Hello CF
We have identified a perceived uncertainty in the CF conventions document which we strongly feel needs clarification. I would like to update the conventions document to remove this ambiguity. In order to update the text we have to agree on the objective.
(a statement of A or B is helpful; context on why will aid the decision making process)
The question is this: "Does a Scalar Coordinate Variable....":
Option A: Represent either a Coordinate Variable or an Auxiliary Coordinate? The presence of a scalar does not mandate the existence of a new dimension; it can imply an undeclared dimension of size one that is not explicitly defined in the file but it does not have to.
Or
Option B: Always represent a Coordinate variable which explicitly declares a dimension of size one, where this dimension is not stated in the file? An exception is provided for string scalar coordinate variables only, which are defined as Auxiliary Coordinates but also mandate a new dimension of size one.
Relevant Extracts from the CF Conventions
1.2 Terminology
http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.6/ch01s02.html
scalar coordinate variable
A scalar variable that contains coordinate data. Functionally equivalent to either a size one coordinate variable or a size one auxiliary coordinate variable.
5.7 Scalar Coordinate Variable
http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.6/ch05s07.html
When a variable has an associated coordinate which is single-valued, that coordinate may be represented as a scalar variable. Since there is no associated dimension these scalar coordinate variables should be attached to a data variable via the coordinates attribute.
...
The new scalar coordinate variable is a convenience feature which avoids adding size one dimensions to variables. Scalar coordinate variables have the same information content and can be used in the same contexts as a size one coordinate variable.
6 Labels and Alternative Coordinates
http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.6/ch06.html#labels
If a character variable has only one dimension (the maximum length of the string), it is regarded as a string-valued scalar coordinate variable, analogous to a numeric scalar coordinate variable (see Section 5.7, ?Scalar Coordinate Variables?<
http://cf-pcmdi.llnl.gov/documents/cf-conventions/1.6/ch05s07.html>)
Example (CDL)
{{{
Dimensions:
d1
d2
s1
s2
Variables
float data(d1,d2) ;
data:coordinates = "alpha, beta,gamma,delta,epsilon" ;
float d1(d1) ;
float d2(d2) ;
float alpha ;
float beta ;
int gamma ;
char delta(s1) ;
char epsilon(s2) ;
}}}
The file is CF compliant, my question is how to interpret the intent from the encoding?
Option A
- The data variable has at least 2 dimensions, defined by the coordinate variables d1 and d2.
- There are 5 variables which may be coordinate variables or auxiliary coordinate variables, the file does not define their type.
- alpha, beta and gamma may each be:
- a coordinate variable defining a new undeclared dimension
- an auxiliary coordinate describing a new undeclared dimension
- an auxiliary coordinate describing an undeclared dimension which is not new (has already been described by another coordinate)
- gamma and epsilon are auxiliary coordinates which may describe:
- a new undeclared dimension
- an undeclared dimension which is not new (has already been described by another coordinate)
Option B
- The data variable has 7 dimensions
- 2 are explicit in the file and defined by the coordinate variables d1 and d2.
- 5 are implicit, due to the presence of the variables alpha - epsilon
- each of these is treated as a coordinate variable defining an explicit dimension of the data
- gamma and epsilon are treated as special cases of string variables which are being used as coordinate variables but are typed as auxiliary coordinates
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Received on Wed May 08 2013 - 09:30:00 BST
