Question

I’ve looked at several StackOverflow posts and Apple documentation on how to implement overlays in MKMapView. For me, I’m interested specifically in displaying MKPolygon objects on my map. I’ve found that fundamentally, the process boils down to the following:

• Link to MapKit and CoreLocation frameworks
• Make an outlet to an MKMapKit object and declare view controller as delegate
• Declare a CLLocationCoordinate2D array containing the points of a polygon and create an MKPolygon object with the class method polygonWithCoordinates:count:
• Call addOverlay: of map and pass the newly created MKPolygon object as the parameter
• Implement (MKOverlayView *)mapView:(MKMapView *)mapView viewForOverlay:(id)overlay

Later on, I’ll likely be having to display 20-30 polygons at a given time on the map. However, in my exploration of how to display overlays (hardcoding test examples right now, rather than reading in data from a file), I’ve found that I can get some overlays to appear, but not others. Reading the Location Awareness Programming Guide by Apple, I came across an example of a polygon overlaid above the state of Colorado. That worked. But when I tried to make a polygon that covered Kansas, I couldn’t get it to work. It seems that any polygon that I tried to make on my own (Embry-Riddle Aeronautical University polygon and Kansas polygon) won’t display, but those that I got online work perfectly. I used Google Earth to create the polygons and then exported them as KML files to get the coordinates.

Code for the implementation of my ViewController is below. Just trying to find out what I may be unintentionally doing wrong to create this problem. Thanks in advance for help.

#import "ViewController.h"
#import <CoreLocation/CoreLocation.h>

@interface ViewController ()

@end

@implementation ViewController

@synthesize mapView;

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    // Array of coordinates for polygon covering state of Colorado ... DISPLAYS PERFECTLY

    CLLocationCoordinate2D points[4];

    points[0] = CLLocationCoordinate2DMake(41.000512, -109.050116);
    points[1] = CLLocationCoordinate2DMake(36.99892, -109.045267);
    points[2] = CLLocationCoordinate2DMake(36.993076, -102.041981);
    points[3] = CLLocationCoordinate2DMake(41.002371, -102.052066);

    MKPolygon *polygon = [MKPolygon polygonWithCoordinates:points count:4];
    [mapView addOverlay:polygon];
    [polygon release];

    // Array of coordinates for polygon covering state of Kansas ... DOESN'T DISPLAY

    CLLocationCoordinate2D kansasPoints[9];

    kansasPoints[0] = CLLocationCoordinate2DMake(-102.0595440241806, 39.99774930940907);
    kansasPoints[1] = CLLocationCoordinate2DMake(-102.0424467175215, 36.99846609483674);
    kansasPoints[2] = CLLocationCoordinate2DMake(-94.62550551403953, 36.98936020770036);
    kansasPoints[3] = CLLocationCoordinate2DMake(-94.58798745384412, 39.11683771419185);
    kansasPoints[4] = CLLocationCoordinate2DMake(-94.79955391183, 39.21290793052091);
    kansasPoints[5] = CLLocationCoordinate2DMake(-95.13489191971419, 39.51613476830012);
    kansasPoints[6] = CLLocationCoordinate2DMake(-94.86553124171813, 39.78380472206268);
    kansasPoints[7] = CLLocationCoordinate2DMake(-95.02618283417986, 39.89072859904893);
    kansasPoints[8] = CLLocationCoordinate2DMake(-95.31904155494097, 39.99390420513669);

    MKPolygon *kansasPolygon = [MKPolygon polygonWithCoordinates:kansasPoints count:9];
    [mapView addOverlay:kansasPolygon];
    [kansasPolygon release];

    // Array of coordinates for polygon covering part of Daytona Beach, FL campus
    // of Embry-Riddle Aeronautical University... DOESN'T DISPLAY

    CLLocationCoordinate2D erauPoints[7];

    erauPoints[0] = CLLocationCoordinate2DMake(-81.05176, 29.18492);
    erauPoints[1] = CLLocationCoordinate2DMake(-81.04409, 29.18801);
    erauPoints[2] = CLLocationCoordinate2DMake(-81.05166, 29.19293);
    erauPoints[3] = CLLocationCoordinate2DMake(-81.05365, 29.19536);
    erauPoints[4] = CLLocationCoordinate2DMake(-81.05465, 29.19493);
    erauPoints[5] = CLLocationCoordinate2DMake(-81.05376, 29.19323);
    erauPoints[6] = CLLocationCoordinate2DMake(-81.05506, 29.19188);

    MKPolygon *erauPolygon = [MKPolygon polygonWithCoordinates:erauPoints count:7];
    [mapView addOverlay:erauPolygon];
    [erauPolygon release];

    // Array of coordinates taken from http://www.shawngrimes.me/2011/04/adding-polygon-map-overlays/
    // for commuter parking lot at Capitol College in Maryland ... DISPLAYS PERFECTLY

    CLLocationCoordinate2D commuterLotCoords[5]={
        CLLocationCoordinate2DMake(39.048019,-76.850535),
        CLLocationCoordinate2DMake(39.048027,-76.850234),
        CLLocationCoordinate2DMake(39.047407,-76.850181),
        CLLocationCoordinate2DMake(39.047407,-76.8505),
        CLLocationCoordinate2DMake(39.048019,-76.850535)
    };

    MKPolygon *commuterPoly1 = [MKPolygon polygonWithCoordinates:commuterLotCoords count:5];
    [mapView addOverlay:commuterPoly1];
    [commuterPoly1 release];
}

- (void)viewDidUnload
{
    [super viewDidUnload];
    // Release any retained subviews of the main view.
}

- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
    return (interfaceOrientation != UIInterfaceOrientationPortraitUpsideDown);
}

- (MKOverlayView *)mapView:(MKMapView *)mapView viewForOverlay:(id<MKOverlay>)overlay {
    if ([overlay isKindOfClass:[MKPolygon class]]) {
        MKPolygonView *polygonView = [[[MKPolygonView alloc] initWithOverlay:overlay] autorelease];
        polygonView.fillColor = [[UIColor cyanColor] colorWithAlphaComponent:0.3f];
        polygonView.strokeColor = [UIColor redColor];
        polygonView.lineWidth = 1.0f;

        return polygonView;
    }

    return nil;
}

@end

Answer 1

It looks like the latitude and longitude parameters of the coordinates for the polygons that don’t display are backwards.

For example, this:

kansasPoints[0] = CLLocationCoordinate2DMake(-102.0595440241806, 39.99774930940907);

should be

kansasPoints[0] = CLLocationCoordinate2DMake(39.99774930940907, -102.0595440241806);

Also, you should not be calling release on the MKPolygon objects you are creating using polygonWithCoordinates since they will be autoreleased.