Task 270: .GRIB File Format
Task 270: .GRIB File Format
1. File Format Specifications for .GRIB
The .GRIB (GRIdded Binary) format is a standard binary format developed by the World Meteorological Organization (WMO) for storing and transmitting gridded meteorological and oceanographic data. It supports both historical and forecast data, with two main editions: GRIB Edition 1 (GRIB1, introduced in 1985) and GRIB Edition 2 (GRIB2, introduced in 2003 as a more flexible and extensible replacement). GRIB2 is the current standard and is backward-incompatible with GRIB1 due to structural changes. Files typically have extensions .grib, .grb, or .gb and consist of one or more self-contained messages, each representing a single 2D grid field (e.g., temperature, wind speed). Messages are concatenated in a file without separators.
Key specifications:
- Binary, self-describing, and compact: Uses bit-oriented packing for efficiency (files are ~1/3 the size of equivalent float binaries). Each message includes metadata for grid, time, variable, and packing without external schema.
- Byte order: Big-endian for multi-octet integers.
- Message structure: 8 fixed sections (0-7), with Section 8 as the end marker. Sections 2 and 6 are optional; sequences of Sections 3-7 can repeat for multi-field messages (though single-field is common).
- Official reference: WMO Manual on Codes (WMO-No. 306), Volume I.2, Part B (FM 92 GRIB specification). GRIB2 uses templates (numbered tables) for extensibility in Sections 3, 4, and 5.
2. List of All Properties Intrinsic to the .GRIB File Format
Based on the GRIB2 specification (as GRIB1 is legacy), here is a comprehensive list of intrinsic properties derived from the format's binary structure. These are mandatory or optional fields encoded in the sections, focusing on metadata (data values themselves are not "properties" but payload). Properties are grouped by section for clarity.
General File Properties:
- Binary format (bit-packed, no text headers).
- One or more concatenated messages.
- Big-endian multi-octet encoding.
- Discipline of processed data (e.g., meteorological=0, oceanographic=10; from Section 0, octet 7).
- Edition number (1 or 2; from Section 0, octet 9).
Section 0: Indicator Section (fixed 16 octets):
- GRIB identifier ("GRIB"; octets 1-4).
- Total length of GRIB message (octets 5-8, unsigned 32-bit integer).
- Length of Section 1 (Identification; octet 10, unsigned 32-bit).
- Length of Section 2 (Local Use; octet 11, unsigned 32-bit; 0 if absent).
- Length of Section 3 (Grid Definition; octet 12, unsigned 32-bit).
- Length of Section 4 (Product Definition; octet 13, unsigned 32-bit).
- Length of Section 5 (Data Representation; octet 14, unsigned 32-bit).
- Length of Section 6 (Bit-Map; octet 15, unsigned 32-bit; 0 if absent).
- Length of Section 7 (Data; octet 16, unsigned 32-bit).
Section 1: Identification Section (variable length, minimum 13 octets):
- Section number (1; octet 2-5 after length).
- Identification of originating/generating center (e.g., NCEP=7; octets 6-7).
- Originating sub-center (octets 8-9).
- GRIB master tables version number (e.g., 2 for GRIB2; octet 10).
- GRIB local tables version number (octet 11).
- Significance of reference time (e.g., analysis=0; octet 12).
- Reference time: Year (octets 13-14, 4-digit), month (15), day (16), hour (17), minute (18), second (19).
- Production status of processed data (e.g., forecast=2; octet 20).
- Type of processed data (e.g., analysis=1; octet 21).
- Optional additional parameters (e.g., data category).
Section 2: Local Use Section (optional, variable):
- Section number (2).
- Center-specific metadata (e.g., NCEP local extensions; unstructured binary).
Section 3: Grid Definition Section (variable):
- Section number (3).
- Source of grid definition (e.g., pre-defined=0; octet 6).
- Number of data points (octets 7-10, unsigned 32-bit).
- List of numbers defining grid points (optional, variable octets).
- Interpreted values of list of numbers (e.g., Ni, Nj for lat/lon grid).
- Grid definition template number (e.g., 0=regular lat/lon; octets 13-14).
- Grid definition template data (variable, template-specific: e.g., latitude/longitude of first/last points, resolution flags, scanning mode).
Section 4: Product Definition Section (variable):
- Section number (4).
- Number of coordinate values after template (octets 7-10, usually 0).
- Product definition template number (e.g., 0=analysis product; octets 11-12).
- Product definition template data (variable, template-specific): Parameter category (e.g., temperature=0), parameter number (e.g., 2m temp=0), type of generating process (e.g., forecast=0), background/analysis process ID, hours/minutes after reference time, indicator of unit of time range (e.g., hour=1), length of time range, number of time ranges, statistical process (e.g., average=1).
Section 5: Data Representation Section (variable):
- Section number (5).
- Number of coordinate values after template (usually 0).
- Data representation template number (e.g., 0=simple packing; octets 11-12).
- Data representation template data (variable, template-specific): Reference value (scaled float), binary scale factor (signed 16-bit), decimal scale factor (signed 16-bit), number of bits used for each packed value (unsigned 8-bit), type of original field values (e.g., floating point=0), first/last values and increment for complex packing, spatial differencing flag, etc.
Section 6: Bit-Map Section (optional, variable):
- Section number (6).
- Bit-map indicator (e.g., 0=provided, 1=consecutive points, 255=missing; octet 6).
- Bit map (variable bits, one per grid point indicating presence/absence of data).
Section 7: Data Section (variable):
- Section number (7).
- Packed binary data values (variable length, using method from Section 5).
Section 8: End Section (fixed 4 octets):
- End marker ("7777").
These properties make GRIB intrinsic to its "file system" as a self-contained, hierarchical binary stream without external indexing or file system dependencies.
3. Two Direct Download Links for .GRIB Files
- Sample GRIB2 file from UCAR GDEX (NCEP GDAS surface flux analysis): https://osdf-data.gdex.ucar.edu/ncar/gdex/d084004/2024/202402/gdas1.sflux.2024020100.f00.grib2
- Another sample GRIB2 file from UCAR GDEX (NCEP GDAS surface flux forecast): https://osdf-data.gdex.ucar.edu/ncar/gdex/d084004/2024/202402/gdas1.sflux.2024020100.f03.grib2
4. Python Class for .GRIB Handling
This class implements a basic GRIB2 parser using struct for binary unpacking. It reads a file, decodes key properties (from the list above), prints them to console, and supports writing the same message back (identity write for verification). Full template parsing is simplified to extract core fields; extend for specific templates.
import struct
import os
class GRIB2Parser:
def __init__(self, filename=None):
self.filename = filename
self.properties = {}
if filename:
self.read()
def read(self):
with open(self.filename, 'rb') as f:
data = f.read()
pos = 0
# Section 0: Indicator
grib_id = data[pos:pos+4].decode('ascii')
if grib_id != 'GRIB':
raise ValueError("Not a GRIB file")
pos += 4
total_len = struct.unpack('>I', data[pos:pos+4])[0]
pos += 4
edition = struct.unpack('B', data[pos:pos+1])[0]
pos += 1
if edition != 2:
raise ValueError("Only GRIB2 supported")
discipline = struct.unpack('B', data[pos:pos+1])[0]
pos += 1 # Skip to lengths
sec1_len = struct.unpack('>I', data[pos:pos+4])[0]
pos += 4
sec2_len = struct.unpack('>I', data[pos:pos+4])[0]
pos += 4
sec3_len = struct.unpack('>I', data[pos:pos+4])[0]
pos += 4
sec4_len = struct.unpack('>I', data[pos:pos+4])[0]
pos += 4
sec5_len = struct.unpack('>I', data[pos:pos+4])[0]
pos += 4
sec6_len = struct.unpack('>I', data[pos:pos+4])[0]
pos += 4
sec7_len = struct.unpack('>I', data[pos:pos+4])[0]
pos += 4
# End marker
end_marker = data[pos:pos+4].decode('ascii')
if end_marker != '7777':
raise ValueError("Invalid end marker")
self.properties = {
'edition': edition,
'discipline': discipline,
'total_length': total_len,
'sec1_length': sec1_len,
'sec2_length': sec2_len,
'sec3_length': sec3_len,
'sec4_length': sec4_len,
'sec5_length': sec5_len,
'sec6_length': sec6_len,
'sec7_length': sec7_len
}
# Section 1: Identification (simplified)
pos = 16 # After Section 0
sec_num = struct.unpack('>I', data[pos+1:pos+5])[0] # Should be 1
pos += 5
center = struct.unpack('>H', data[pos:pos+2])[0]
pos += 2
sub_center = struct.unpack('>H', data[pos:pos+2])[0]
pos += 2
master_version = struct.unpack('B', data[pos:pos+1])[0]
pos += 1
local_version = struct.unpack('B', data[pos:pos+1])[0]
pos += 1
ref_time_sig = struct.unpack('B', data[pos:pos+1])[0]
pos += 1
year = struct.unpack('>H', data[pos:pos+2])[0]
pos += 2
month = struct.unpack('B', data[pos:pos+1])[0]
pos += 1
day = struct.unpack('B', data[pos:pos+1])[0]
pos += 1
hour = struct.unpack('B', data[pos:pos+1])[0]
pos += 1
minute = struct.unpack('B', data[pos:pos+1])[0]
pos += 1
second = struct.unpack('B', data[pos:pos+1])[0]
pos += 1
production_status = struct.unpack('B', data[pos:pos+1])[0]
pos += 1
data_type = struct.unpack('B', data[pos:pos+1])[0]
self.properties.update({
'center': center,
'sub_center': sub_center,
'master_version': master_version,
'local_version': local_version,
'ref_time_sig': ref_time_sig,
'ref_year': year,
'ref_month': month,
'ref_day': day,
'ref_hour': hour,
'ref_minute': minute,
'ref_second': second,
'production_status': production_status,
'data_type': data_type
})
# Section 3: Grid (simplified, assume template 0)
pos += sec1_len - 21 # Skip rest of sec1
if sec2_len > 0:
pos += sec2_len
sec_num3 = struct.unpack('>I', data[pos+1:pos+5])[0]
pos += 5
grid_source = struct.unpack('B', data[pos:pos+1])[0]
pos += 1
num_points = struct.unpack('>I', data[pos:pos+4])[0]
pos += 4
# Skip list if present
grid_template_num = struct.unpack('>H', data[pos:pos+2])[0]
self.properties.update({
'grid_source': grid_source,
'num_points': num_points,
'grid_template_num': grid_template_num
})
# Section 4: Product (simplified)
pos += sec3_len - 6 - 4 # Approximate skip
sec_num4 = struct.unpack('>I', data[pos+1:pos+5])[0]
pos += 5
coord_vals = struct.unpack('>I', data[pos:pos+4])[0]
pos += 4
product_template_num = struct.unpack('>H', data[pos:pos+2])[0]
pos += 2
# Assume template 0: param category, param number
param_cat = struct.unpack('B', data[pos:pos+1])[0]
param_num = struct.unpack('B', data[pos+1:pos+2])[0]
self.properties.update({
'coord_vals_after_template': coord_vals,
'product_template_num': product_template_num,
'param_category': param_cat,
'param_number': param_num
})
# Section 5: Data Rep (simplified)
pos += sec4_len - 7 # Approximate
sec_num5 = struct.unpack('>I', data[pos+1:pos+5])[0]
pos += 5
coord_vals5 = struct.unpack('>I', data[pos:pos+4])[0]
pos += 4
data_template_num = struct.unpack('>H', data[pos:pos+2])[0]
pos += 2
ref_value = struct.unpack('>d', data[pos:pos+8])[0] # Scaled float
pos += 8
bin_scale = struct.unpack('>h', data[pos:pos+2])[0]
pos += 2
dec_scale = struct.unpack('>h', data[pos:pos+2])[0]
pos += 2
num_bits = struct.unpack('B', data[pos:pos+1])[0]
pos += 1
orig_field_type = struct.unpack('B', data[pos:pos+1])[0]
self.properties.update({
'coord_vals_after_data_template': coord_vals5,
'data_template_num': data_template_num,
'reference_value': ref_value,
'binary_scale_factor': bin_scale,
'decimal_scale_factor': dec_scale,
'num_bits_packed': num_bits,
'orig_field_type': orig_field_type
})
# Section 6: Bit-map (if present)
if sec6_len > 0:
pos += sec5_len - 15 # Approximate
bitmap_indicator = struct.unpack('B', data[pos+6:pos+7])[0] # Octet 6 after len/sec
self.properties['bitmap_indicator'] = bitmap_indicator
# Section 7: Data (length known, but skip unpacking data)
# End Section 8 already checked
def print_properties(self):
for k, v in self.properties.items():
print(f"{k}: {v}")
def write(self, output_filename):
with open(output_filename, 'wb') as f:
with open(self.filename, 'rb') as infile:
f.write(infile.read()) # Identity write for now; extend for modified properties
# Usage
parser = GRIB2Parser('sample.grib2')
parser.print_properties()
parser.write('output.grib2')
5. Java Class for .GRIB Handling
This Java class uses ByteBuffer for binary parsing. It reads, decodes, prints properties, and writes identically.
import java.io.*;
import java.nio.*;
import java.nio.channels.FileChannel;
import java.util.HashMap;
import java.util.Map;
public class GRIB2Parser {
private String filename;
private Map<String, Object> properties = new HashMap<>();
public GRIB2Parser(String filename) {
this.filename = filename;
if (filename != null) {
read();
}
}
public void read() {
try (RandomAccessFile file = new RandomAccessFile(filename, "r");
FileChannel channel = file.getChannel()) {
ByteBuffer buffer = ByteBuffer.allocate((int) channel.size());
channel.read(buffer);
buffer.flip();
int pos = 0;
// Section 0
byte[] gribId = new byte[4];
buffer.get(gribId, pos, 4);
pos += 4;
if (!new String(gribId).equals("GRIB")) {
throw new IllegalArgumentException("Not a GRIB file");
}
ByteBuffer lenBuf = buffer.slice();
lenBuf.position(pos);
lenBuf.limit(pos + 4);
int totalLen = lenBuf.getInt();
pos += 4;
byte edition = buffer.get(pos);
pos += 1;
if (edition != 2) {
throw new IllegalArgumentException("Only GRIB2 supported");
}
pos += 1; // Skip to discipline
byte discipline = buffer.get(pos);
pos += 1;
// Lengths
int sec1Len = buffer.getInt(pos);
pos += 4;
int sec2Len = buffer.getInt(pos);
pos += 4;
int sec3Len = buffer.getInt(pos);
pos += 4;
int sec4Len = buffer.getInt(pos);
pos += 4;
int sec5Len = buffer.getInt(pos);
pos += 4;
int sec6Len = buffer.getInt(pos);
pos += 4;
int sec7Len = buffer.getInt(pos);
pos += 4;
// End marker
byte[] endMarker = new byte[4];
buffer.get(endMarker, pos, 4);
if (!new String(endMarker).equals("7777")) {
throw new IllegalArgumentException("Invalid end marker");
}
properties.put("edition", (int) edition);
properties.put("discipline", (int) discipline);
properties.put("total_length", totalLen);
properties.put("sec1_length", sec1Len);
properties.put("sec2_length", sec2Len);
properties.put("sec3_length", sec3Len);
properties.put("sec4_length", sec4Len);
properties.put("sec5_length", sec5Len);
properties.put("sec6_length", sec6Len);
properties.put("sec7_length", sec7Len);
// Section 1 (simplified)
pos = 16;
pos += 5; // Length + sec num
short center = buffer.getShort(pos);
pos += 2;
short subCenter = buffer.getShort(pos);
pos += 2;
byte masterVer = buffer.get(pos);
pos += 1;
byte localVer = buffer.get(pos);
pos += 1;
byte refSig = buffer.get(pos);
pos += 1;
short year = buffer.getShort(pos);
pos += 2;
byte month = buffer.get(pos);
pos += 1;
byte day = buffer.get(pos);
pos += 1;
byte hour = buffer.get(pos);
pos += 1;
byte minute = buffer.get(pos);
pos += 1;
byte second = buffer.get(pos);
pos += 1;
byte prodStatus = buffer.get(pos);
pos += 1;
byte dataType = buffer.get(pos);
properties.put("center", (int) center);
properties.put("sub_center", (int) subCenter);
properties.put("master_version", (int) masterVer);
properties.put("local_version", (int) localVer);
properties.put("ref_time_sig", (int) refSig);
properties.put("ref_year", (int) year);
properties.put("ref_month", (int) month);
properties.put("ref_day", (int) day);
properties.put("ref_hour", (int) hour);
properties.put("ref_minute", (int) minute);
properties.put("ref_second", (int) second);
properties.put("production_status", (int) prodStatus);
properties.put("data_type", (int) dataType);
// Similar simplifications for Sections 3-5 (omitted for brevity; follow Python logic with ByteBuffer.get*)
// e.g., for Section 3:
pos += sec1Len - 21;
if (sec2Len > 0) pos += sec2Len;
pos += 5;
byte gridSource = buffer.get(pos);
pos += 1;
int numPoints = buffer.getInt(pos);
pos += 4;
short gridTplNum = buffer.getShort(pos);
properties.put("grid_source", (int) gridSource);
properties.put("num_points", numPoints);
properties.put("grid_template_num", (int) gridTplNum);
// ... (extend for full sections as in Python)
} catch (IOException e) {
e.printStackTrace();
}
}
public void printProperties() {
for (Map.Entry<String, Object> entry : properties.entrySet()) {
System.out.println(entry.getKey() + ": " + entry.getValue());
}
}
public void write(String outputFilename) {
try (FileInputStream fis = new FileInputStream(filename);
FileOutputStream fos = new FileOutputStream(outputFilename)) {
byte[] buffer = new byte[1024];
int len;
while ((len = fis.read(buffer)) != -1) {
fos.write(buffer, 0, len);
}
} catch (IOException e) {
e.printStackTrace();
}
}
// Usage: GRIB2Parser parser = new GRIB2Parser("sample.grib2"); parser.printProperties(); parser.write("output.grib2");
}
6. JavaScript Class for .GRIB Handling
This class uses FileReader and DataView for browser/Node.js. Embed in HTML for drag-drop. Reads/decodes/prints/writes properties.
class GRIB2Parser {
constructor(filename = null) {
this.filename = filename;
this.properties = {};
if (filename) {
this.readFile(filename);
}
}
async readFile(filePath) {
return new Promise((resolve, reject) => {
const reader = new FileReader();
reader.onload = (e) => {
const arrayBuffer = e.target.result;
const dataView = new DataView(arrayBuffer);
let pos = 0;
// Section 0
const gribId = String.fromCharCode(dataView.getUint8(pos), dataView.getUint8(pos+1), dataView.getUint8(pos+2), dataView.getUint8(pos+3));
pos += 4;
if (gribId !== 'GRIB') throw new Error('Not a GRIB file');
const totalLen = dataView.getUint32(pos, false); // Big-endian
pos += 4;
const edition = dataView.getUint8(pos);
pos += 1;
if (edition !== 2) throw new Error('Only GRIB2 supported');
pos += 1; // Skip
const discipline = dataView.getUint8(pos);
pos += 1;
const sec1Len = dataView.getUint32(pos, false);
pos += 4;
const sec2Len = dataView.getUint32(pos, false);
pos += 4;
const sec3Len = dataView.getUint32(pos, false);
pos += 4;
const sec4Len = dataView.getUint32(pos, false);
pos += 4;
const sec5Len = dataView.getUint32(pos, false);
pos += 4;
const sec6Len = dataView.getUint32(pos, false);
pos += 4;
const sec7Len = dataView.getUint32(pos, false);
pos += 4;
const endMarker = String.fromCharCode(dataView.getUint8(pos), dataView.getUint8(pos+1), dataView.getUint8(pos+2), dataView.getUint8(pos+3));
if (endMarker !== '7777') throw new Error('Invalid end marker');
this.properties = {
edition,
discipline,
total_length: totalLen,
sec1_length: sec1Len,
sec2_length: sec2Len,
sec3_length: sec3Len,
sec4_length: sec4Len,
sec5_length: sec5Len,
sec6_length: sec6Len,
sec7_length: sec7Len
};
// Section 1 (simplified)
pos = 16;
pos += 5;
const center = dataView.getUint16(pos, false);
pos += 2;
const subCenter = dataView.getUint16(pos, false);
pos += 2;
const masterVer = dataView.getUint8(pos);
pos += 1;
const localVer = dataView.getUint8(pos);
pos += 1;
const refSig = dataView.getUint8(pos);
pos += 1;
const year = dataView.getUint16(pos, false);
pos += 2;
const month = dataView.getUint8(pos);
pos += 1;
const day = dataView.getUint8(pos);
pos += 1;
const hour = dataView.getUint8(pos);
pos += 1;
const minute = dataView.getUint8(pos);
pos += 1;
const second = dataView.getUint8(pos);
pos += 1;
const prodStatus = dataView.getUint8(pos);
pos += 1;
const dataType = dataView.getUint8(pos);
Object.assign(this.properties, {
center,
sub_center: subCenter,
master_version: masterVer,
local_version: localVer,
ref_time_sig: refSig,
ref_year: year,
ref_month: month,
ref_day: day,
ref_hour: hour,
ref_minute: minute,
ref_second: second,
production_status: prodStatus,
data_type: dataType
});
// Sections 3-5 similar to Python/JS logic (extend as needed)
// e.g., for Section 3:
pos += sec1Len - 21;
if (sec2Len > 0) pos += sec2Len;
pos += 5;
const gridSource = dataView.getUint8(pos);
pos += 1;
const numPoints = dataView.getUint32(pos, false);
pos += 4;
const gridTplNum = dataView.getUint16(pos, false);
Object.assign(this.properties, { grid_source: gridSource, num_points: numPoints, grid_template_num: gridTplNum });
// ... (full extension)
resolve();
};
reader.onerror = reject;
const file = new File([filePath], 'temp'); // For Node, use fs.readFileSync
reader.readAsArrayBuffer(file);
});
}
printProperties() {
Object.entries(this.properties).forEach(([k, v]) => console.log(`${k}: ${v}`));
}
write(outputFilename) {
// For browser, create download; for Node, fs.writeFileSync
const blob = new Blob([this.arrayBuffer]); // Assume stored from read
const url = URL.createObjectURL(blob);
const a = document.createElement('a');
a.href = url;
a.download = outputFilename;
a.click();
}
}
// For Ghost blog embed: HTML + JS for drag-drop
/*
<div id="drop-zone">Drag & drop GRIB file here</div>
<div id="output"></div>
<script>
const dropZone = document.getElementById('drop-zone');
const output = document.getElementById('output');
dropZone.addEventListener('drop', (e) => {
e.preventDefault();
const file = e.dataTransfer.files[0];
const parser = new GRIB2Parser();
parser.readFile(file).then(() => {
parser.printProperties(); // Or append to output.innerHTML
let html = '';
Object.entries(parser.properties).forEach(([k, v]) => { html += `<p>${k}: ${v}</p>`; });
output.innerHTML = html;
});
});
dropZone.addEventListener('dragover', (e) => e.preventDefault());
</script>
*/
7. Ghost Blog Embedded HTML JavaScript for Drag & Drop
Embed this full HTML snippet in a Ghost blog post (use HTML card). It creates a drag-drop zone, parses the dropped .GRIB file using the JS class above, and dumps properties to a screen div.
Drag & drop a .GRIB file here to analyze its properties
8. C Class (Struct) for .GRIB Handling
This C implementation uses fread for binary reading. Compile with gcc grib2_parser.c -o grib2_parser. Reads, prints properties, writes identically.
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
typedef struct {
uint8_t edition;
uint8_t discipline;
uint32_t total_length;
uint32_t sec1_length;
uint32_t sec2_length;
uint32_t sec3_length;
uint32_t sec4_length;
uint32_t sec5_length;
uint32_t sec6_length;
uint32_t sec7_length;
uint16_t center;
uint16_t sub_center;
uint8_t master_version;
uint8_t local_version;
uint8_t ref_time_sig;
uint16_t ref_year;
uint8_t ref_month;
uint8_t ref_day;
uint8_t ref_hour;
uint8_t ref_minute;
uint8_t ref_second;
uint8_t production_status;
uint8_t data_type;
uint8_t grid_source;
uint32_t num_points;
uint16_t grid_template_num;
// Add more as needed
} GRIBProperties;
typedef struct {
FILE* file;
GRIBProperties props;
} GRIB2Parser;
void read_grib2(GRIB2Parser* parser, const char* filename) {
parser->file = fopen(filename, "rb");
if (!parser->file) {
perror("File open error");
return;
}
fseek(parser->file, 0, SEEK_END);
long size = ftell(parser->file);
fseek(parser->file, 0, SEEK_SET);
uint8_t* buffer = malloc(size);
fread(buffer, 1, size, parser->file);
int pos = 0;
// Section 0
if (memcmp(buffer + pos, "GRIB", 4) != 0) {
fprintf(stderr, "Not a GRIB file\n");
free(buffer);
return;
}
pos += 4;
parser->props.total_length = (buffer[pos] << 24) | (buffer[pos+1] << 16) | (buffer[pos+2] << 8) | buffer[pos+3];
pos += 4;
parser->props.edition = buffer[pos];
pos += 1;
if (parser->props.edition != 2) {
fprintf(stderr, "Only GRIB2 supported\n");
free(buffer);
return;
}
pos += 1; // Skip
parser->props.discipline = buffer[pos];
pos += 1;
parser->props.sec1_length = (buffer[pos] << 24) | (buffer[pos+1] << 16) | (buffer[pos+2] << 8) | buffer[pos+3];
pos += 4;
parser->props.sec2_length = (buffer[pos] << 24) | (buffer[pos+1] << 16) | (buffer[pos+2] << 8) | buffer[pos+3];
pos += 4;
parser->props.sec3_length = (buffer[pos] << 24) | (buffer[pos+1] << 16) | (buffer[pos+2] << 8) | buffer[pos+3];
pos += 4;
parser->props.sec4_length = (buffer[pos] << 24) | (buffer[pos+1] << 16) | (buffer[pos+2] << 8) | buffer[pos+3];
pos += 4;
parser->props.sec5_length = (buffer[pos] << 24) | (buffer[pos+1] << 16) | (buffer[pos+2] << 8) | buffer[pos+3];
pos += 4;
parser->props.sec6_length = (buffer[pos] << 24) | (buffer[pos+1] << 16) | (buffer[pos+2] << 8) | buffer[pos+3];
pos += 4;
parser->props.sec7_length = (buffer[pos] << 24) | (buffer[pos+1] << 16) | (buffer[pos+2] << 8) | buffer[pos+3];
pos += 4;
if (memcmp(buffer + pos, "7777", 4) != 0) {
fprintf(stderr, "Invalid end marker\n");
free(buffer);
return;
}
// Section 1 (simplified)
pos = 16;
pos += 5;
parser->props.center = (buffer[pos] << 8) | buffer[pos+1];
pos += 2;
parser->props.sub_center = (buffer[pos] << 8) | buffer[pos+1];
pos += 2;
parser->props.master_version = buffer[pos];
pos += 1;
parser->props.local_version = buffer[pos];
pos += 1;
parser->props.ref_time_sig = buffer[pos];
pos += 1;
parser->props.ref_year = (buffer[pos] << 8) | buffer[pos+1];
pos += 2;
parser->props.ref_month = buffer[pos];
pos += 1;
parser->props.ref_day = buffer[pos];
pos += 1;
parser->props.ref_hour = buffer[pos];
pos += 1;
parser->props.ref_minute = buffer[pos];
pos += 1;
parser->props.ref_second = buffer[pos];
pos += 1;
parser->props.production_status = buffer[pos];
pos += 1;
parser->props.data_type = buffer[pos];
// Sections 3-5 similar (bit shifts for big-endian)
pos += parser->props.sec1_length - 21;
if (parser->props.sec2_length > 0) pos += parser->props.sec2_length;
pos += 5;
parser->props.grid_source = buffer[pos];
pos += 1;
parser->props.num_points = (buffer[pos] << 24) | (buffer[pos+1] << 16) | (buffer[pos+2] << 8) | buffer[pos+3];
pos += 4;
parser->props.grid_template_num = (buffer[pos] << 8) | buffer[pos+1];
free(buffer);
}
void print_properties(GRIBProperties* props) {
printf("edition: %d\n", props->edition);
printf("discipline: %d\n", props->discipline);
printf("total_length: %u\n", props->total_length);
// ... print all fields similarly
printf("center: %d\n", props->center);
// Extend for all
}
void write_grib2(GRIB2Parser* parser, const char* output) {
FILE* out = fopen(output, "wb");
if (!out) return;
fseek(parser->file, 0, SEEK_SET);
char buf[1024];
size_t len;
while ((len = fread(buf, 1, sizeof(buf), parser->file)) > 0) {
fwrite(buf, 1, len, out);
}
fclose(out);
}
int main(int argc, char** argv) {
if (argc < 2) return 1;
GRIB2Parser parser = {0};
read_grib2(&parser, argv[1]);
print_properties(&parser.props);
write_grib2(&parser, "output.grib2");
fclose(parser.file);
return 0;
}