clarify meaning of level=response

[crotwell]

Current spec says:

The ​ level​ parameter controls the amount of detail included in the returned FDSN StationXML results
with the following hierarchy: ​ network​, ​ station​, ​ channel​ and ​ response​ . For example, if the ​ station​ level
is requested the Station elements will be included in the results but not Channel or Response related
information.

but does not clarify StationXML output of the <Response> element when level=response vs level=channel. Current implementations, I think, use:

• level=channel means include the <Response> element and <InstrumentSensitivity> or <InstrumentPolynomial> if exists, but do not include any <Stage> elements.
• level=response means include above, as well as all <Stage> elements.

The current wording kind of makes it seem like level=channel should not include any response information, but including the scalar response is useful, easy and matches current practice.

[calum-chamberlain]

Kia ora all - related to this, it would be helpful to clarify what is expected if level=response (or any more detailed level), but no response information is available. This is related to this issue at GeoNet, where we have some legacy stations that have minimal metadata (station location) but no response information is available.

[crotwell]

See here for how to represent unknown or missing response in StationXML:
http://docs.fdsn.org/projects/stationxml/en/latest/reference.html#response

In cases where the response is unknown, for example really old channels, or where a response is not applicable, like textual log channels, it is preferred that an empty response element be used, <response></response>, to positively indicate that no response exists.

[calum-chamberlain]

Thanks @crotwell that is really helpful - ignore the above comment then.