Game Development Reference
In-Depth Information
//Calculate deltas
float dLat = lat2 - lat1;
float dLon = lon2 - lon1;
//find delta phi
float deltaPhi = log(tan(lat2/2+(M_PI)/4)/tan(lat1+M_PI/4))
float q=(deltaPhi==0 ? dlat/deltaPhi : cos(lat1); //avoids division by 0
if (abs(dLon) > M_PI){
dLon = (dLon>0 ? −(2*(M_PI-dLon):(2*M_PI+dLon));
}
float D = sqrt(dLat*dLat + q*q*dLon*dLo)* 6371;
float theta = atan2f(dLon, deltaPhi);
//now convert to compass heading
float bearing = theta * (180 / M_PI); //radians to degrees
bearing = ( bearing > 0 ? bearing : (360.0 + bearing)); //fix range
return bearing;
}
There are a few things worth pointing out. First is that we are using a ternary function
in the line commented by “avoids division by 0” to take care of the case when delta
Phi is equal to 0. If it is 0, q is set to cos(lat1) ; if not, then it is set to dlat/deltaPhi .
The if statement immediately following ensures that if dLon is greater than π (180°),
hence putting us on a longer-than-required rhumb line, then we should correct the
value to correspond to the shortest route. This is achieved via the ternary, which ensures
that dLon is less than π and nonnegative. Lastly, we convert from a normalized radian
answer to a compass direction.
Now that you have a good idea about how to calculate position and distance in the
geographic coordinate system, you can use the earlier chapters to determine other
quantities like speed and acceleration.
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