Hello Everyone,
To expand on some previous points...
* The WMO does maintain machine-readable versions of the tables, for
both BUFR and GRIB, at
http://www.wmo.int/pages/prog/www/WMOCodes/TDCFtables.html Thanks
to Paolo for pointing this out in his earlier email. Different
versions of the tables can be downloaded from this site. For BUFR
at least, table version 13 is a superset of all previous versions,
so it can be used to decode BUFR messages from all previous
versions. This is no longer true beginning with version 14, where
it's possible that some deprecated items have now been removed or
that some descriptor characteristics have been modified from
previous versions of the table. So for BUFR, decoding centers
should maintain copies of tables 13, 14 and onward in whatever
format(s) are required by their local processing software.
* John rightly points out that the proper use of this table version
number by message originators would eliminate the problem outlined
in his paper. In my experience, this problem stems mostly from a
casual attitude by originators in ensuring they've used the proper
version number in their messages. Many originators use software
where this number is often hardcoded and so becomes, at best, an
afterthought. This is an education issue that WMO is working hard
to address among its members.
* There's also a concerted effort among members to develop BUFR
templates for certain types of commonly-reported data such as
SYNOP, BUOY, TEMP/PILOT, CLIMAT, etc. This is a by-product of
WMO's ongoing migration from these old alphanumeric fixed-field
formats to BUFR. The list of templates is available at
http://www.wmo.int/pages/prog/www/WMOCodes/TemplateExamples.html#Regulations
, and while their use isn't mandatory, it does make things a lot
simpler for downstream codes which have to interpret the decoded
output, and which is another point that John made in his paper.
* If anyone has a requirement for a new BUFR or GRIB2 descriptor
which they feel would be reasonable to propose as a new official
WMO descriptor (vs. just using their own local descriptor number),
please let me know. I represent the U.S. to the WMO codes group
which reviews and approves these types of requests. Depending on
the nature of the request, there are fast-track procedures
available which can lead to formal approval within a matter of 2-3
months.
* As originally envisioned, BUFR and GRIB2 weren't designed to be
formats for archive storage, but rather for efficient real-time
exchange of meteorological data. Nevertheless, this doesn't mean
they can't be used as archive formats. We do this here at NCEP,
and the approach we use involves storing a copy of the applicable
table with each archived dataset. Note that, for BUFR at least,
table information can be encoded into BUFR messages using
descriptors from Class 0 of Table B. When this is done, the
necessary table information can be easily retained alongside the
data in a very compact and efficient manner, using one or two
additional BUFR messages at the head of each archived file. Such
an approach could even be used when exchanging real-time data sets
between centers, at the cost of one or two additional BUFR
messages. This would eliminate the problem of receiving centers
having to "guess" whether the table version number in each data
message was encoded properly.
* In my opinion, when everyone follows the rules (e.g. using
official descriptors with proper table version numbers), the
process works very well. The trick of course is to get everyone
(and their software) to pay attention to the rules. But this is
true of any format and is not unique to BUFR and GRIB.
With best regards,
-Jeff
On 3/29/2011 6:26 AM, Enrico Zini wrote:
[resending because the first attempt apparently did not make it to the
list]
On Wed, Mar 09, 2011 at 12:10:31PM -0700, John Caron wrote:
Apologies for the long hiatus on this list.
I have written a brief report about BUFR/GRIB with a (possibly
controversial) recommendation. Feel free to forward to anyone who
might be interested.
http://www.unidata.ucar.edu/staff/caron/bufr/Summary.html
Hello,
from the experience[1][2][3] I have with BUFR messages, I see a few
problems with your proposal:
1. it would imply that BUFR decoding can only happen when/where there
is network connectivity and the central server is working. I am not
comfortable in tying a long lived archive to the existance of a 3rd
party server;
2. alternatively, the archive needs to store and maintain up to date an
entire mirror of all the tables mentioned by all the BUFRs it
contains, and that more or less what we already have, barring the
proposal to standardise a file format for storing tables.
But if you retrofit the system that we have now with a standard file
format for tables and a working central repository, you basically
fix it without the need for hash codes;
3. 16bits (0-65535) are imo not that big a hash space: when you allow
everyone to create new tables at will, things may degenerate
quickly.
But the biggest problem I have is this: you do need to maximise reuse of
BUFR table codes, otherwise the problem of making sense of the decoded
data is not machine computable anymore.
I am maintaining software that not only decodes BUFR bulletins, but also
tries to make sense of them: for example, it can understand that a given
decoded value is a temperature, that it is sampled at a given vertical
level and that it went through a given kind of statistical processing.
That is, it can decode a bulletin and say:
"There is a temperature reading at 2 meters above ground, maximum over
12 hours."
This interpreted information can be used by meteorologists without
having to be aware that temperatures can come as B12001, B12101, B12111,
B12112, B12114..B12119 or what else. Where I work, the possibility to do
this is considered a very valuable resource, as it allows to uniformly
compare readings from different sources.
If you have a process where data sharing across centers has to use some
well standardised, well known tables (as well as some reasonable
standards, or even just practices, for laying out BUFR templates), you
can code (I have coded) that sort of interpretation in software. If
instead anyone can at any point start distributing BUFRs that can use
any B code they want to represent temperature, then the only way to make
sense of a decoded bulletin is to have it personally read by an
experienced meteorologist.
Even if you don't want machine interpretation of the bulletins, if the
lifetime of the archive is long enough then its data can potentially
outlive the availability of experienced meteorologists who can remember
how to make sense of them.
To have a long lived archive, IMO what is needed are pervasive
standards, stable over time. Instead of designing for chaos, I'd rather
see how to make coordination work: propose a standard file format for
distributing tables; propose the creation of a repository where to
download the WMO standard table; propose a process for submission of new
table entries, akin to what happens with submissions of new code points
to UTF-8, or new locales to ISO. My feeling is that something like UTF-8
is more like the kind of thing to model BUFR tables on.
Of course chaos should still be supported, because scientists have to
have full freedom of experimentation. But there are already local table
numbers that can be used for that, and after the experiments are
successful the new entries can be submitted to a new version of the
shared tables, so that the shared language can grow.
[1] http://www.arpa.emr.it/dettaglio_documento.asp?id=2927&idlivello=64
[2] http://www.arpa.emr.it/dettaglio_documento.asp?id=514&idlivello=64
[3] http://www.arpa.emr.it/dettaglio_documento.asp?id=1172&idlivello=64
Ciao,
Enrico
