NXdata: errors on auxiliary signals

[woutdenolf]

We have the VARIABLE_errors fields to specify the errors on all axes but we only have one errors field to specify the errors on the signal. I would propose DATA_errors to specify errors on all signals (auxiliary and default).

data:NXdata
   @axes=["x"]
   @signal="data1"
   @auxiliary_signals=["data2", "data3"]
   x: [1024]
   data1: [1024]
   data2: [1024]
   data3: [1024]
   x_errors: [1024]
   data1_errors: [1024]
   data2_errors: [1024]
   data3_errors: [1024]

data1_errors is errors in the current definition and data2_errors, data3_errors do not exist in the current definition.

[woutdenolf]

Looking very closely at the definition it seems that VARIABLE_errors can refer to any field

<field name="VARIABLE_errors" type="NX_NUMBER" nameType="any">

But we also have

<field name="VARIABLE" type="NX_NUMBER" nameType="any">

which is for axes only. So probably we should just rename VARIABLE_errors to FIELD_errors and in the comments say it applies to any field, whether it is an axis, signal or auxiliary signal.

[woutdenolf]

Hmm it's more complicated. It says

"Errors" (actually *uncertainties*) associated with axis ``VARIABLE``. 

so VARIABLE_errors applies to axes only. But then you have this paragraph

This pattern of using ``VARIABLE_errors`` can be used ...
to associate uncertainties with a dataset named ``VARIABLE``.

So the _errors pattern might apply to auxiliary signals, depending on how you interpret this text.

definitions/base_classes/NXdata.nxdl.xml

Lines 302 to 313 in a10bd5e

	<field name="VARIABLE_errors" type="NX_NUMBER" nameType="any">
	<doc>
	"Errors" (actually *uncertainties*) associated with axis ``VARIABLE``.
	Client is responsible for defining the dimensions of the data.
	The name of this field may be changed to fit the circumstances
	but is matched with the *VARIABLE* field with ``_errors`` appended.
	This pattern of using ``VARIABLE_errors`` can be used
	throughout a NeXus data file to associate uncertainties
	with a dataset named ``VARIABLE``. This pattern also
	applies to other relationships such as ``VARIABLE_resolutions``
	to connect additional data with a certain dataset.

[woutdenolf]

To summarize: currently NXdata does not allow errors on auxiliary signals.

Proposal:

• renaming the VARIABLE field to AXISNAME
• updating the doc of VARIABLE_errors to say it applies to axes, signal or auxiliary signals: this is the actual change, although it is already somewhat support by NeXus as described here.
• deprecate errors in favor of VARIABLE_errors for the signal field

[benajamin]

NIAC agrees that this is just a documentation bug that should be fixed. Also the errors field was already decided to be deprecated in 2018.