GRIB学习笔记：样例文件分析 - Section 7
目录
本文通过分析一个GRIB2消息介绍GRIB2格式。
样例文件来自中国气象局数值预报中心GRAPES全球模式的预报结果，选用2019年5月26日00时次的000时次850hPa温度场。
Section 7: Data Section
包含数据值。GRIB2数据有多种压缩格式：简单封装、复杂封装、空间差分、JPEG2000、PNG等。。内容如下：
| Octets | Key | Type | Content |
|---|---|---|---|
| 1-4 | section7Length | unsigned | Length of section in octets (nn) |
| 5 | numberOfSection | unsigned | Number of section (7) |
| 6-nn | Data in a format described by Data Template 7.x, where x is the Data Representation Template number given in octets 10-11 of Section 5. |
grib_dump示例
====================== SECTION_7 ( length=624877, padding=0 ) ======================
1-4 section7Length = 624877
5 numberOfSection = 7
6-624877 codedValues = (1036800,624872) {
2.7052777344e+02, 2.7043777344e+02, 2.7054777344e+02, 2.7047777344e+02, 2.7043777344e+02, 2.7054777344e+02, 2.7046777344e+02, 2.7047777344e+02,
2.7048777344e+02, 2.7048777344e+02, 2.7047777344e+02, 2.7048777344e+02, 2.7049777344e+02, 2.7047777344e+02, 2.7048777344e+02, 2.7047777344e+02,
2.7048777344e+02, 2.7048777344e+02, 2.7047777344e+02, 2.7048777344e+02, 2.7047777344e+02, 2.7048777344e+02, 2.7048777344e+02, 2.7046777344e+02,
2.7048777344e+02, 2.7049777344e+02, 2.7047777344e+02, 2.7048777344e+02, 2.7047777344e+02, 2.7047777344e+02, 2.7048777344e+02, 2.7047777344e+02,
2.7047777344e+02, 2.7048777344e+02, 2.7046777344e+02, 2.7048777344e+02, 2.7047777344e+02, 2.7047777344e+02, 2.7047777344e+02, 2.7048777344e+02,
2.7047777344e+02, 2.7047777344e+02, 2.7047777344e+02, 2.7048777344e+02, 2.7046777344e+02, 2.7047777344e+02, 2.7048777344e+02, 2.7047777344e+02,
2.7047777344e+02, 2.7048777344e+02, 2.7047777344e+02, 2.7047777344e+02, 2.7046777344e+02, 2.7048777344e+02, 2.7046777344e+02, 2.7047777344e+02,
2.7048777344e+02, 2.7046777344e+02, 2.7047777344e+02, 2.7048777344e+02, 2.7046777344e+02, 2.7047777344e+02, 2.7048777344e+02, 2.7046777344e+02,
2.7047777344e+02, 2.7048777344e+02, 2.7046777344e+02, 2.7047777344e+02, 2.7047777344e+02, 2.7046777344e+02, 2.7048777344e+02, 2.7047777344e+02,
2.7046777344e+02, 2.7047777344e+02, 2.7046777344e+02, 2.7048777344e+02, 2.7046777344e+02, 2.7046777344e+02, 2.7048777344e+02, 2.7045777344e+02,
2.7048777344e+02, 2.7046777344e+02, 2.7046777344e+02, 2.7047777344e+02, 2.7047777344e+02, 2.7046777344e+02, 2.7048777344e+02, 2.7046777344e+02,
2.7046777344e+02, 2.7047777344e+02, 2.7047777344e+02, 2.7046777344e+02, 2.7047777344e+02, 2.7046777344e+02, 2.7046777344e+02, 2.7048777344e+02,
2.7045777344e+02, 2.7047777344e+02, 2.7046777344e+02, 2.7046777344e+02
... 1036700 more values
} # data_jpeg2000_packing codedValues
说明
样例文件本节从 0xAC 开始
1-4: section长度
5: section序号
本节长度为 0x988ED，下一节起始位置是 0xAC + 0x988ED = 0x98899。
可以看到下一节是Section 8。
6-nn: 由Template 7.x定义，x是Section 5中octets 10-11定义的dataRepresentationTemplateNumber。
样例文件使用Template 7.40，Grid point data - JPEG 2000 Code Stream Format Note: For most templates, details of the packing process are described in regulation 92.9.4。
Template 7.40 内容如下：
| Octets | Key | Type | Content |
|---|---|---|---|
| 6-nn | JPEG 2000 Code Stream as described in Part1 of the JPEG 2000 standard. (ISO/IEC 15444- 1:2000) |
样例文件使用JPEG 2000标准压缩数据。我还不了解JPEG 2000格式，以后再补充这方面的内容。
