MARS Key Mapping

The importer reads the following MARS namespace keys from each GRIB message using ecCodes’ read_key_dynamic API.

Keys Extracted

Identification

GRIB Key	Description	Example
`class`	MARS class	`"od"` (operational)
`type`	Data type	`"an"` (analysis), `"fc"` (forecast)
`stream`	Data stream	`"oper"`, `"enfo"`
`expver`	Experiment version	`"0001"`

Parameter

GRIB Key	Description	Example
`param`	Parameter ID	`"2t"` (2m temperature)
`shortName`	Short name	`"2t"`
`name`	Full name	`"2 metre temperature"`
`paramId`	Numeric ID	`167`
`discipline`	WMO discipline	`0`
`parameterCategory`	WMO category	`0`
`parameterNumber`	WMO number	`0`

Vertical

GRIB Key	Description	Example
`level`	Level value	`500`
`typeOfLevel`	Level type	`"isobaricInhPa"`
`levtype`	MARS level type	`"pl"` (pressure level)

Temporal

GRIB Key	Description	Example
`date` / `dataDate`	Reference date	`20260404`
`time` / `dataTime`	Reference time	`1200`
`stepRange` / `step`	Forecast step	`"0"`, `"6"`, `"0-6"`
`stepUnits`	Step units	`1` (hours)

Spatial

GRIB Key	Description	Example
`gridType`	Grid type	`"regular_ll"`
`Ni`, `Nj`	Grid dimensions	`360`, `181`
`numberOfPoints`	Total grid points	`65160`
`latitudeOfFirstGridPointInDegrees`	First latitude	`90.0`
`longitudeOfFirstGridPointInDegrees`	First longitude	`0.0`
`latitudeOfLastGridPointInDegrees`	Last latitude	`-90.0`
`longitudeOfLastGridPointInDegrees`	Last longitude	`359.0`
`iDirectionIncrementInDegrees`	Longitude step	`1.0`
`jDirectionIncrementInDegrees`	Latitude step	`1.0`

Other

GRIB Key	Description	Example
`bitsPerValue`	Packing precision	`16`
`packingType`	GRIB packing	`"grid_simple"`
`centre`	Originating centre	`"ecmf"`
`subCentre`	Sub-centre	`0`
`generatingProcessIdentifier`	Process ID	`148`

Storage in Tensogram

Given N GRIB messages in merge-all mode:

Extract all MARS keys from each message using read_key_dynamic
Store ALL keys for each GRIB message in the corresponding base[i]["mars"] entry independently
There is no common/varying partitioning in the output — each base[i] entry is self-contained

graph TD
    A[N GRIB messages] --> B[Extract MARS keys from each]
    B --> C["Store in base[i] independently"]
    C --> D["base[0]: all keys from GRIB msg 0"]
    C --> E["base[1]: all keys from GRIB msg 1"]
    C --> F["base[N-1]: all keys from GRIB msg N-1"]

If you need to extract commonalities after decoding (e.g. for display), use the compute_common() utility in software.

Sentinel Handling

ecCodes uses sentinel values for missing keys:

String: "MISSING" or "not_found" → skipped
Integer: 2147483647 or -2147483647 → skipped
Float: NaN or Inf → skipped

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Tensogram