Determine an object is in front of you or not

[rayhklee]

Assume I can get a lat/lon data of an object, for iPhone 3Gs model, how can I determine the object is in front of me or not, so I can take a video

[RobotWoods]

From a theoretical perspective, couldn't you determine the angle between north, and the line formed by your current location and the location of the object, then use the compass to determine if the phone heading is the same as the calculated angle? And you'd probably want to determine the distance between the points so that if the object is very far away, you can ignore it. I have not used the compass myself, but have heard that the accuracy is not the best...so I'd probably make that distance pretty small so that you can give a decent margin of error on the angles aligning, and still have a good chance of getting the video you need.

[coulls]

The easiest solution is this:

1. Get your lat/lon. (lat1/lon1)
2. Get the lat/lon of the object (lat2/lon2).
3. Calculate the absolute bearing from lat1/lon1 to lat2/lon2.
4. Get the compass bearing of your device. Subtract the bearing of the device from the bearing of the object. This is your "relative bearing".

Code for step 4 is this:

- (float)relativeBearingFromBearing:(float)bearingOfLocation:(float)bearingOfCompassHeading
{
	// bearingOfLocation is the rhumb line from where you are physically.
	// bearingOfCompassHeading is the direction your compass is pointing physically.
	
	//Example:
	//if object is 0 deg (N) of you and you are facing west (-270), then object is actually
	//90 deg to your right.
	
	float relativeBearing = (bearingOfLocation + bearingOfCompassHeading);
	if (relativeBearing > 360) {
		//Correct it.
		relativeBearing -= 360;
	}
	return relativeBearing;
}

If the bearing is +/- say 20 degrees, then the object is roughly in front of you.

Hope that helps.

Cheers,

Jase