[CF-metadata] axis attribute

Sebastien Villaume sebastien.villaume at ecmwf.int
Wed Apr 5 02:36:29 MDT 2017


Hi,

I am also against assigning an "axis" attribute to a 2-D variables. 

>From a mathematical point of view an axis is one dimension, has an origin, a reference unit and a direction. For instance a 3D Cartesian coordinates system has three dimensions defined by 3 axes, each axis is defined by a unit vector (reference unit and direction) and an origin (it happens that they all share the same origin but it is not a requirement in principle). A spherical coordinate system has also 3 dimensions, defined by 1 axis and 2 angles, the axis is defined like in Cartesian coordinates, the 2 angles are defined by an origin (0deg), a reference unit (1/360 of a circle) and a direction ( increasing degrees is anti clockwise).

Clearly 2-D latitude and longitude do not fulfil this. In my case both are simply variables in a 2D space that needs to be defined somehow. This is exactly why I am trying to define 2 "supporting" 1-D variables with the clear intention to put an axis attribute on them. I could name this 2 supporting variables x and y , or i and j or whatever. 

Is it acceptable to put an "axis = x/y" on variables with standard names containing i/j? would it be acceptable to put axis = i/j instead? 

More generally when you have a dataset in a different coordinates system, i.e. spherical coordinates, do you put axis x/y/z on r, theta, phi? if you have a dataset in a grid point space: i/j/k? of in a lattice space (admittedly with limited usage for earth science)?  Would it be more logical to have different type of variable attributes for different type of dimensions? like "axis", "angle", etc.

This is a more general discussion for the CF convention experts, what I only need is two standard names to describe my lat/lon supporting axes!

____________________________________ 

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 23:43:58
Subject: Re: [CF-metadata] axis attribute

Karl, 

Don't allow the attribute on 2D variables. :) I feel like it's a pretty far stretch to get to your 2D example. 


Jim 

On 4/4/17 6:41 PM, Karl Taylor wrote: 


Hi all, 
I don't think the issue of 2-d auxiliary coordinates entered the discussion leading to their allowance by CF 1.6 (but I only quickly reviewed the discussion). I think that allowing the axis attribute to be attached to an auxiliary coordinate that is 1-d can be useful (e.g., when a balloon records temperature as a function of time and we want to record its lat and lon positions as a function of time; one could conceivably want to plot the temperature as a function of latitude and/or longitude, with one or the other of them providing the positions along a coordinate axis). 

I agree that saying lat(x,y) is a "y axis" seems rather odd, but if you consider each x,y pair to define an index, then it might be tolerable to say they each could be regarded as parametric axes defined as a function of an index. 

In both cases, of course, the axis values may not be monotonic, so they wouldn't be considered coordinates axes themselves. 

It's really not a pretty situation. Not sure what can be done about it. 

best regards, 
Karl 


On 4/4/17 1:49 PM, Jim Biard wrote: 




Jonathan, 

But was the axis attribute intended for use on 2D auxiliary coordinate variables? Perhaps that was before my time, but I don't recall seeing any discussion where that use was advocated. 

Jim 

On 4/4/17 4:30 PM, Jonathan Gregory wrote: 



Dear David and Jim

Before CF 1.6, the axis attribute was allowed only for (Unidata) coordinate
variables (i.e. the 1D ones whose name equals their dimension name). In CF 1.6
it was generalised to be allowed for auxiliary coordinate variables, as
described in the preamble of sect 5. I wasn't really in favour of this change,
but the majority was.

Best wishes

Jonathan

----- Forwarded message from Jim Biard <jbiard at cicsnc.org> ----- 



Date: Fri, 31 Mar 2017 12:44:11 -0400
From: Jim Biard <jbiard at cicsnc.org> CC: CF Metadata <cf-metadata at cgd.ucar.edu> Subject: Re: [CF-metadata] CF compliant tripolar grid representation
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:45.0)
	Gecko/20100101 Thunderbird/45.8.0

David,

Yes. I think the wording could stand to be clearer. What I wonder is
what use is there for identifying a 2D grid of latitude values as
being an axis? I do a lot of satellite swath imagery and have worked
with polar stereographic data, and latitude is not an axis of my
measurement variable grid in either case.

I think part of the confusion arises from a somewhat unclear
definition of coordinate. I tend to use the phrase "true coordinate"
for one that is1-D, has a variable name equal to its dimension name,
is monotonic, has no fill values, etc, versus "auxiliary coordinate"
for one that doesn't meet one or more of those requirements. I
generally assume that true coordinates are being referred to when I
see the word coordinate in the Conventions unless it's made clear
that is not the case (as in Section 5 paragraph 6). With that
reading, the coordinate type and dimension type are one in the same
in Section 4 paragraph 2, since only true coordinate variables are
being discussed.

Grace and peace,

Jim

On 3/31/17 12:28 PM, David Hassell wrote: 



Hi Jim,

I agree you with in spirit, but the conventions do say that the
axis attribute as being there to identify the *coordinate* type,
rather than the *dimension* type (section 4, paragraph 2). Perhaps
the wording here could be tightened up to say dimension type? I
wonder how the axis attribute has been used over the last 6 years
since 1.6 was released?

All the best,

David

On 31 March 2017 at 17:04, Jim Biard < jbiard at cicsnc.org <mailto:jbiard at cicsnc.org> > wrote:

   David,

   As I read the Conventions, the axis attribute is to be applied to
   coordinate variables (Section 4. Coordinate Types and Section 5.
   Coordinate systems) to indicate that this variable can be treated
   as representing an dimensional axis of corresponding variable
   grids. Section 5 paragraph 6 talks about how it is still possible
   to figure out that an auxiliary coordinate variable is a
   spatiotemporal dimension of the if the axis attribute is not
   present. I don't think a 2D auxiliary coordinate variable can be
   considered to be a dimensional axis, can it?

   Grace and peace,

   Jim


   On 3/31/17 11:52 AM, David Hassell wrote: 



Hello Sébastien and Jim,

       You are right to feel weird about identifying 2D lat and lon
       as Y and X axes. The axis attribute should never be applied
       to 2D variables. It is only valid for 1D "true" coordinate
       variables.

   The axis attribute can be attached to auxiliary coordinate
   variables with any number of dimensions. I would agree, though,
   that attaching the axis=X attribute to a 2-d longitude auxiliary
   coordinate variable is likely to confuse. The axis attribute's
   purpose is merely to make identification easier, but as long
   there are units of degrees_east (mandatory) and a standard name
   of longitude (optional), humans and software alike should be happy.

   All the best,

   David


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Cooperative Institute for Climate and Satellites NC 
North Carolina State University 
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formerly NOAA’s National Climatic Data Center 
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e: jbiard at cicsnc.org 
o: +1 828 271 4900 

Connect with us on Facebook for climate and ocean and geophysics information, and follow us on Twitter at @NOAANCEIclimate and @NOAANCEIocngeo . 

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