Convert decimal latitude/longitude coordinates to iPad screen coordinates - ios

Question

I have a latitude/longitude decimal coordinates that I would like to convert to points that will fit within the area of the iPad coordinate system.
My code takes in a latitude/longitude coordinates and converts them to cartesian coordinates (based on this question: convert to cartesian coordinates) The results of the conversion are (2494.269287, 575.376465).
Here is my code (no compile or runtime errors):
#define EARTH_RADIUS 6371
- (void)drawRect:(CGRect)rect
{
CGPoint latLong = {41.998035, -116.012215};
CGPoint newCoord = [self convertLatLongCoord:latLong];
NSLog(#"Cartesian Coordinate: (%f, %f)",newCoord.x, newCoord.y);
//Draw dot at coordinate
CGColorRef darkColor = [[UIColor colorWithRed:21.0/255.0
green:92.0/255.0
blue:136.0/255.0
alpha:1.0] CGColor];
CGContextRef context = UIGraphicsGetCurrentContext();
CGContextSetFillColorWithColor(context, darkColor);
CGContextFillRect(context, CGRectMake(newCoord.x, newCoord.y, 100, 100));
}
-(CGPoint)convertLatLongCoord:(CGPoint)latLong
{
CGFloat x = EARTH_RADIUS * cos(latLong.x) * cos(latLong.y);
CGFloat y = EARTH_RADIUS * cos(latLong.x) * sin(latLong.y);
return CGPointMake(x, y);
}
How can I convert the decimal coordinates to coordinates that will show up on an iPad screen?

Answer 1

Your convertLatLongCoord: method makes no attempt to adjust the resulting point to screen coordinates. As written, you can get x and y values in the range -EARTH_RADIUS to +EARTH_RADIUS. These need to be scaled to fit the screen.
Something like the following should help:
- (CGPoint)convertLatLongCoord:(CGPoint)latLong {
CGFloat x = EARTH_RADIUS * cos(latLong.x) * cos(latLong.y) * SCALE + OFFSET;
CGFloat y = EARTH_RADIUS * cos(latLong.x) * sin(latLong.y) * SCALE + OFFSET;
return CGPointMake(x, y);
}
SCALE and OFFSET should be a value determined as follows:
CGSize screenSize = [UIScreen mainScreen].applicationFrame.size;
CGFloat SCALE = MIN(screenSize.width, screenSize.height) / (2.0 * EARTH_RADIUS);
CGFloat OFFSET = MIN(screenSize.width, screenSize.height) / 2.0;
This assumes you want the map to fill the smallest screen dimension.