Some older GRIB decoders used to work on a number-based table to retrieve all the information from the message. This approach forced the user either to learn a code table or to use the documentation intensively. With ecCodes a key name based access is provided so that all the information contained in the GRIB message is retrieved through alphanumeric names.
Some key names are built from the official WMO documentation on the GRIB edition 1 and edition 2 coding standard removing the spaces in the key description and capitalizing the initials so that the caption:
is transformed into the key name:
Some short names (aliases) are also provided, e.g. "centre" is an alias for identificationOfOriginatingGeneratingCentre. The names are always easily related to the meaning of their value.
A different set of keys is available for each message because the content is different. It is easy to find the keys available in a message by using the GRIB tools (grib_dump) or the library (keys_iterator.c).
Coded and Computed keys
There are two different types of keys: coded and computed.
The coded keys are directly linked to octets of the GRIB message and their value is obtained by only decoding the octets. A list of all the coded keys in a message can be obtained using grib_dump without any option (use the -a option to obtain also their aliases).
The computed keys are obtained by combining other keys (coded or computed) and when their value is set all the related keys are set in a cascade process.
These keys provide a synthesis of the information contained in the GRIB message and are a safe way to set complex attributes such as the type of grid or the type of packing. They are also helpful in the interpretation of some octets such as the scanning mode whose bits are related to the way of scanning the grid. In this case the computed keys:
will provide access to single bits of the scanning mode octet hiding its structure from the user.
The keys can also have some attributes such as read only, which means that the key cannot be set (e.g. 7777 at the end of the message), or edition specific that is the attribute of all the keys having different values in the two editions (e.g. longitudeOfFirstGridPoint) or being present in one edition only (e.g. alternativeRowScanning).
Moreover there are some computed keys that cannot be "get" and can be considered as functions acting on the GRIB message in some way. These keys are always characterised by a predicate in their name (e.g. setDecimalPrecision, setBitsPerValue).
For the computed keys we provide the following preliminary documentation.
All MARS keywords are available. Some examples are:
Angles in degrees
All the angle variables are provided in two versions, a native one with the units coded into the GRIB file and an edition-independent one in degrees. It is always better to work with the "in degrees" version that is always provided through the key which has the same name of the native version with the suffix "InDegrees":
The type of grid computed from the grid description section.
- For both editions:
- For edition 2 only:
The algorithm used to pack data into the GRIB message.
- For GRIB edition 1:
- For GRIB edition 2:
is a function key used to set the decimal precision. See the grib_set page for usage.
The number of values coded into the data section of the GRIB message