1.2.3WGS84->LLA转换问题

在分析其日志的过程中，遇到一个问题，就是TimGPS二进制日志保存的坐标数据是以WGS84大地坐标系为准的，大地坐标系的XYZ轴如下：

要得到经纬度和海拔(LLA：Longitude/Latitude/Altitude)坐标，需要一些椭球体、基准面及地图投影的知识。

后来从网上找到一个Fortune77写的WGS84->GLL换算函数，将其转成C/C++的函数:

void wgsxyz2lla(double x,double y,double z,double *lat,double *lon,double *alt)

{

         double pi = 3.14159265357;

         longA_EARTH = 6378137;

         double  flattening = 1/298.257223563;

         double NAV_E2 = (2-flattening)*flattening; // also e^2

         double rad2deg = 180/pi;

         double wlon,wlat,walt,rhosqrd,rho,templat,tempalt,rhoerror,zerror;

         double slat,clat,q,r_n,drdl,invdet,aa,bb,cc,dd;

         if ((x == 0.0) & (y == 0.0))

                   wlon = 0.0;

         else

                   wlon = atan2(y, x)*rad2deg;

         if ((x == 0.0) & (y == 0.0) & (z == 0.0))

         {

                   printf("WGS xyz at center of earth");

                   wlon = 360;

                   wlat = 360;

                   walt = 6378137;

         }

         else

         {

                   rhosqrd = x*x + y*y;

                   rho = sqrt(rhosqrd);

                   templat = atan2(z, rho);

                   tempalt = sqrt(rhosqrd + z*z) - A_EARTH;

                   rhoerror = 1000.0;

                   zerror   = 1000.0;

                   while ((Dabs(rhoerror) > 1e-6) | (Dabs(zerror) > 1e-6))

                   {

                            slat = sin(templat);

                            clat = cos(templat);

                            q = 1 - NAV_E2*slat*slat;

                            r_n = A_EARTH/sqrt(q);

                            drdl = r_n*NAV_E2*slat*clat/q; // d(r_n)/d(latitutde)

                            rhoerror = (r_n + tempalt)*clat - rho;

                            zerror   = (r_n*(1 - NAV_E2) + tempalt)*slat - z;

                            aa = drdl*clat - (r_n + tempalt)*slat;

                            bb = clat;

                            cc = (1 - NAV_E2)*(drdl*slat + r_n*clat);

                            dd = slat;

                            invdet = 1.0/(aa*dd - bb*cc);

                            templat = templat - invdet*(+dd*rhoerror -bb*zerror);

                            tempalt = tempalt - invdet*(-cc*rhoerror +aa*zerror);

                   }

                   wlat = templat*rad2deg;

                   walt = tempalt;

         }

         if(lon )*lon = wlon;

         if(lat )*lat = wlat;

         if(alt )*alt = walt;

}

结语：开发GPS应用软件的要点在于理解相关的协议和接口，仔细处理协议字段。

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