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General Philosophy of Table Driven Code Forms

General Philosophy of Table Driven Code Forms. Simon Elliott , EUMETSAT E va Červená, CHMI. Code Structures BUFR and CREX Tables Features Common to BUFR and CREX Differences Between BUFR and CREX. Table driven codes.

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General Philosophy of Table Driven Code Forms

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  1. General Philosophy of Table Driven Code Forms Simon Elliott, EUMETSAT Eva Červená, CHMI Training on Meteorological Telecommunications Alanya, Turkey, 22-30 September 2010

  2. Code Structures • BUFR and CREX Tables • Features Common to BUFR and CREX • Differences Between BUFR and CREX Training on Meteorological Telecommunications Alanya, Turkey, 22-30 September 2010

  3. Table driven codes • GRIB - Processed data in the form of grid-point values expressed in binary form. • Forecast, analysis, climatology, satellite images and products • BUFR - Binary universal form for the representation of meteorological data. • Observations, satellite products • CREX - Character form for the representation and exchange of data. • Like simple BUFR but human readable ASCII Training on Meteorological Telecommunications Alanya, Turkey, 22-30 September 2010

  4. General structure • Indicator section: GRIB/BUFR/CREX • Identification section: Date, time, originator, table versions ... • Optional section: Metadata (potentially XML), private data … • Data description: What sort of data follows • Data section: actual data here • Closure: “7777” Table driven codes generally have this structure  Training on Meteorological Telecommunications Alanya, Turkey, 22-30 September 2010

  5. Some features • Data are self defining: type, content, edition, section lengths,etc. • Data identification is at fixed offsets for quick access without decoding • Binary data (GRIB and BUFR) are not really human readable • Optional section can contain anything Training on Meteorological Telecommunications Alanya, Turkey, 22-30 September 2010

  6. Consider BUFR • Table B - Element descriptors • Start with ‘0’, temperature, pressure, etc. • Table C - Operator descriptors • Start with ‘2’, change of scale, qc follows, etc. • Table D - Sequence descriptors • Start with ‘3’, wind sequence, high accuracy location sequence, etc. • Code and flag tables - • Type of cloud, vertical sounding significance, etc. Training on Meteorological Telecommunications Alanya, Turkey, 22-30 September 2010

  7. Table B Example: 0-20-010 Cloud cover (total) % 0 0 7 Element descriptor Entry in class Unit Reference value Class 20, observed phenomena Element name Scale Data width (bits) Training on Meteorological Telecommunications Alanya, Turkey, 22-30 September 2010

  8. Scale, reference value and data width Encoding: value = (obs x [10^”scale”]) - “reference” e.g.: Let scale=1, reference=-50, width=7 then, if obs=-1.5, encoded value is 35 allows obs from -5.0 to +7.6, steps of 0.1 Decoding: obs = (value + “reference”) ÷ [10^”scale”] e.g.: Let scale=-2, reference=5, width=4 then, if value=14, decoded value is 1900 allows obs from 500 to 1900, steps of 100 Training on Meteorological Telecommunications Alanya, Turkey, 22-30 September 2010

  9. Table C Operator descriptors for more complex tasks, e.g. • Change width • Change scale • Change reference • Quality control information • First order statistics follow • Define “data present” bit map • Re-use “data present” bit map • Cancel “data present” bit map Training on Meteorological Telecommunications Alanya, Turkey, 22-30 September 2010

  10. Table D • Not strictly required • Saves space in Section 3 (list of descriptors) • One table D entry represents an agreed list of table B and/or C and/or D descriptors • 3-01-011 = 0-04-001 Year 0-04-002 Month 0-04-003 Day Training on Meteorological Telecommunications Alanya, Turkey, 22-30 September 2010

  11. BUFR is binary, better for computers BUFR has a compression scheme Encoded values are non-negative in BUFR Wide range of operators is available in BUFR BUFR mostly uses SI units (for OPMET data, kt, ft, ˚C are allowed) CREX is ASCII, easier for humans to read CREX has no explicit compression scheme Encoded values may be negative in CREX Only a subset of BUFR operators is used in CREX CREX allows other units (temperature can be Celsius or Kelvin) Differences between BUFR and CREX In general, BUFR and CREX are quite similar. CREX may be used as an interim solution if computers are not available or telecoms are inadequate ... Training on Meteorological Telecommunications Alanya, Turkey, 22-30 September 2010

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