首先先了解下什么是NMEA码，这里有很好的解释，就不直接搬运了

http://www.gpsbaby.com/wz/nmea.html

首先要找到包含GPS信息的文本行，即字符串GPGGA所在行

$GPGGA,052551.00,3409.341502,N,10853.663318,E,1,05,1.2,459.4,M,-28.0,M,,*4E

在实际的工程应用中，可能由于各种原因（比如设备在室内时就无法获取GPS信息），GPS信息并不是实时都可以获取准确的完整的值，因此在GPS.log文件内，会存在一些不完整的GPS信息。

$GPGGA,,,,,,0,,,,,,,,*66　　//这是我在室内定位的GPS信息

所以要确定一个文本行确实存在完整的GPS信息，必须确认GPGGA, N/S, W/E 三个字符串均存在。然后再定位纬度所在位置，提取数值即可。

实现代码如下

#include
#include
#include
#include
#include
#include
#include
#include
#include

#define FILE_PATH "/home/mr_han/code/exp/GPS.log"
#define READ_MAX 150
#define GPS_DATA 15
#define GPS_KEY "GPGGA"
#define bool char
#define true 1
#define false 0
void gps_cal(double p, char q, int *angle, float *min)//将原始GPS信息转换为度分形式
{
*angle = p/100;
*min = (p-*angle*100)/60;

}
int Search_comma(char *p, int L)//由GPGGA纬度字段前的逗号即可找到得到纬度GPS信息
{
while(p[L-1]!=',')
{
--L;
}
return L;
}
/*从文件中c查找完整GPS信息，参数依次为， 文件路径(名)，纬度值，经度值， 经度，纬度*/
bool get_gps_data(const char *file_name, double *p, double *q, char *longitude, char *latitude)
{
char *rw = NULL;
int location = 0, i = 0, N=0,S=0,E=0,W=0;
char read_line_buff[READ_MAX], gps[GPS_DATA];
memset(gps, 0, GPS_DATA);
FILE *fd = fopen(FILE_PATH, "r");
if(fd == NULL)
{
printf("Open file error!\n");
return false;
}
while(!feof(fd))
{
memset(read_line_buff, 0, READ_MAX);
rw = fgets(read_line_buff, READ_MAX, fd);//每次从文件读取一行，
if(rw == NULL)
return false;
if(strstr(read_line_buff, GPS_KEY)!=NULL)
{

        N = (strchr(read\_line\_buff,'N')==NULL ? 0 : 1);  
        S = (strchr(read\_line\_buff, 'S')==NULL ? 0 : 2);  
        E = (strchr(read\_line\_buff, 'E')==NULL ? 0 : 4);  
        W = (strchr(read\_line\_buff, 'W')==NULL ? 0 : 7);/  
        i = N+S+E+W;  
        /\*完整的GPS,信息只可能存在 NE东经北纬、NW西经北纬、SE东经南纬、SW西经南纬，分别给四种组合不同的权值，用以确定是那种情况\*/  
        switch(i)//这里case可以调用函数用来代替goto语句  
        {  
            case 5:  
                \*longitude = 'E';  
                \*latitude = 'N';  
                i = 0;  
                fclose(fd);  
                goto NE;  
            case 6:  
                \*longitude = 'E';  
                \*latitude = 'S';  
                i = 0;  
                fclose(fd);  
                goto SE;  
            case 8:  
                \*longitude = 'W';  
                \*latitude = 'N';  
                i = 0;  
                fclose(fd);  
                goto NW;  
            case 9:  
                \*longitude = 'W';  
                \*latitude = 'S';  
                i = 0;  
                fclose(fd);  
                goto SW;  
            default: continue;  
        }  
    }  
}  

NE:
rw = strchr(read_line_buff, 'N');//strchr函数会在第一个参数字符串内找第二个参数字符，找到返回一个地址，找不到返回空
location = rw - read_line_buff - 1; //地址相减，得到数字，这一步主要是定位到纬度前的逗号，$GPGGA,052551.00,3409.341502,N location即N前的逗号
location = Search_comma(read_line_buff,location); //再向前定位上一个逗号即可找到GPS信息
while(read_line_buff[location+1] != 'N')
gps[i++] = read_line_buff[location++];
*p = atof(gps);
memset(gps, 0, GPS_DATA);
i = 0;
location = location+3;//3409.341502,N,10853.663318,E,纬度赋值完成后，location在N前逗号出，只需向后跳三位即可找到经度信息
while(read_line_buff[location+1] != 'E')
gps[i++] = read_line_buff[location++];
*q = atof(gps);
return true;

SE:
rw = strchr(read_line_buff, 'S');
location = rw - read_line_buff - 1;
location = Search_comma(read_line_buff, location);
while(read_line_buff[location+1] != 'S')
gps[i++] = read_line_buff[location++];
*p = atof(gps);
memset(gps, 0, GPS_DATA);
i = 0;
location = location + 3;
while(read_line_buff[location+1] != 'E')
gps[i++] = read_line_buff[location++];
*q = atof(gps);
return true;
NW:
rw = strchr(read_line_buff, 'N');
location = rw - read_line_buff - 1;
location = Search_comma(read_line_buff, location);
while(read_line_buff[location+1] != 'N')
gps[i++] = read_line_buff[location++];
*p = atof(gps);
memset(gps, 0, GPS_DATA);
i = 0;
location =location + 3;
while(read_line_buff[location+1] != 'W')
gps[i++] = read_line_buff[location++];
*q = atof(gps);
return true;
SW:
rw = strchr(read_line_buff, 'S');
location = rw - read_line_buff - 1;
location = Search_comma(read_line_buff, location);
while(read_line_buff[location+1] != 'S')
gps[i++] = read_line_buff[location++];
*p = atof(gps);//atof函数可将字符串转为double后返回，转换失败返回0
memset(gps, 0, GPS_DATA);
i = 0;
location =location + 3;
while(read_line_buff[location+1] != 'W')
gps[i++] = read_line_buff[location++];
*q = atof(gps);
return true;

}
char *print(char p)
{
switch(p)
{
case 'S':return "南纬";
case 's':return "南纬";
case 'N':return "北纬";
case 'n':return "北纬";
case 'W':return "东经";
case 'w':return "东经";
case 'E':return "西经";
case 'e':return "西经";
default: return "经纬错误";
}
}
int main()
{
int angle=0;
float min=0;
char longitude=0, latitude=0;
double gps_g=0, gps_i=0;//gps_g经度 gps_i纬度
get_gps_data(FILE_PATH, &gps_i, &gps_g, &longitude, &latitude);
printf("gps: %lf %c, %lf %c\n",gps_i, latitude, gps_g, longitude);

/\*gps\_cal(gps\_g, longitude, &angle, &min);  
printf("%s %d度%f分", print(longitude), angle, min);  
gps\_cal(gps\_i, latitude, &angle, &min);  
printf("    %s %d度%f分\\n", print(latitude), angle, min);\*/  
gps\_cal(gps\_i, latitude, &angle, &min);  
printf("%s %d度%f分", print(latitude), angle, min);  
gps\_cal(gps\_g, longitude, &angle, &min);  
printf("    %s %d度%f分\\n", print(longitude), angle, min);  
return 0;  

}

　　效果如下

附测试用的GPS.log文件部分信息

$GPVTG,,T,,M,,N,,K,N*2C
$GPGSA,A,1,,,,,,,,,,,,,,,*1E
$GPGGA,,,,,,0,,,,,,,,*66
$GPRMC,,V,,,,,,,,,,N*53
$GPGSV,3,1,10,10,49,322,50,12,15,119,40,20,78,352,49,25,09,151,39*7F
$GPGSV,3,2,10,32,28,272,48,14,13,260,,15,25,063,,21,41,203,*7B
$GPGSV,3,3,10,24,51,053,,27,,,*49
$GPVTG,,T,,M,,N,,K,N*2C
$GPGSA,A,1,,,,,,,,,,,,,,,*1E
$GPGGA,,,,,,0,,,,,,,,*66
$GPRMC,,V,,,,,,,,,,N*53
$GPGSV,3,1,11,10,49,322,51,12,15,119,40,20,78,352,49,25,09,151,38*7E
$GPGSV,3,2,11,32,28,272,47,14,13,260,,15,25,063,,21,41,203,*75
$GPGSV,3,3,11,24,51,053,,27,,,,40,,,36*49
$GPGGA,052550.00,3409.338831,N,10853.658643,E,1,04,1.4,467.2,M,-28.0,M,,*43
$GPVTG,1.5,T,4.6,M,0.3,N,0.6,K,A*20
$GPRMC,052550.00,A,3409.338831,N,10853.658643,E,0.3,1.5,121218,3.1,W,A*25
$GPGSA,A,2,10,12,20,32,,,,,,,,,1.6,1.4,0.9*39
$GPGSV,3,1,11,10,49,322,51,12,15,119,40,14,13,260,29,15,25,063,30*77
$GPGSV,3,2,11,20,78,352,49,21,41,203,22,24,51,053,33,25,09,151,39*7E
$GPGSV,3,3,11,32,28,272,47,27,,,,40,,,38*4C
$GPGGA,052551.00,3409.341502,N,10853.663318,E,1,05,1.2,459.4,M,-28.0,M,,*4E
$GPVTG,1.5,T,4.6,M,0.0,N,0.0,K,A*25
$GPRMC,052551.00,A,3409.341502,N,10853.663318,E,0.0,1.5,121218,3.1,W,A*27
$GPGSA,A,2,10,12,20,24,32,,,,,,,,1.4,1.2,0.8*3A
$GPGSV,3,1,11,10,49,322,51,12,15,119,40,14,13,260,30,15,25,063,30*7F
$GPGSV,3,2,11,20,78,352,49,21,41,203,22,24,51,053,33,25,09,151,39*7E
$GPGSV,3,3,11,32,28,272,47,27,,,,40,,,38*4C
$GPGGA,052552.00,3409.340891,N,10853.662052,E,1,05,1.2,462.9,M,-28.0,M,,*42
$GPVTG,1.5,T,4.6,M,0.0,N,0.0,K,A*25
$GPRMC,052552.00,A,3409.340891,N,10853.662052,E,0.0,1.5,121218,3.1,W,A*2E
$GPGSA,A,2,10,12,20,24,32,,,,,,,,1.4,1.2,0.8*3A
$GPGSV,3,1,11,10,49,322,51,12,15,119,40,14,13,260,30,15,25,063,27*79
$GPGSV,3,2,11,20,78,352,49,21,41,203,20,24,51,053,31,25,09,151,39*7E
$GPGSV,3,3,11,32,28,272,47,27,,,,40,,,38*4C
$GPGGA,052553.00,3409.340173,N,10853.661041,E,1,05,1.2,465.3,M,-28.0,M,,*4A
$GPVTG,1.5,T,4.6,M,0.0,N,0.0,K,A*25
$GPRMC,052553.00,A,3409.340173,N,10853.661041,E,0.0,1.5,121218,3.1,W,A*2B
$GPGSA,A,2,10,12,20,24,32,,,,,,,,1.4,1.2,0.8*3A
$GPGSV,3,1,11,10,49,322,50,12,15,119,41,14,13,260,30,15,25,063,27*79
$GPGSV,3,2,11,20,78,352,49,21,41,203,20,24,51,053,32,25,09,151,39*7D
$GPGSV,3,3,11,32,28,272,46,27,,,,40,,,39*4C
$GPGGA,052554.00,3409.339925,N,10853.660214,E,1,05,1.2,466.6,M,-28.0,M,,*4D
$GPVTG,1.5,T,4.6,M,0.0,N,0.0,K,A*25
$GPRMC,052554.00,A,3409.339925,N,10853.660214,E,0.0,1.5,121218,3.1,W,A*2A
$GPGSA,A,2,10,12,20,24,32,,,,,,,,1.4,1.2,0.8*3A
$GPGSV,3,1,11,10,49,322,51,12,15,119,40,14,13,260,31,15,25,063,28*77
$GPGSV,3,2,11,20,78,352,49,21,41,203,21,24,51,053,32,25,09,151,39*7C
$GPGSV,3,3,11,32,28,272,46,27,,,,40,,,39*4C
$GPGGA,052555.00,3409.340026,N,10853.659458,E,1,05,1.2,466.2,M,-28.0,M,,*48
$GPVTG,1.5,T,4.6,M,0.0,N,0.0,K,A*25
$GPRMC,052555.00,A,3409.340026,N,10853.659458,E,0.0,1.5,121218,3.1,W,A*2B
$GPGSA,A,2,10,12,20,24,32,,,,,,,,1.4,1.2,0.8*3A
$GPGSV,3,1,11,10,49,322,51,12,15,119,40,14,13,260,30,15,25,063,28*76
$GPGSV,3,2,11,20,78,352,49,21,41,203,22,24,51,053,32,25,09,151,39*7F
$GPGSV,3,3,11,32,28,272,46,27,,,,40,,,39*4C
$GPGGA,052556.00,3409.339490,N,10853.658951,E,1,05,1.2,465.8,M,-28.0,M,,*40
$GPVTG,1.5,T,4.6,M,0.0,N,0.0,K,A*25