title: 解码已加载的 GRIB 消息

weight: 3

目标尽可能地少解码。永远不要解码整个已加载的 GRIB 消息。可以使用 grib_dump -D <grib_file> 查看到底有多少个 ecCodes key。

用于解码的主要函数：

  1. int codes_get_long (codes_handle *h, const char *key, long *value);
  2. int    codes_get_double (codes_handle *h, const char *key, double *value)
  3. int    codes_get_string (codes_handle *h, const char *key, char *mesg, size_t *length)
  5. int codes_get_long_array (codes_handle *h, const char *key, long *vals, size_t *length)
  6. int    codes_get_double_array (codes_handle *h, const char *key, double *vals, size_t *length)

帮助函数

获取一个（数组）数据的大小：

  1. int codes_get_size(codes_handle* h, const char* key,size_t *size);
  2. int grib_get_size(grib_handle* h, const char* key,size_t *size);

详细输出 grib 消息的内容：

  1. void codes_dump_content(codes_handle* h,FILE* out,const char* mode, unsigned long option_flags,void* arg);
  2. void grib_dump_content(grib_handle* h,FILE* out,const char* mode, unsigned long option_flags,void* arg);

检查某个 key 是否被定义（存在）：

  1. int codes_is_defined(codes_handle* h, const char* key);
  2. int grib_is_defined(grib_handle* h, const char* key);

检查某个 key 的值是否是缺失值：

  1. int codes_is_missing(codes_handle* h, const char* key, int* err);
  2. int grib_is_missing(grib_handle* h, const char* key, int* err);

统计文件中的消息个数：

  1. int codes_count_in_file(codes_context* c, FILE* f,int* n);
  2. int grib_count_in_file(grib_context* c, FILE* f,int* n);

示例：codes_get

  1. #include <iostream>
  3. #include <eccodes.h>
  5. using namespace std;
  7. int main(int argc, char** argv)
  8. {
  9.     if(argc < 2)
  10.     {
  11.         cout<<"Usage: "<<argv[0]<<" grib_file_path";
  12.         return 1;
  13.     }
  15.     const char* file_path = argv[1];
  17.     FILE* in = fopen(file_path, "rb");
  18.     if(!in)
  19.     {
  20.         cout<<"ERROR: unable to open file "<<file_path<<endl;
  21.         return 1;
  22.     }
  24.     int err = 0;
  25.     codes_handle *h = nullptr;
  27.     while((h=codes_handle_new_from_file(nullptr, in, PRODUCT_GRIB, &err)) != nullptr)
  28.     {
  29.         long data_date = -1;
  30.         codes_get_long(h, "dataDate", &data_date);
  32.         long type_of_level = -1;
  33.         codes_get_long(h, "typeOfLevel", &type_of_level);
  35.         long level = -1;
  36.         codes_get_long(h, "level", &level);
  38.         size_t size;
  39.         codes_get_size(h, "values", &size);
  41.         double *values = new double[size];
  42.         codes_get_double_array(h, "values", values, &size);
  44.         cout<<data_date<<" "<<type_of_level<<" "<<level<<" "<<size<<" "<<values[size-1]<<endl;
  46.         delete [] values;
  47.         codes_handle_delete(h);
  48.     }
  50.     fclose(in);
  52.     return 0;
  53. }

示例：grib_get

将上面的代码替换为以 grib_ 开头的函数

  1. #include <iostream>
  3. #include <grib_api.h>
  5. using namespace std;
  7. int main(int argc, char** argv)
  8. {
  9.     if(argc < 2)
  10.     {
  11.         cout<<"Usage: "<<argv[0]<<" grib_file_path";
  12.         return 1;
  13.     }
  15.     const char* file_path = argv[1];
  17.     FILE* in = fopen(file_path, "rb");
  18.     if(!in)
  19.     {
  20.         cout<<"ERROR: unable to open file "<<file_path<<endl;
  21.         return 1;
  22.     }
  24.     int err = 0;
  25.     grib_handle *h = nullptr;
  27.     while((h=grib_handle_new_from_file(nullptr, in, &err)) != nullptr)
  28.     {
  29.         long data_date = -1;
  30.         grib_get_long(h, "dataDate", &data_date);
  32.         long type_of_level = -1;
  33.         grib_get_long(h, "typeOfLevel", &type_of_level);
  35.         long level = -1;
  36.         grib_get_long(h, "level", &level);
  38.         size_t size;
  39.         grib_get_size(h, "values", &size);
  41.         double *values = new double[size];
  42.         grib_get_double_array(h, "values", values, &size);
  44.         cout<<data_date<<" "<<type_of_level<<" "<<level<<" "<<size<<" "<<values[size-1]<<endl;
  46.         delete [] values;
  47.         grib_handle_delete(h);
  48.     }
  50.     fclose(in);
  52.     return 0;
  53. }

ecCodes 可以做更多的事情

ecCodes 的宗旨是提供高层次的 key 集合和不同的函数，用于检索或计算已加载 GRIB 消息中的额外信息。

对于不同的应用：

  • 只读key，返回数据值的均值和最值，分布经纬度信息
  • 计算经纬度、数据值的函数： codes_get_data
  • 获取指定坐标点最近的数据值 codes_grib_nearest_find
  • 根据 index 列表获取数据值 codes_get_double_elements