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Ticket Resolution Summary Owner Reporter
#104 fixed Clarify the interpretation of scalar coordinate variables davidhassell jonathan
Description

David Hassell and I propose the following changes to the CF standard document to clarify the interpretation of scalar coordinate variables. We do not think this is a material change to the convention, which already implies this interpretation, and we believe this interpretation is what was intended when scalar coordinate variables were introduced.

Section 5.7, Scalar coordinate variables.

The convention contains the following sentences:

Under COARDS the method of providing a single valued coordinate was to add a dimension of size one to the variable, and supply the corresponding coordinate variable. 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.

These sentences are OK as they stand, we think, but it would be better to describe the current situation without emphasising its history, so we would propose to replace them with the following. In addition, we propose to add a bit extra, as shown, to the second sentence below:

The use of scalar coordinate variables is a convenience feature which avoids adding size one dimensions to variables. A scalar coordinate variable has the same information content and can be used in the same contexts as a size one coordinate variable, if numeric, or a size one auxiliary coordinate variable, if a string (Section 6.1).

The next sentence would be unchanged; it mentions how this situation relates to that of the COARDS convention.

At the end of the section, after the example, we propose to append the following sentences:

If a data variable has two or more scalar coordinate variables, they are regarded as though they were all independent coordinate variables with dimensions of size one. If two or more single-valued coordinates are not independent, but have related values (for instance, time and forecast period, or vertical coordinate and model level number, Section 6.2), they should be stored as coordinate or auxiliary coordinate variables of the same size one dimension, not as scalar coordinate variables.

We think the above interpretation and implications are consistent with the convention already, which says in this section that, "Scalar coordinate variables have the same information content and can be used in the same contexts as a size one coordinate variable". However, it would be an improvement to spell it out.

Section 6.1, Labels

Replace the last sentence

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).

with

If a character variable has only one dimension (the maximum length of the string), it is a string-valued scalar coordinate variable (see Section 5.7, Scalar Coordinate Variables). As such, it has the same information content and can be used in the same contexts as a string-valued auxiliary coordinate variable of a size one dimension. This is a convenience feature which avoids adding the size one dimension to the data variable.

The last part is a repetition of what Section 5.7 says. The reason for the change is that the existing wording is careless in implying that a string could be a coordinate variable; in fact, this is not possible, since string- value coordinates must be auxiliary coordinate variables.

The interpretation of scalar coordinate variables in Section 9 may be different from the above, and this may require further clarification if the above is agreed.

#100 fixed Clarifications to the preamble of sections 4 and 5 davidhassell jonathan
Description

Dear all

I have the honour of opening ticket number 100. In this ticket, Steve Hankin and I have a number of changes to propose to the text of section 5, with the intention of clarifying it, not changing what it means. Therefore this is a defect ticket, but please object if you think it is changing the meaning, or making it any less clear!

Cheers

Jonathan

Replace this text in the first paragraph

A variable's spatiotemporal dimensions are used to locate data values in time and space. This is accomplished by associating these dimensions with the relevant set of latitude, longitude, vertical, and time coordinates.

with

A data variable's dimensions are used to locate data values in time and space or as a function of other independent variables. This is accomplished by associating these dimensions with the relevant set of latitude, longitude, vertical, time and any non-spatiotemporal coordinates.

These changes clarify that coordinate systems belong to data variables (rather than any other type of variable in a CF-netCDF file) and recognise the use of non-spatiotemporal dimensions.

Replace this text in the second paragraph

All of a variable's dimensions that are latitude, longitude, vertical, or time dimensions (see Section 1.2, "Terminology") must have corresponding coordinate variables, i.e., one-dimensional variables with the same name as the dimension

with

Any of a variable's dimensions that is an independently varying latitude, longitude, vertical, or time dimension (see Section 1.2, "Terminology") and that has a size greater than one must have a corresponding coordinate variable, i.e., a one-dimensional variable with the same name as the dimension

These changes remove the implication that spatiotemporal scalar coordinates might be prohibited, and allow for discrete axes (section 4.5, used extensively in section 9).

Replace this text in the third paragraph

All of a variable's spatiotemporal dimensions that are not latitude, longitude, vertical, or time dimensions are required to be associated with the relevant latitude, longitude, vertical, or time coordinates via the new coordinates attribute of the variable

with

Any longitude, latitude, vertical or time coordinate which depends on more than one spatiotemporal dimension must be identified by the coordinates attribute of the data variable.

This change is for clarity.

Replace this text in the fifth paragraph

The use of coordinate variables is required whenever they are applicable. That is, auxiliary coordinate variables may not be used as the only way to identify latitude and longitude coordinates that could be identified using coordinate variables.

with

If the longitude, latitude, vertical or time coordinate is multi-valued and varies in only one dimension, it is not permitted to store it as an auxiliary coordinate variable.

This change is for simplicity and clarity. The use of coordinate variables where applicable is already required by the second paragraph.

Append to the fifth paragraph

If the longitude, latitude, vertical or time coordinate is single-valued, it may be stored either as a coordinate variable with a dimension of size one, or as a scalar coordinate variable (Section 5.7).

This change recognises the use of spatiotemporal scalar coordinates.

#150 duplicate Clarification of use of standard region names in "region" variables. cf-conventions@… martin.juckes
Description

The CF standard name region has the current description "A variable with the standard name of region contains strings which indicate geographical regions. These strings must be chosen from the standard region list." This description implies that the variable should be of character type, but it is often more convenient to have an integer variable and make a clear link to the region names using flag_values and flag_meanings. The proposal is to clarify the definition so that either usage is acceptable and include an example of the latter usage in the convention text. It is also proposed that an appendix be added to the CF Convention text to state clearly any constraints on file meta-data which are implied by the CF Standard Name definitions, so that it is possible to test such constraints in the CF checker.

New description for CF standard name "region"

A variable with the standard name of region must have values associated with geographical regions from the CF standard region list, either as a character variable using the region names directly or as an integer variable with values linked to region names through flag_values and flag_meanings attributes.

New usage example in CF Convention text

The following should be placed at the end of 6.1.1, after example 6.2

A variable with standard name 'region' may also be of integer type and use 'flag_values' and 'flag_meanings' attributes to express the relationship between the integers and the region names:

integer basin(basin);
       standard_name: region;
       flag_values: '1 2 3';
       flag_meanings:'atlantic_arctic_ocean indo_pacific_ocean global_ocean';
......
values::
   basin: 1, 2, 3;

New Appendix Section

Change "Appendix B: Standard Name Table Format" to:

Appendix B: Standard Names

B.1: Standard Name Table Format

.....

and

B.2 Constraints for specific standard names

B.2.1: region

Variables with standard name region must be one of:

  • type character, with values taken from the CF standard region list;
  • type integer, with flag_values and flag_meanings attributes. The flag_meanings attributes must be a space separatd list of values from the CF standard region list (see example 6.2).
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