[WMO Codes - DRC #291] (Validated) Addition of a new note to GRIB2 code table 3.2 ? Shape of the Earth

This is a step foreward and I consider it harmless, though the discussion is slightly uncomfortable for me --- we can't touch poles, and the humans have had different CRSs with respect to latitude, not only for longitude.

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Issue #291 has been updated by Jitsuko Hasegawa.

• Status changed from ET discussion to Validated
• Target version set to InterCBS2016

Final Report of IPET-DRMM-III 2015 Beijing says:

Several entries in GRIB Code table 3.2 (Manual on Codes, Volume I.2 (WMO-No.306)) specify the shape of the Earth without sufficient information to interpret coordinates, signifying positions on the earth's surface. This allows GRIBs to be encoded with latitude/longitude coordinates which may or may not be understood as the encoding user intended, because the reference prime meridian is left to the user's interpretation.

Ms Sibylle Krebber, Deutcher Wetter Dienst (DWD) presented her proposal, highlighting the issue of ambiguity in a coordinate system used in GRIB messages.

Historically, easting coordinates based on differing easting origins have led to confusion in many circumstances. Although the latitudinal origin is easily defined based on the earth's rotational characteristics, the longitudinal origin is a matter of arbitrary definition, creating the need for the definition and maintenance by an external authority, which are referred in the Code figures 2, 4, 5, 8 and 9 in the Code table 3.2, but the other code figures remain ambiguous.

The meeting reviewed the current Code table 3.2 and agreed that the new note proposed by Ms Krebber will facilitate WMO Members to understand the ambiguity of some coordinate systems in the GRIB Code table 3.2, which is to be approved by the procedure for adoption between CBS sessions as shown in the Annex to this paragraph.

Dr Enrico Fucile offered ECMWF's assistance of the validation.

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DRC #291: Addition of a new note to GRIB2 code table 3.2 – Shape of the Earth

• Author: Jitsuko Hasegawa
• Status: Validated
• Priority: Normal
• Assignee: Eizi TOYODA
• Category: GRIB2 structure
• Target version: InterCBS2016

The proposal was made to add a new note to GRIB2 code table 3.2 – Shape of the Earth, as agenda item 2.1(1) of IPET-DRMM-III, to reduce ambiguity in interpreting coordinates.

Please see linked document for further background.

Proposed additional note
(2) Coordinates can only be unambiguously interpreted if the coordinate reference system in which they are embedded is known. Therefore, defining the shape of the earth alone without coordinate system axis origins is ambiguous. Generally, the prime meridian defined in the geodetic system WGS84 can be safely assumed to be the longitudinal origin. However, because these code figures do not specify the longitudinal origin explicitly, it is suggested to contact the originating center if high precision coordinates are needed in order to obtain the precise details of the coordinate system used.

Document: IPET-DRMM-III Doc 2.1.1

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