Modifying CLLocations

[coulls]

Hi all,

I have a line object made up of two CLLocation's (startPoint and endPoint). I am adding a tolerance set in metres (say 3000 metres) around an MBR (Minimum Bounding Rectangle) derived from these two points.

The question I have is how would I add say 3000 metres to a CLLocation and arrive at a new CLLocation, especially given that there's no function like this?

-(CLLocation *) addDistanceToLocation:(CLLocation*)location:(float)metresLatitude:(float)metresLongitude;

What I'd ideally like to do is something like this:

startPoint = addDistanceToLocation:startPoint:3000:-3000

EDIT: I've changed my MBR code to read like this:

	MBR *mbr = [MBR alloc];
	CLLocation *x1y1 = [[CLLocation alloc] init];
	CLLocation *x2y2 = [[CLLocation alloc] init];
	x1y1.coordinate.latitude = MAX(startPoint.coordinate.latitude, [endPoint.coordinate.latitude]);
	x1y1.coordinate.longitude = MIN(startPoint.coordinate.longitude, [endPoint.coordinate.longitude]);
	x2y2.coordinate.latitude = MIN(startPoint.coordinate.latitude, [endPoint.coordinate.latitude]);
	x2y2.coordinate.longitude = MAX(startPoint.coordinate.longitude, [endPoint.coordinate.longitude]);
	mbr.topLeft = x1y1;
	mbr.bottomRight = x2y2;

Thought it might be easier to work with two CLLocations (topLeft and bottomRight). Still can't see how to add distances to these and arrive at a new set of points.

Thanks,

Jason

[coulls]

Found the solution, so don't worry...

Just incase anyone else hits this, here's how I did it.

-(CLLocation*) offsetLocation:(CLLocation*)startLocation:(double)offsetMeters:(double)bearing
{
	
	double EARTH_MEAN_RADIUS_METERS = 6372796.99;
	double lat2 = asin( sin(startLocation.coordinate.latitude) * cos(offsetMeters/EARTH_MEAN_RADIUS_METERS) + cos(startLocation.coordinate.latitude) * sin(offsetMeters/EARTH_MEAN_RADIUS_METERS) * cos(bearing) );
	double lon2 = startLocation.coordinate.longitude + atan2( sin(bearing) * sin(offsetMeters/EARTH_MEAN_RADIUS_METERS) * cos(startLocation.coordinate.latitude), cos(offsetMeters/EARTH_MEAN_RADIUS_METERS) - sin(startLocation.coordinate.latitude) * sin(lat2));
	CLLocation *tempLocation = [[CLLocation alloc] initWithLatitude:lat2 longitude:lon2];

	return tempLocation;
}

So, if I wanted to move something 3000 meters south (180 degree bearing).

newLocation = [self offsetLocation: oldLocation :3000 :180];

Cheers,

Jason

[b.dot]

Originally Posted by coulls

	
	double lat2 = asin( sin(startLocation.coordinate.latitude) * cos(offsetMeters/EARTH_MEAN_RADIUS_METERS) + cos(startLocation.coordinate.latitude) * sin(offsetMeters/EARTH_MEAN_RADIUS_METERS) * cos(bearing) );

@coulls

Thanks for helping me with a starting point. I'm building a bounding box, and trying to determine the maximum latitude north. This is the exact calculation that I need, however given the following:

offsetLocation: latitude 37.331688999999997, longitude -122.030731
offsetMeters: 1000
bearing: 0

lat2 returns -0.36726592610659514

Any suggestions? I've coded around this particular formula until now and this one has me stumped.

b.dot

[Duncan C]

Originally Posted by coulls

Hi all,

I have a line object made up of two CLLocation's (startPoint and endPoint). I am adding a tolerance set in metres (say 3000 metres) around an MBR (Minimum Bounding Rectangle) derived from these two points.

The question I have is how would I add say 3000 metres to a CLLocation and arrive at a new CLLocation, especially given that there's no function like this?

-(CLLocation *) addDistanceToLocation:(CLLocation*)location:(float)metresLatitude:(float)metresLongitude;

What I'd ideally like to do is something like this:

startPoint = addDistanceToLocation:startPoint:3000:-3000

EDIT: I've changed my MBR code to read like this:

	MBR *mbr = [MBR alloc];
	CLLocation *x1y1 = [[CLLocation alloc] init];
	CLLocation *x2y2 = [[CLLocation alloc] init];
	x1y1.coordinate.latitude = MAX(startPoint.coordinate.latitude, [endPoint.coordinate.latitude]);
	x1y1.coordinate.longitude = MIN(startPoint.coordinate.longitude, [endPoint.coordinate.longitude]);
	x2y2.coordinate.latitude = MIN(startPoint.coordinate.latitude, [endPoint.coordinate.latitude]);
	x2y2.coordinate.longitude = MAX(startPoint.coordinate.longitude, [endPoint.coordinate.longitude]);
	mbr.topLeft = x1y1;
	mbr.bottomRight = x2y2;

Thought it might be easier to work with two CLLocations (topLeft and bottomRight). Still can't see how to add distances to these and arrive at a new set of points.

Thanks,

Jason

I wrestled with this too. You can't change the values in a CLLocation. What you have to do is calculate the new coordinate you want, and then initialize a new one with the parameters from the old one.

You use the call initWithCoordinate:altitude:horizontalAccuracy:ver ticalAccuracy:timestamp: to create a new location using the values from the old location.

That lets you copy most, but not all, of the settings from the old location.


Regards,

Duncan C
WareTo

[coulls]

Originally Posted by b.dot

@coulls

Thanks for helping me with a starting point. I'm building a bounding box, and trying to determine the maximum latitude north. This is the exact calculation that I need, however given the following:

offsetLocation: latitude 37.331688999999997, longitude -122.030731
offsetMeters: 1000
bearing: 0

lat2 returns -0.36726592610659514

Any suggestions? I've coded around this particular formula until now and this one has me stumped.

b.dot

I've not had a chance to come back to this one yet. I hope to attack it fairly soon though! :-)

Coulls

[coulls]

Originally Posted by Duncan C

I wrestled with this too. You can't change the values in a CLLocation. What you have to do is calculate the new coordinate you want, and then initialize a new one with the parameters from the old one.

Thanks - I sort of arrived at that conclusion. The bigger problem for me next was the accuracy of the offset in metres when dealing with longitude. Still haven't quite had the time to sit down and figure that one out.

Coulls

[coulls]

OK - Had to fix that bug, and it's looking good now... although much different to the first implementation. Much head scratching got me ready to throw the Mac out the window....

Add this at the top of your .m file. (Note the ArcCos hack I had to make as couldn't get the quiet_nan to work).

#include <math.h>
#define KmPerDegree		111.12000071117
#define	DegreesPerKm	(1.0/KmPerDegree)
#define PI				M_PI
#define TwoPI			(M_PI+M_PI)
#define HalfPI			M_PI_2
#define RadiansPerDegree	(PI/180.0)
#define	DegreesPerRadian	(180.0/PI)
#define copysign(x,y)		(((y)<0.0)?-fabs(x):fabs(x))
#define NGT1(x)		(fabs(x)>1.0?copysign(1.0,x):(x))
//#define ArcCos(x)	(fabs(x)>1?quiet_nan():acos(x)) //Hack
#define ArcCos(x)	(acos(x))
#define hav(x)		((1.0-cos(x))*0.5)		/* haversine */
#define ahav(x)		(ArcCos(NGT1(1.0-((x)*2.0))))	/* arc haversine */
#define sec(x)		(1.0/cos(x))			/* secant */
#define csc(x)		(1.0/sin(x))			/* cosecant */

The new function (take a deep breath - I kept the function signature the same, though) looks like this:

//Moves a CLLocation from one point to another in the bearing specified by the meters specified in that direction.
-(CLLocation*) moveLocation:(CLLocation*)startLocation:(double)movementInMeters:(double)movementBearing
{
	
	double	dist = (movementInMeters / 1000);	/* -> great-circle distance (km) */
	double	course = movementBearing;			/* -> initial great-circle course (degrees) */
	double	slt = startLocation.coordinate.latitude;	/* -> starting decimal latitude (-S) */
	double	slg = startLocation.coordinate.longitude;	/* -> starting decimal longitude(-W) */
	double	xlt = 0;	/* <- ending decimal latitude (-S) */
	double	xlg = 0;	/* <- ending decimal longitude(-W) */
	
	double	c, d, dLo, L1, L2, coL1, coL2, l;
		
	if (dist > KmPerDegree*180.0) {
		course -= 180.0;
		if (course < 0.0) course += 360.0;
		dist    = KmPerDegree*360.0-dist;
	}
	
	if (course > 180.0) course -= 360.0;
	c    = course*RadiansPerDegree;
	d    = dist*DegreesPerKm*RadiansPerDegree;
	L1   = slt*RadiansPerDegree;
	slg *= RadiansPerDegree;
	coL1 = (90.0-slt)*RadiansPerDegree;
	coL2 = ahav(hav(c)/(sec(L1)*csc(d))+hav(d-coL1));
	L2   = HalfPI-coL2;
	l    = L2-L1;
	if ((dLo=(cos(L1)*cos(L2))) != 0.0)
		dLo  = ahav((hav(d)-hav(l))/dLo);
	if (c < 0.0) dLo = -dLo;
	slg += dLo;
	if (slg < -PI)
		slg += TwoPI;
	else if (slg > PI)
		slg -= TwoPI;
		
	xlt = L2*DegreesPerRadian;
	xlg = slg*DegreesPerRadian;
		
	CLLocation *tempLocation = [[CLLocation alloc] initWithLatitude:xlt longitude:xlg];
	return tempLocation;
}

The source of this solution is here:
http://sam.ucsd.edu/sio210/propseawater/ppsw_c/gcdist.c

Cheers,

Coulls