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Clearer terminology for coordinate variables #1296

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Mar 15, 2017
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Clearer terminology for coordinate variables #1296

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Clearer terminology for coordinate variables
fmaussion Mar 5, 2017
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Update terms
fmaussion Mar 6, 2017
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44 changes: 31 additions & 13 deletions doc/data-structures.rst
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Expand Up @@ -199,6 +199,8 @@ Coordinates can also be set or removed by using the dictionary like syntax:
del foo['ranking']
foo.coords

For more details, see :ref:`coordinates` below.

Dataset
-------

Expand Down Expand Up @@ -472,19 +474,35 @@ objects in the ``coords`` attribute:
ds.coords

Unlike attributes, xarray *does* interpret and persist coordinates in
operations that transform xarray objects.

One dimensional coordinates with a name equal to their sole dimension (marked
by ``*`` when printing a dataset or data array) take on a special meaning in
xarray. They are used for label based indexing and alignment,
like the ``index`` found on a pandas :py:class:`~pandas.DataFrame` or
:py:class:`~pandas.Series`. Indeed, these "dimension" coordinates use a
:py:class:`pandas.Index` internally to store their values.

Other than for indexing, xarray does not make any direct use of the values
associated with coordinates. Coordinates with names not matching a dimension
are not used for alignment or indexing, nor are they required to match when
doing arithmetic (see :ref:`coordinates math`).
operations that transform xarray objects. There are two types of coordinates
in xarray:

- **dimension coordinates** are one dimensional coordinates with a name equal
to their sole dimension (marked by ``*`` when printing a dataset or data
array). They are used for label based indexing and alignment,
like the ``index`` found on a pandas :py:class:`~pandas.DataFrame` or
:py:class:`~pandas.Series`. Indeed, these "dimension" coordinates use a
:py:class:`pandas.Index` internally to store their values.

- **non-dimension coordinates** are variables that contain coordinate
data, but are not a dimension coordinate. They can be multidimensional
(see :ref:`examples.multidim`), and there is no relationship between the
name of a non-dimension coordinate and the name(s) of its dimension(s).
Non-dimension coordinates can be useful for indexing or plotting; otherwise,
xarray does not make any direct use of the values associated with them.
They are not used for alignment or automatic indexing, nor are they required
to match when doing arithmetic
(see :ref:`coordinates math`).

.. note::

xarray's terminology differs from the `CF terminology`_, where the
"dimension coordinates" are called "coordinate variables", and the
"non-dimension coordinates" are called "auxiliary coordinate variables"
(see :issue:`1295` for more details).

.. _CF terminology: http://cfconventions.org/cf-conventions/v1.6.0/cf-conventions.html#terminology


Modifying coordinates
~~~~~~~~~~~~~~~~~~~~~
Expand Down