[CF-metadata] CF compliant tripolar grid representation

Hedley, Mark mark.hedley at metoffice.gov.uk
Thu Apr 6 06:32:10 MDT 2017


With regard to the Rotated Pole, as raised by Dan, I agree with David's intent here, but perhaps I would use slightly different terminology in agreeing with him.

I would describe the Rotated Pole as a Derived Geodetic Coordinate Reference System.  It uses another Geodetic Coordinate Reference System as its Base, in many useful cases this turns out to be WGS84, but other base GeodCRS instances may be used.
The derived conversion may then be parameterised by the location of the North Pole, and a subsequent axis rotation.
A system of coordinates (rotated_longitude and rotated_latitude) with comprehensible units (degrees) can now be used to describe position with respect to this Derived CRS

The crucial part is that I can define coordinate values with respect to this derived CRS and use well understood mathematical operations to ask questions like
  - where is this location on a PlateCarree Projection defined with respect to WGS84
  - which cell is this defined location (in WGS84 longitude and latitude ) contained within

It is these kinds of question that I think will be challenging to provide answers to when it comes to the tri-polar grid and index based coordinate values, with no associated unit of measure

mark


__________________________________________________________

From: David Hassell [david.hassell at ncas.ac.uk]

Sent: 06 April 2017 10:24

To: Hollis, Dan

Cc: Hedley, Mark; cf-metadata at cgd.ucar.edu

Subject: Re: [CF-metadata] CF compliant tripolar grid representation







Hi Dan,




I wonder if the rotated_latitude_longitude grid mapping is a CRS - it's a CRS which is the same as the latitude_longitude grid mapping CRS (the "usual" one) apart from having a different datum
 for its horizontal coordinates - i.e. not at [0 east, 0 north].




All the best,




David




On 6 April 2017 at 10:10, Hollis, Dan <dan.hollis at metoffice.gov.uk>
 wrote:


Hi Mark,



I support your concerns. There is a difference between stating which geographic and/or projected CRS is being used and defining which specific points are included in the data set. The issue of describing a tripolar grid seems to relate to the latter not the
 former.



>From a quick glance at Appendix F I would say that the odd one out is the rotated pole. Describing a grid as 'rotated pole' is surely just convenient shorthand for describing which points in a lat-long CRS you are using. It is certainly not a CRS and, arguably,
 not a grid mapping. Is it possible, for example, to declare that a grid is 'rotated pole' _and_ that the CRS is WGS84?



Perhaps tripolar grids and rotated pole grids are two parts of the same problem.



Regards,



Dan





-----Original Message-----

From: CF-metadata [mailto:cf-metadata-bounces at cgd.ucar.edu] On Behalf Of Hedley, Mark

Sent: 06 April 2017 09:28

To: 
cf-metadata at cgd.ucar.edu

Subject: Re: [CF-metadata] CF compliant tripolar grid representation



I would like to restate my concern about describing the tri-polar grid as a coordinate reference system, using the 'grid mapping' defined in approach in 5.6. Horizontal Coordinate Reference Systems, Grid Mappings, and Projections and Appendix F Grid Mappings.



I do not think that the tri-polar grid is a Coordinate Reference System, unlike all other entities defined in Appendix F.  I think that adding a tri-polar definition here fundamentally alters the interpretation of 'grid mapping' and has potentially significant
 implications for software providing capabilities to work with grid mapping entities.

This would represent a significant change of scope for these aspects of the Conventions document.



Currently all entries in Appendix F are either a Geographic Coordinate Reference System or a Projected Coordinate Reference System.

The tri-polar grid is neither of these.

The mechanics of looking up the required longitude and latitude values for a given x/i and y/j coordinate index are not mathematically similar to the calculations for geographic or projected space



I think the CF community would do well to consider where else information defining a tri-polar grid may be encoded in a file, and to keep the scope of the Grid Mapping section constrained.



I saw Appendix D: Parametric Vertical Coordinates mentioned as well, all be it in passing.  The tri-polar grid does not seem like a case of parametric coordinates either, to me, even if the 'vertical' was relaxed.



I realise I am raising objections but not proposing solutions, I do not have an easy answer here, I am afraid.



all the best

mark





________________________________________

From: CF-metadata [cf-metadata-bounces at cgd.ucar.edu] on behalf of Sebastien Villaume [sebastien.villaume at ecmwf.int]

Sent: 05 April 2017 19:20

To: Gregory, Jonathan

Cc: 
cf-metadata at cgd.ucar.edu

Subject: Re: [CF-metadata] CF compliant tripolar grid representation



Dear Jonathan,



I will try to look at Appendix F and come back with a proposal. I have a rough idea of what I need but I have no clue what would be the proper terms for those: extra attributes to describe north pole 1 and north pole 2, latitude separating the "regular" from
 the irregular region, etc.



____________________________________



Dr. Sébastien Villaume

Analyst

ECMWF

Shinfield Park,

Reading RG2 9AX, UK

+44 7825 521592

sebastien.villaume at ecmwf.int

____________________________________



----- Original Message -----

From: "Jonathan Gregory" <j.m.gregory at reading.ac.uk>

To: 
cf-metadata at cgd.ucar.edu

Sent: Wednesday, 5 April, 2017 16:15:55

Subject: Re: [CF-metadata] CF compliant tripolar grid representation



Dear Sebastien



Apologies. I meant Appendix F (grid mappings) not D. Could you describe your species of tripolar grid as one of these? Maybe there aren't any parameters to be recorded.



Best wishes



Jonathan



----- Forwarded message from Sebastien Villaume <sebastien.villaume at ecmwf.int> -----



> Date: Wed, 5 Apr 2017 08:47:57 +0000

> From: Sebastien Villaume <sebastien.villaume at ecmwf.int>

> To: 
cf-metadata at cgd.ucar.edu

> Subject: Re: [CF-metadata] CF compliant tripolar grid representation

> X-Mailer: Zimbra 8.6.0_GA_1200 (ZimbraWebClient - FF50

> (Linux)/8.6.0_GA_1200)

>

> Hi all,

>

> I could try to draft an new entry in grid_mapping or a new entry in

> Appendix D (it will not be a dimensionless "vertical" coordinate but a

> dimensionless "horizontal" coordinate)

>

> Could we agree first on what I need to define? I don't want to invest too much time in defining something before everyone agree on the way forward.

>

> thanks

>

> ____________________________________

>

> Dr. Sébastien Villaume

> Analyst

> ECMWF Shinfield Park,

> Reading RG2 9AX, UK

> +44 7825 521592

> 
sebastien.villaume at ecmwf.int

> ____________________________________

>

> ----- Original Message -----

> From: "Jim Biard" <jbiard at cicsnc.org>

> To: 
cf-metadata at cgd.ucar.edu

> Sent: Tuesday, 4 April, 2017 21:47:36

> Subject: Re: [CF-metadata] CF compliant tripolar grid representation

>

> Hi.

>

> I tend to agree with Jonathan about the use of the grid_mapping variable, although it would probably be necessary to provide a clear distinction between this sort of information about mapping grid indices to lats and lons and providing information about mapping
 projected coordinate axis values to lats and lons. This new use is probably more appropriate for the name of the variable ( grid _mapping). Having said that, the potential for confusion and complication makes me wonder if a new construct isn't needed.

>

> The problem that I see with x/y_coordinate_index is that the indices are very likely indices to lat/lon coordinates, not x/y coordinates. They function as a sort of unitless, non-geographic x and y, but I think it would better to avoid overloading concepts.
 It's also possible that these indices could be indices to x and y coordinates, so it seems to me that lat/lon_coordinate_index would be no better. This is what led me to the names in my list that didn't use x, y, lat, or lon. They could be useful in other
 scenarios, such as satellite swath imagery, which have axes of scan and sample, so I didn't want to constrain the terms too closely to the mesh grid scenario that this discussion started with.

>

> Grace and peace,

>

> Jim

>

> On 4/4/17 4:25 PM, Jonathan Gregory wrote:

>

>

>

> Dear Sébastien et al.

>

> From what you say I understand that the translation of indices to

> coordinate values is rather ad-hoc, rather than being done by the same

> formulae for all sorts of tripolar grid. You could identify the grid

> construction, if that would be useful, in a non-standard way in some

> attribute such as "comment". To provide a standardised description, I

> still think grid_mapping would be the right place, but evidently "tripolar" would not be a sufficient definition.

> Instead you would need different entries in Appendix D for the

> different sorts of tripolar grid in use. In these entries you could

> certainly give URLs to documentation, I think, as well as a

> description. The aim of putting it in Appendix D would be to provide a

> source of information about how the indices are related to coordinate values.

>

> I suggested [xy]_coordinate_index because these phrases are already

> used in standard names (one of each). If we don't like them now, we

> ought to change the existing names, since we should be consistent. I

> think the phrase "coordinate index" means "the index to a coordinate

> value". Just "index" would be less informative, I feel.

>

> Best wishes

>

> Jonathan

>

>

> ----- Forwarded message from Sebastien Villaume

> <sebastien.villaume at ecmwf.int> -----

>

>

>

> Date: Mon, 3 Apr 2017 13:56:40 +0000

> From: Sebastien Villaume <sebastien.villaume at ecmwf.int> To:

> 
cf-metadata at cgd.ucar.edu Subject: Re: [CF-metadata] CF compliant

> tripolar grid representation

> X-Mailer: Zimbra 8.6.0_GA_1200 (ZimbraWebClient - FF50

> (Linux)/8.6.0_GA_1200)

>

>

> Hi Mark,

>

> I agree that we need to find the best way to describe these grids (with the appropriate controlled metadata) and not necessarily use an existing concept (crs, grid_mapping) if it does not fit the purpose and generates confusion.

>

> These tripolar grids are tricky and I guess this is why there is no standard systematic way to describe them.

>

> Reading more on it, I realized that some of them are not always "regular grids" (by regular I mean monotonic increase of lat and lon when increasing i and j indices): it seems that some NEMO configurations reuse some of the i and j indices that are over land
 (large parts of Asia and Africa) and relocate them over specific water regions to locally increase the grid resolution!

>

> This can be seen here: 
http://www.nemo-ocean.eu/content/download/7538/40914/file/meshmask_grid.pdf Some of these grids do not have a simple analytical description since it is a composite of several local descriptions. How can I then properly reference/identify them?
 using an attribute like "model_grid_mapping" or "model_mesh_mapping" or simply "mesh_mapping" instead of "grid_mapping" and points to an URN/URI?

>

> AMy main issue is that I can not derive directly from the metadata the type of grid used. I have to plot it to know what it is and this is not satisfactory.

>

> Regardless of the preferred solution (if one exists), I would still like to have a proper standard name for my 1-D mesh indices i and j.

>

> thanks

> ____________________________________

>

> Dr. Sébastien Villaume

> Analyst

> ECMWF Shinfield Park,

> Reading RG2 9AX, UK

> +44 7825 521592

sebastien.villaume at ecmwf.int

> +____________________________________

>

> ----- Original Message -----

> From: "Hedley, Mark" <mark.hedley at metoffice.gov.uk> To: "Jim Biard"

> <jbiard at cicsnc.org> ,

cf-metadata at cgd.ucar.edu Sent: Monday, 3 April,

> 2017 10:28:05

> Subject: Re: [CF-metadata] CF compliant tripolar grid representation

>

> Hello All,

>

> I'd like to pick up on an earlier comment from Jim:

> If I'm not mistaken, we would need to propose a new grid_mapping to be

> added to the Conventions that would define a Tripolar Coordinate

> Reference System, along with any attributes that don't currently exist

> that are needed to complete the definition. I did a search for a

> standard tripolar CRS in proj4 or epsg, and was unable to find one. Is

> it possible to make such a definition?

> I don't think this is the correct approach

>

> In my opinion, the tri-polar grid is described with respect to a Geographic Coordinate Reference System: typically the one used to co-locate the observations for assimilation, by spatial coordinates.

> The 'Grid' is not a projection and it is not a coordinate reference system: it is the description of a model grid.

> In data files I have seen, each spatial location is defined by a location in latitude, longitude and depth, with respect to a suitable geodetic datum.

>

>

> I agree with your more recent comment Jim:

>

> I'm wondering if x and y have too strong an association to projected

> coordinate systems. I also like u/v, but that may be too strongly

> associated for some people with vector components (wind, for example).

>

> I think that describing grid indices should be carefully distinguished from spatial coordinates. Put a different way, I don't think a grid index can be georeferenceable.

>

> I think that a good deal of care not to confuse the grid indices with any interpretation of 'grid_mapping' relations is required here.

> I don't think that a CF grid mapping should be used to connect any description of model index space with geographic space in these cases.

>

> Sebastien states:

>

> I would like to propose for addition standard names to support the mesh indices/coordinates:

>

> "mesh_grid_i/j_index" suggested by Jim or "x/y_coordinate_index"

> suggested by Jonathan

>

> The mixing of the terms coordinate and index gives me pause for thought. What information is being encoded here?

>

> A key question I have is about the expectation for values of these indices, under operations such as sub-setting. I have seen many files which do not have coordinate variables for the x-like and y-like dimensions, the only horizontal spatial metadata is contained
 in auxiliary coordinates.

> Clearly I can perform index operations on these arrays, but I don't consider the index values important and I don't preserve them.

>

> Sebastien:

> Is it the case that you would like to ensure that model index space values are preserved, for example when removing a regional subset from a tri-polar ocean model?

> Would you like to be able to encode a result where it is clear that a regional subset of 50 <= x < 150, 70 <= x < 120 has been taken from a larger extent model?

>

> If standard names are provided to encode such information, I would

> advocate clear descriptive text stating that there is no mathematical

> relationship between such index coordinates (i still don't like mixing

> these terms) and projection coordinates or geographic coordinates

>

> Sebastien states:

> I have checked both IPSL and CNRM CMIP5 datasets. It is indeed NEMO

> datasets and it is probably a ORCA tripolar grid in both cases. I

> write "probably" because it is not clear and conclusive without plotting the datasets: lat and lon are 2D fields, the datasets define 2 extra 1D coordinates "i" and "j"

> to be used as mesh indices (but without a proper standard name).

> The datasets also have bounds for lat and lon, defined as

> "lat_vertices" and "lon_vertices" which I think is one solution to

> describe the tripolar grid. I would prefer something more standardized

> and documented so that one can quickly identify from the metadata that

> it is a tripolar grid (defining the resolution, where are the poles,

> how it is derived, etc.)

>

> I appreciate the desire to have a standardised approach to defining

> such a model grid. I would not advocate trying to use grid mapping variables and relationships for this, I think this could do more harm than good.

> I don't have a better suggestion to hand, I'm sad to say.

>

> I am not raising principled objections to this conversation or the direction of travel; I am raising waryness and caution about introducing further confusion or implying stronger relationships than can be provided.

>

> all the best

> mark

>

>

> From: CF-metadata [ 
cf-metadata-bounces at cgd.ucar.edu ] on behalf of

> Jim Biard [ 
jbiard at cicsnc.org ]

> Sent: 31 March 2017 23:26

> To: 
cf-metadata at cgd.ucar.edu Subject: Re: [CF-metadata] CF compliant

> tripolar grid representation

>

>

>

> Hi.

>

> I like the more generic x/y_coordinate_index name, but I'm wondering if x and y have too strong an association to projected coordinate systems. I also like u/v, but that may be too strongly associated for some people with vector components (wind, for example).
 What do the rest of you think? Here are some names that come to mind. Feel free to suggest something better!

>

>

>     * mesh_grid_i_index, mesh_grid_j_index

>     * grid_i_index, grid_j_index

>     * grid_i_coordinate, grid_j_coordinate

>     * x_coordinate_index, y_coordinate_index

>     * index_x_coordinate, index_y_coordinate (this ordering matches the projection_x/y_coordinate naming)

>     * u_coordinate, v_coordinate

>     * i_coordinate, j_coordinate

>     * grid_row_coordinate, grid_column_coordinate

>     * row_coordinate, column_coordinate

>

>

> The more I look at these, the more I like the last two.

>

>

> As for a definitions, how about something like this variation on the ones for the projection_x/y_coordinate?

>

>

>

>

> column_coordinate: "column" indicates the fastest-changing dimension of a two-dimensional grid, when this is not associated with a spatial coordinate dimension such as longitude or projected X, positive with increasing column. The column coordinate, possibly
 in conjunction with the row coordinate, serves as a parametric driver mapping abstract grid positions to spatial coordinates such as latitude and longitude.

>

>

> row_coordinate: "row" indicates the the slowest-changing dimension of

> a 2-dimensional grid, when this is not associated with a spatial

> coordinate dimension such as latitude or projected Y, positive with

> increasing row. The row and column coordinates serve as a parametric

> driver mapping abstract grid positions to spatial coordinates such as

> latitude and longitude. Grace and peace,

>

> Jim

>

> On 3/31/17 5:37 PM, Sebastien Villaume wrote:

>

>

>

> Hi all,

>

> I have checked both IPSL and CNRM CMIP5 datasets. It is indeed NEMO

> datasets and it is probably a ORCA tripolar grid in both cases. I

> write "probably" because it is not clear and conclusive without

> plotting the datasets: lat and lon are 2D fields, the datasets define

> 2 extra 1D coordinates "i" and "j" to be used as mesh indices (but

> without a proper standard name). The datasets also have bounds for lat

> and lon, defined as "lat_vertices" and "lon_vertices" which I think is

> one solution to describe the tripolar grid. I would prefer something

> more standardized and documented so that one can quickly identify from

> the metadata that it is a tripolar grid (defining the resolution,

> where are the poles, how it is derived, etc.)

>

> I would like to propose for addition standard names to support the mesh indices/coordinates:

>

> "mesh_grid_i/j_index" suggested by Jim or "x/y_coordinate_index"

> suggested by Jonathan

>

> I let the experts in standard names decide which pair suits best the present case.

>

> Regarding tripolar grids characteristics, I did some research and came to the conclusion that "Murray tripolar grids" are not identical to "ORCA/NEMO tripolar grids". This is true even without considering characteristics like the grid resolution, the location
 of the poles or where the latitude boundary is placed between the modified and unmodified parts.

>

> The Murray tripolar grid (used by GFDL) has its "north" poles on the boundary as shown here:

https://www.gfdl.noaa.gov/wp-content/uploads/pix/user_images/mw/bipolar.gif The ORCA/NEMO tripolar grids have the "north" poles within the modified regions but not on the boundary as shown in my original post:

http://www.geomar.de/typo3temp/pics/globe_grid2_14_b8edb639ae.png This complicates things...

>

>

> ____________________________________

>

> Dr. Sébastien Villaume

> Analyst

> ECMWF Shinfield Park,

> Reading RG2 9AX, UK

> +44 7825 521592

sebastien.villaume at ecmwf.int

> +____________________________________

>

> ----- Original Message -----

> From: "James Orr" <James.Orr at lsce.ipsl.fr> To: "Karl Taylor"

> <taylor13 at llnl.gov> Cc:

cf-metadata at cgd.ucar.edu Sent: Thursday, 30

> March, 2017 23:01:54

> Subject: Re: [CF-metadata] CF compliant tripolar grid representation

>

> The IPSL and CNRM cimate models that participated in CMIP5 both used

> the NEMO model (ORCA2 and ORCA1 configurations) with tripolar grids.

> Both provided output the was CF compliant.

>

> James

>

> On Thu, 30 Mar 2017, Karl Taylor wrote:

>

>

>

> Hi Sebastien,

>

> More than one group stored output on a tripolar grid in CMIP5.  I'm

> pretty sure they did it in a CF-conforming way.  I know at least some

> of the GFDL model output was reported on a tripolar grid, as described

> at 
http://nomads.gfdl.noaa.gov/CM2.X/oceangrid.html (or search on

> "tripolar grid" for additional links).  You could look to their example, and see if you think it is done correctly.

>

> I don't think extensions or modifications to CF are needed for

> tripolar grids.

>

> best regards,

> Karl

>

> On 3/30/17 9:42 AM, Jim Biard wrote:

>

>

>

> Sébastien,

>

> If I'm not mistaken, we would need to propose a new grid_mapping to be

> added to the Conventions that would define a Tripolar Coordinate

> Reference System, along with any attributes that don't currently exist

> that are needed to complete the definition. I did a search for a

> standard tripolar CRS in proj4 or epsg, and was unable to find one. Is

> it possible to make such a definition?

>

> Regarding the standard names for your X and Y coordinate variables, I

> think you could use "projection_x/y_coordinate" once a grid_mapping

> has been defined. Of course you could always leave the attribute off,

> since a standard_name attribute is not a requirement.

>

> If making a new grid_mapping is not feasible, you could request

> standard names along the lines of mesh_grid_i_index and

> mesh_grid_j_index. These standard names would (on reading their

> definitions) make it clear that the measurements are on a mesh grid

> for which there is no CRS. At least that's what comes to mind at the moment.

>

> Grace and peace,

>

> Jim

>

> On 3/30/17 11:52 AM, Sebastien Villaume wrote:

>

>

>

> Hello all,

>

> I am looking for the best approach to describe in a CF compliant way

> the tripolar grids usually used in NEMO configurations.

>

> Basically, the difference with a usual bipolar grid (north pole-south

> pole) is that the north pole is split into 2 poles moved over Canada

> and Russia (to have distortions/singularities not over the ocean). A

> good visual representation can be found here:

> 
http://www.geomar.de/typo3temp/pics/globe_grid2_14_b8edb639ae.png

> everything south of the green line (40degN) is identical to a regular

> grid, but everything north of it is computed using a technique

> described

> here:

>

> Madec, G. and M. Imbard, 1996 : A global ocean mesh to overcome the

> north pole singularity. Clim. Dyn., 12, 381-388.

>

>

> The usual NEMO output of the grid looks like this:

>

>      float longitude(y, x) ;

>          longitude:standard_name = "longitude" ;

>          longitude:units = "degrees_east" ;

>          longitude:long_name = "longitude" ;

>      float latitude(y, x) ;

>          latitude:standard_name = "latitude" ;

>          latitude:units = "degrees_north" ;

>          latitude:long_name = "latitude" ;

>

>

> Basically both latitudes and longitudes need to be specified for each

> grid point, hence lat and lon are 2D arrays. This is not a problem

> itself but I would like to give more information through maybe

> grid_mapping or crs so it is clear that the grid is tripolar. This is

> useful information if one want to project/interpolate this back to a more regular representation.

>

> Looking at the CF conventions, I can see that grids can be fairly

> nicely documented but nothing for tripolar grids.

>

> Is there some documentation/guidelines on how to derive a proper

> grid_mapping/crs with valid attributes for tripolar grids?

>

> I would also like to add to my netcdf file a way to better describe axes:

>

>      double y(y) ;

>          y:units = "1" ;

>          y:long_name = "j-index of mesh grid" ;

>          y:standard_name = ??? ;

>      double x(x) ;

>          x:units = "1" ;

>          x:long_name = "i-index of mesh grid" ;

>          x:standard_name = ??? ;

>

> what would be the standard name of these?

>

> Thanks,

>

> ____________________________________

>

> Dr. Sébastien Villaume

> Analyst

> ECMWF Shinfield Park,

> Reading RG2 9AX, UK

> +44 7825 521592 
sebastien.villaume at ecmwf.int

> +____________________________________

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http://mailman.cgd.ucar.edu/mailman/listinfo/cf-metadata





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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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