How to calibrate altitude depending on position above sea level?

Question

I'm building a simple altimeter app and I want to be able to calibrate the altitude depending on where I am located above sea level.

So at the moment I am at 300m but I want to be able calibrate that so it says 0m, and make my location "sea level".

How would I implement the calibration?

MainView.h

import

import

import

@interface MainView : UIView { IBOutlet UILabel *altitude;

CLLocationManager *locmanager;

}

• (void)locationManager:(CLLocationManager *)manager didUpdateToLocation:(CLLocation *)newLocation fromLocation:(CLLocation *) oldLocation ;
• (void)locationManager:(CLLocationManager *)manager didFailWithError:(NSError *) error;

@end

And the .m file

MainView.m

@implementation MainView

-(void)awakeFromNib {

locmanager = [[CLLocationManager alloc] init]; [locmanager setDelegate:self]; [locmanager setDesiredAccuracy:kCLLocationAccuracyBest];

[locmanager startUpdatingLocation];

}

• (void)locationManager:(CLLocationManager *)manager didUpdateToLocation:(CLLocation *)newLocation fromLocation:(CLLocation *)oldLocation {

  altitude.text = [NSString stringWithFormat: @"%.2f m", newLocation.altitude]; }

• (void)locationManager:(CLLocationManager *)manager didFailWithError:(NSError *)error { altitude.text = @"0.00 m"; }

• (id)initWithFrame:(CGRect)frame { if (self = [super initWithFrame:frame]) { // Initialization code } return self; }

• (void)drawRect:(CGRect)rect { // Drawing code }

• (void)dealloc { [super dealloc]; }

@end