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NCEPLIBS-g2c

## Introduction

This document briefly describes the routines available for encoding/decoding GRIB Edition 2 (GRIB2) messages with the NCEPLIBS-g2c library.

A GRIB Edition 2 message is a machine independent format for storing one or more gridded data fields. Each GRIB2 message consists of the following sections:

    SECTION 0 - Indicator Section
    SECTION 1 - Identification Section
    SECTION 2 - (Local Use Section) - optional                           }
    SECTION 3 - Grid Definition Section                     }            }
    SECTION 4 - Product Definition Section    }             }            }(repeated)
    SECTION 5 - Data Representation Section   }             }(repeated)  }
    SECTION 6 - Bit-map Section               }(repeated)   }            }
    SECTION 7 - Data Section                  }             }            }
    SECTION 8 - End Section                   }             }            }

Sequences of GRIB sections 2 to 7, 3 to 7, or sections 4 to 7 may be repeated within a single GRIB message. All sections within such repeated sequences must be present and shall appear in the numerical order noted above. Unrepeated sections remain in effect until redefined.

For detailed information on GRIB2 see the NCEP WMO GRIB2 Documentation.

## GRIB2 File-Based API

Starting with version 2.0.0 of the NCEPLIBS-g2c library, a file-based API is supported.

To open a GRIB2 file, call g2c_open() or g2c_open_index(). All opened files must be closed with g2c_close().

Once opened, inquiry functions such as g2c_inq(), g2c_inq_msg(), g2c_inq_msg_time(), g2c_inq_prod(), and g2c_inq_dim() can be used to learn about the messages in the file.

Use g2c_get_prod() to read data from a message.

## GRIB2 Message Encoding Routines

Since a GRIB2 message can contain gridded fields for many parameters on a number of different grids, several routines are used to encode a message. This should give users more flexibility in how to organize data within one or more GRIB2 messages.

To start a new GRIB2 message, call function g2_create(). It encodes Sections 0 and 1 at the beginning of the message. This routine must be used to create each message.

Function g2_addlocal() can be used to add a Local Use Section (Section 2). Note that section is optional and need not appear in a GRIB2 message.

Function g2_addgrid() is used to encode a grid definition into Section 3 - The Grid Definition Section. This grid definition defines the geometry of the the data values in the fields that follow it. Function addgrid() can be called again to change the grid definition describing subsequent data fields.

Each data field is added to the GRIB2 message using routine g2_addfield(), which adds Sections 4, 5, 6, and 7 to the message.

After all desired data fields have been added to the GRIB2 message, a call to routine g2_gribend() is needed to add the final section 8 to the message and to update the length of the message. A call to g2_gribend() is required for each GRIB2 message.

## GRIB2 Message Decoding Routines

Function g2_info() can be used to find out how many Local Use sections and data fields are contained in a given GRIB2 message. In addition, this routine also returns the number of octets of the largest Local Use section in the message.

g2_getfld() can be used to get all information pertaining to the nth data field in the message. An option exists that lets the user decide if the function should unpack the Bit-map (if applicable) and the data values or just return the field description information.

## GRIB2 Tables/Templates

Use g2c_get_grid_template(), g2c_get_pds_template(), and g2c_get_drs_template() to get GRIB2 template information.

## GRIB2 Documentation

NCO Provides documentation on WMO GRIB2 at https://www.nco.ncep.noaa.gov/pmb/docs/grib2/grib2_doc/

WMO's GRIB2 specification "FM 92-XII GRIB - General Regularly-distributed Information in Binary Form" contains descriptions of each template and code table information. This document can be found at https://codes.wmo.int/grib2/_codeflag (PDF and MSWord formats are available).

## Documentation for Previous Versions of NCEPLIBS-g2c

- NCEPLIBS-g2c Version 2.0.0
- NCEPLIBS-g2c Version 1.9.0
- NCEPLIBS-g2c Version 1.8.0
- NCEPLIBS-g2c Version 1.7.0
- NCEPLIBS-g2c Version 1.6.4