issue_comments: 419187692

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https://github.com/pydata/xarray/issues/2368#issuecomment-419187692 https://api.github.com/repos/pydata/xarray/issues/2368 419187692 MDEyOklzc3VlQ29tbWVudDQxOTE4NzY5Mg== 1197350 2018-09-06T18:02:19Z 2018-09-06T18:02:19Z MEMBER

@dopplershift - thanks for the clarifications! I agree that it's good for netCDF to be as open-ended as possible.

So I guess my quarrel is with the CDM. This is what it says about variables and dimensions:

A Variable is a container for data. It has a DataType, a set of Dimensions that define its array shape, and optionally a set of Attributes. Any shared Dimension it uses must be in the same Group or a parent Group.

A Dimension is used to define the array shape of a Variable. It may be shared among Variables, which provides a simple yet powerful way of associating Variables. When a Dimension is shared, it has a unique name within the Group. If unlimited, a Dimension's length may increase. If variableLength, then the actual length is data dependent, and can only be found by reading the data. A variableLength Dimension cannot be shared or unlimited.

then later

A Variable can have zero or more Coordinate Systems containing one or more CoordinateAxis. A CoordinateAxis can only be part of a Variable's CoordinateSystem if the CoordinateAxis' set of Dimensions is a subset of the Variable's set of Dimensions. This ensures that every data point in the Variable has a corresponding coordinate value for each of the CoordinateAxis in the CoordinateSystem.

A Coordinate System has one or more CoordinateAxis, and zero or more CoordinateTransforms.

A CoordinateAxis is a subtype of Variable, and is optionally classified according to the types in AxisType.

These are the rules which restrict which Variables can be used as Coordinate Axes:

Shared Dimensions: All dimensions used by a Coordinate Axis must be shared with the data variable. When a variable is part of a Structure, the dimensions used by the parent Structure(s) are considered to be part of the nested Variable.

I have a very hard time understanding what all of this means. Can the same variable be a "Dimension" and a "CoordinateAxis" in CDM?

It seems much simpler to me to use the CF approach to describe the physical coordinates of the data using "auxiliary coordinate variables" and to keep the dimensions as purely 1D "coordinate variables".

IMO, xarray is being overly pedantic here.

What would you like xarray to do with these datasets, given the fact that orthogonality of dimensions is central to its data model?

 
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