[webservices] A question of location ID, how to represent empty IDs in XML?

Tue Aug 12 10:53:26 PDT 2014

I've been following this thread, and thought it was time to chime in.

IMHO, the FDSN web services should follow the SEED convention.
The SEED convention states that station, network, channel, and location
are all blank-padded fields of fixed lengths.
To me, this means that that we should either use the full blank-padded
fields for ALL of these identifiers, or for none of them.

eg:

<Network code="G " >
<Station code="KIP  ">
<Channel locationCode="  " code="BHZ">

or

<Network code="G" >
<Station code="KIP">
<Channel locationCode="" code="BHZ">

Personally I think the latter (blank trimmed) is better.

I agree that the blank location code is a pain when dealing with
Oracle, white-space delimited fields such as command lines, etc,
but unless we change the SEED convention, I don't see that making
an aliases of "-" or "--" in FDSN station XML improves the situation.

AFAIK, the ONLY reason that we struggle with the two-blank issue is
that certain software (eg Oracle) cannot distinguish between the
the empty string (string of length 0) and NULL.  Therefore, the DMC,
NCEDC, AQMS, etc have been forced to not use a blank-trimmed string
for the location code.

Unless we propose to change the SEED standard, all of our data in
our archives, and all of our current acquisition systems, I think
that we have to live with "emtpy" location codes.

I have not seen any compelling argument for representing a blank (empty)
location code in FDSN station XML as anything but the empty string.

If you want to have "" and "  " be equivalent in FDSN station XML,
you can simply change the schema definition of the field to be a "token"
rather than a "string", in which case any representation with blanks will
be reduced to the empty string.  Problem solved?

I note that the NCEDC implementation currently uses 1 blank " "
for empty location code.  I have no problem changing this if we can
agree on a convention.

I also note ironically that the TA network run by IRIS is one of the
largest networks in terms of stations, and uses blank location codes.

My 2 cents...

- Doug N

On 07/23/2014 10:30 AM, Chad Trabant wrote:
>
> Hello WS users and developers,
>
> A recent discussion between FDSN data centers is centered on
> representation of empty location IDs in StationXML, the default
> format returned by the fdsnws-station web service. The DMC may be
> changing how it represents location ID in XML and text formats based
> on these discussions. We are asking for input as any such change will
> effect users of our metadata service.
>
> Some background: In the SEED channel naming scheme there is a
> hierarchy of network, station, location and channel identifiers. Of
> these, it is only the location ID that is commonly accepted to be
> empty. In the SEED format the location ID is a two-character field,
> where the value is left justified and padded with spaces if needed.
> When the value is empty the field is simply two spaces of padding.
>
> Historically, and presumably to avoid having an empty location ID,
> the DMC has represented “empty” location IDs as a string of two
> spaces. Following this practice, we express this in StationXML by
> setting the locationCode attribute to a string of two spaces. We have
> done this so long we sometimes forget that it is not compliant with a
> strict reading of SEED, at best it falls into the vagaries of SEED,
> on the other hand we have been doing it for years with no apparent
> problems (in fact it has helpfully avoided an empty core
> identifier).
>
> There now exists another fdsnws-station implementation that returns
> StationXML with the locationCode attribute set to an empty string
> when the SEED value is empty. The justification is that this follows
> the SEED rules of trimming the padding spaces from the values.
>
> Unfortunately this means there are now flavors of StationXML that are
> incompatible in the core channel name identifiers. In other words,
> two StationXML documents for the same SEED channel appear, without
> extra field translation, to be different channels.
>
> As most of you are users of SEED and StationXML metadata (at some
> level) and some of you have written code to parse these formats and
> manage the data returned by the DMC and other FDSN data centers, we
> are asking for your input regarding the potential solutions.
>
> Here are the options being considered for mapping an empty location
> ID in SEED to StationXML:
>
> 1) Set locationCode to two spaces. While the DMC and users have been
> using this for a long while, it is not precisely the SEED value (but
> the mapping could be formalized). Also, whitespace in attributes does
> have some theoretical challenges: the wonky rules for XML attributes
> related to whitespace handling require removal of spaces in some
> cases (we have never heard of problems though).
>
> 2) Set locationCode to an empty string. This would match the strict
> value present in SEED, an empty identifier.
>
> 3) Set locationCode to “--“ (two dashes). This avoids issues with
> whitespace in XML attribute values and avoids issues with an empty
> identifier. Also, this matches the request mechanisms where “--“ is
> accepted as a synonym for an empty location ID.
>
> All of these solutions are viable in that we can make them work in
> code, it is a matter of choosing one for future FDSN metadata, pick
> your poison so to speak.
>
> In my personal opinion, an empty location ID is an unfortunate quirk
> of SEED that we should rectify in StationXML. An empty identifier can
> be confused for “unknown” if the programmer is not careful, which is
> semantically very different than “set to empty”. The two-space
> strings that the DMC is currently using are also not ideal, they are
> hard for humans to read and potentially weird with XML rules. The
> dashed location ID avoids these issues but requires the most change.
> I also think requiring all readers of StationXML to translate (e.g.
> remove padding) is a bad idea, the values in SEED should be uniquely
> mapped to values in StationXML.
>
> Thanks for reading this far.  Your opinion and input is appreciated.
>
> regards,
> Chad
>
>
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>

-- 
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Doug Neuhauser                  University of California, Berkeley
doug at seismo.berkeley.edu        Berkeley Seismological Laboratory
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