# SparkFun Forums

### Sirf Protocol Help

Tips and questions relating to the GPS modules from SFE

### Sirf Protocol Help#50182

By jordi
#50182
Hello Everybody:

Im trying to use the binary mode in this GPS: http://www.sparkfun.com/commerce/produc ... ts_id=8621

The problem is that i have no idea how to convert the position XYZ data (in meters),
to degrees (lat and lon) and altitude...

To give an idea what im talking about, please check the page 48 on this data sheet:
On message ID #4...

Any idea?

Thanks very much JOrdi

### #50202

By gm
#50202
It looks like message ID 41 (page 87) contains the lat and lon you are looking for.

Hope this helps.

gm

### #50562

By coyotebush
#50562
If you want to tackle it head on, here is some code based on Kai and Borre's "Linear algebra, Geodesy and GPS". The routine converts xyz coordinates to lat/lon in radians. (The open source "gps toolkit" also has a version.)

"a" and "f" are constants describing the shape of the earth ellipsoid. For WGS84, they are set to 6378137.0 and 1.0/298.2572235630 respectively.

void XYZToGeod(double a, double f,
double x, double y, double z,
double& phi, double& lambda, double& h)
{
debug("XYZToGeod a=%.3f f=%.6f x=%.3f y=%.3f z=%.3f\n", a,f,x,y,z);

// set up some constants
double r = sqrt(x*x + y*y);

// Iterate. Converges quickly. Do a fixed number of iterations for now.
h = 0;
double sinphi = 0;
for (int i=0; i<10; i++) {
double N = a / sqrt(1 - f*(2-f)*sinphi*sinphi);
phi = atan2(z, r*(1 - f*(2-f)*N/(N+h)));
h = r / cos(phi) - N;
sinphi = sin(phi);
debug(" N=%.3f h=%.3f phi=%.6f\n", N, h, RadToDeg(phi));
}

// calculate longitude
lambda = atan2(y,x);

debug(" lambda=%.3f phi=%.3f h=%.3f\n",