I understand it, the CF convention does not recognize axis as a
> valid
>
>
>
>
>
> object on its own like for "dimensions" and the various type of
> "variables"
>
>
>
>
>
> and the convention seems to make it mandatory to attach to it a
> variable
>
>
>
>
>
> that becomes a "coordinate" variable. Note that I say that it is the
> coordinate variable that is attached to the axis and not the opposite.
>
> From a mathematical point of view, it is perfectly possible to define
> an
>
>
>
>
>
> axis without a coordinate on it (arguably it is not that useful). The
> common case is that a 1-D array defines positions on that axis (the
> coordinate). Then your 1-D data points are positioned with the help of
> the
>
>
>
>
>
> coordinate, itself attached to the axis.
>
> If you have one more axis, you can define a new coordinate on it. This
> creates a 2-D space. Now you have the choice on how you represent your
> 2-D
>
>
>
>
>
> data points:
> if the dataset is totally irregular you will have a 1-D array of "n"
> data
>
>
>
>
>
> points associated with a 1-D array of "n" positions for the first
> dimension
>
>
>
>
>
> and a 1-D array of "n" positions for the second dimension. It works,
> it is
>
>
>
>
>
> still a 2-D dataset stored in a long one dimensional vector.
>
> Imagine that you realize that your dataset is not as irregular as you
> thought, it is in fact a regular grid! you identify that you only have
> i
>
>
>
>
>
> possible values of the first coordinate and j possible values for the
> second coordinate, you also notice that i*j=n. Great you can now
> represent
>
>
>
>
>
> your dataset with 2 coordinates of length i and j respectively, each of
> them associated with 2 axes x and y and your data is now a 2-D array of
> size (i,j). you can position your data using the coordinates, it is
> mapped
>
>
>
>
>
> using the indices within each coordinate array. Now you have a 2-D
> spatial
>
>
>
>
>
> dataset sored in a 2-D array with 2 supporting 1-D spatial coordinates
> stored in one dimensional vectors.
>
> Lets say now that you take this regular grid and you distort it... your
> regular grid is gone you can no longer use i and j for partitioning!
> really? well no, nobody says that you can not slice your "n" long
> vectors
>
>
>
>
>
> into i*j arrays! you could choose whatever you want for i and j as
> long as
>
>
>
>
>
> i*j=n. Of course if you choose (2)*(n/2) or (n/2)*(2), it is a bit
> useless,
>
>
>
>
>
> but you can also choose meaningful i and j because even if your grid
> became
>
>
>
>
>
> irregular, it is not random points, it is still a grid of size i*j .
> This
>
>
>
>
>
> is exactly my use case! And in that situation your coordinates can be
> arranged in arrays of size i*j. What I need is 2 axes and 2
> coordinates of
>
>
>
>
>
> dimension 2 with lengths i and j. The catch here is that I have 2-D
> arrays
>
>
>
>
>
> to store one "spatial" dimension! It is another case of overlapped
> concepts, dimension is used transparently for the dimension of arrays,
> dimension of the geometrical space, and sometimes for the size of one
> of
>
>
>
>
>
> the dimensions of an array!!
>
> Anyway, I should be able to define my axes like this:
>
> int x;
>     x:axis = "X";
>     x:standard_name = "x_axis" ; // no standard name exists...
>     x:units = "1" ; // no units, it will come with the coordinate
> int y;
>     y:axis = "Y";
>     y:standard_name = "y_axis" ; // no standard name exists...
>     y:units = "1" ; // no units, it will come with the coordinate
> float longitude(j,i);
>     longitude:standard_name = "longitude" ;
>     longitude:units = "degrees" ;
>     longitude:positive = "east" ;
>     longitude:long_name = "longitude" ;
>     longitude:axis_mapping = "X" ;
> float latitude(j,i);
>     latitude:standard_name = "latitude" ;
>     latitude:units = "degrees" ;
>     latitude:positive = "north" ;
>     latitude:long_name = "latitude" ;
>     latitude:axis_mapping = "Y" ;
> float sit(j, i) ;
>     sit:units = "m" ;
>     sit:standard_name = "sea_ice_thickness" ;
>     sit:long_name = "Ice thickness" ;
>     sit:coordinates = "latitude longitude" ;
>
> several comments:
> notice how one could tell on which axis the coordinate should go using
> for
>
>
>
>
>
> instance a "axis_mapping" attribute. Not a "coordinate" attribute,
> this one
>
>
>
>
>
> should be used to tell the coordinates of my data variable!
> I find this approach clearer and more flexible as it can probably cater
> for any situation of axes, coordinates, etc.
>
> But because in CF one cannot create bare axis, I follow the rules and
> creates:
>
> double x(i);
>     x:axis = "X";
>     x:standard_name = "..." ; // not an axis anymore, give me a
> standard
>
>
>
>
>
> name
>     x:units = "1" ;
>     y:long_name = "i-index of mesh grid" ;
> double y(j);
>     y:axis = "Y";
>     y:standard_name = "..." ; // not an axis anymore, give me a
> standard
>
>
>
>
>
> name
>     y:units = "1" ;
>     y:long_name = "j-index of mesh grid" ;
>
> and I have the choice of what I put in those arrays since it is somehow
> artificial.
>
> I could populate the "primary" coordinates with 1 to i and 1 to j which
> would represent the indices and if I subset the grid, I then retain the
> information that the domain has been cropped because the indices left
> will
>
>
>
>
>
> not be 1 to i/j but n to m.
> I don' t really like this but what can I do?
>
> If we follow this idea, it means introducing a clear concept from
> "axis"
>
>
>
>
>
> besides the other types of variables, defining new attribute to
> "attach"
>
>
>
>
>
> coordinates to axes, etc.
>
> Another solution, much less disturbing, would be to heavily modify the
> proper chapters in the CF document to:
> - completely decouple the concepts of "axis" and "coordinate": a
> coordinate is not an axis and vice versa.
> - completely decouple the concepts of spatio temporal dimension from
> array
>
>
>
>
>
> dimension from the size the array dimension
> - continue to use the "axis" attribute  but on n-D array coordinates:
> the
>
>
>
>
>
> array has n-D dimensions but the coordinate map to 1 axis/spatial
> dimension
>
>
>
>
>
> only!
> - Whatever the dimensions of the array for the coordinate, all the
> values
>
>
>
>
>
> contained in the array must be mapped on one given axis, the one
> defined in
>
>
>
>
>
> axis attribute. For instance, a 2-D latitude only contains values that
> are
>
>
>
>
>
> latitudes and will only map on one axis.
> - In principle one could have in the same file several coordinates of
> possibly different "array" dimensions, different sizes and different
> units
>
>
>
>
>
> defined for one axis. This means that the attribute "axis=z" for
> instance
>
>
>
>
>
> can appears more than once in the file. The only restriction I see is
> that
>
>
>
>
>
> 2 data variables can be only plotted simultaneously if all their
> coordinates share the same units (the coordinate mapped on one axis of
> the
>
>
>
>
>
> first data variable must have the same units than the coordinate
> mapped on
>
>
>
>
>
> the same axis for the other data variable). This allow 2 data variables
> defined on two different grid sharing the same units to be in the same
> file
>
>
>
>
>
> and plotted together.
> - X and Y should be clearly decoupled from longitude and latitude. X
> and Y
>
>
>
>
>
> are the axes, longitude and latitude are the coordinates!
>
>
> @Jonathan:
> I think the whole confusion here comes from the overlapping of
> concepts:
>
>
>
>
>
> axes and coordinates on one hand and dimension of arrays and spatial
> dimensions on the other hand. If the relevant chapters are rewritten
> carefully to separate axes from coordinates and array dimensions from
> spatio-temporal dimensions we are good.  think
>
> @all: Reading more through the Trac tickets system, I noticed the nice
> Trac ticket 117 about "multiple" time axis. This is a nice example of
> mixing axes, coordinates, dimensions of arrays, the time dimension,
> etc!
>
>
>
>
>
> /Sébastien
>
> ----- Original Message -----
> From: "Jonathan Gregory" <j.m.gregory at reading.ac.uk> To:
> cf-metadata at cgd.ucar.edu Sent: Thursday, 6 April, 2017 16:49:56
> Subject: Re: [CF-metadata] axis attribute
>
> Dear Jim and Sebastien
>
> The original intention of axis was to label the independent variables
> as 1D
>
>
>
>
>
> xyzt axes of the data variables.  This can be deduced from other
> attributes,
> but it's more effort. It's partly a plotting hint, but also it's
> because
>
>
>
>
>
> you
> might reasonable want to tell software, "give me the z-axis
> coordinates",
>
>
>
>
>
> or
> "calculate a mean over the x-direction". The latter is often a zonal
> mean,
>
>
>
>
>
> but
> it isn't with a rotated-pole or tripolar grid, yet the operation is
> still
>
>
>
>
>
> performed sometimes.
>
> It's useful that you've pointed out the confusion of purpose. If it
> were
>
>
>
>
>
> regarded as an acceptable backwards-incompatibility, which I'm nervous
> about,
> I'd be happy if we returned "axis" to its original purpose of
> identifying
>
>
>
>
>
> 1D
> axes, and also for scalar coordinate variables (which are equivalent to
> axes
> of size one), and provided another attribute to label aux coords as
> horizontal.
>
> I agree that if we have 1D x and y, with 2D lat and lon, the 1D
> variables
>
>
>
>
>
> are
> the axes. That's consistent with the original purpose of the axis
> attribute.
>
>
>
> I also find the units of latitude and longitude confusing: it looks
> like
>
>
>
>
>
> it was a way to squeeze the direction of the coordinate inside the
> units. I
>
>
>
>
>
> have the same observation for the time coordinate that has its origin
> in
>
>
>
>
>
> the units!
>
> This convention was kept in CF for backwards-compatibility with
> COARDS. CF
>
>
>
>
>
> does
> not use units in any other case to identify the quantity or sense.
>
>
>
> It was done correctly for z coordinate using "units" and "positive",
> probably because there are many types of z coordinates with various
> origin
>
>
>
>
>
> and directions, and no real consensus. I note however that often the
> origin
>
>
>
>
>
> is not always clearly defined.
>
> The positive attribute was also kept for backwards-compatibility with
> COARDS.
> It has the advantage of being useful to identify the vertical axis, but
> this
> can also be done with axis="Z". CF standard names provide information
> which
>
>
>
>
>
> indicates the sign convention.
>
> If coordinate_index is confusing, I think standard_names containing
> x_index
>
>
>
>
>
> or y_index would be OK, provided we change the existing standard names
>   magnitude_of_derivative_of_position_wrt_x_coordinate_index
>   magnitude_of_derivative_of_position_wrt_y_coordinate_index
> to remove "_coordinate".
>
> Best wishes
>
> Jonathan
> _______________________________________________
> CF-metadata mailing list CF-metadata at cgd.ucar.edu
> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
> _______________________________________________
> CF-metadata mailing list CF-metadata at cgd.ucar.edu
> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
> --
> David Hassell
> National Centre for Atmospheric Science
> Department of Meteorology, University of Reading,
> Earley Gate, PO Box 243, Reading RG6 6BB
> Tel: +44 118 378 5613 http://www.met.reading.ac.uk/
> ----- End forwarded message -----
> _______________________________________________
> CF-metadata mailing list CF-metadata at cgd.ucar.edu
> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
> --
> David Hassell
> National Centre for Atmospheric Science
> Department of Meteorology, University of Reading,
> Earley Gate, PO Box 243, Reading RG6 6BB
> Tel: +44 118 378 5613 http://www.met.reading.ac.uk/
> ----- End forwarded message -----
> _______________________________________________
> CF-metadata mailing list CF-metadata at cgd.ucar.edu
> http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata
>
> --
>
>         Visit us on
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-- 
David Hassell
National Centre for Atmospheric Science
Department of Meteorology, University of Reading,
Earley Gate, PO Box 243, Reading RG6 6BB
Tel: +44 118 378 5613
http://www.met.reading.ac.uk/



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