How can I reduce the number of annotations on a map?

Question

I\'m coding a map view with around 900 annotations. Having this many annotations on a map make the performance suffer, so I\'d like to reduce it to about 300 at a time. The

Answer 1

One approach is that before placing a new pin, check if there's another pin already placed within distance d of the new pin. If there is, don't place the new pin. You need to vary d based on the current zoom level.

You can reduce the number of pins you check against by considering only pins in a bounding box centered on the new pin. The box could be d x d degrees on a side (with d varying based on zoom level).

Answer 2

Two options I can think of:

• If you have points of interest to work with (Eg, cities), you can simply group all pins by the POI they're closest to, in lower zoom levels.
• You can use K-means clustering to group pins into clusters and represent them with a midpoint pin.

Answer 3

Here's a code snippet that takes an MKAnnotation's coordinates, convert it to a CGPoint relative to the MKMapView, and logs what's the underlying view at that CGPoint.

CGPoint pinPoint = [mapView convertCoordinate:pinView.annotation.coordinate toPointToView:mapView];
NSLog(@"pointing to %@", [[mapView hitTest:pinPoint withEvent:nil] description]);

Put that inside a loop that iterates through all your pins. If the underlying view is another MKAnnotation instance, then hide that pin.

if([[mapView hitTest:pinPoint withEvent:nil] isKindOfClass:[FFMapPinView class]])
    pinView.hidden = YES;

For this to work properly, you need the pinsArray to be ordered so that index 0 is the frontmost pin.

Answer 4

Considering that many pins in densely-populated areas will be on the same street, you could consider making "super pins" that list the pins on a particular street, instead of on each individual address.

-S!

Answer 5

If a commercial, third party library is an option, check out Superpin (license costs $199). It is an iOS Framework that internally uses quadtrees for annotation storage and performs grid-based clustering. The algorithm is quite fast, the included sample app is showing airports of the world (more than 30k+ annotations) and it's running quite smooth on an 3G iPhone.

You may also want to check http://revolver.be/blog/mapkit-clustering-with-ios/, another ready-made solution, which is free for non-commerical projects.

Disclaimer: I'm one of the Superpin developers

Answer 6

Late to the party, I know, but you may find this routine useful. It comes from this file, which is part of a FOSS project.

/**************************************************************//**
 \brief This function looks for meetings in close proximity to each
        other, and collects them into "red markers."
 \returns an NSArray of BMLT_Results_MapPointAnnotation objects.
 *****************************************************************/
- (NSArray *)mapMeetingAnnotations:(NSArray *)inResults ///< This is an NSArray of BMLT_Meeting objects. Each one represents a meeting.
{
#ifdef DEBUG
    NSLog(@"BMLTMapResultsViewController mapMeetingAnnotations - Checking %d Meetings.", [inResults count]);
#endif
    NSMutableArray  *ret = nil;

    NSInteger   displayIndex = 1;

    if ( [inResults count] )
        {
        NSMutableArray  *points = [[NSMutableArray alloc] init];
        for ( BMLT_Meeting *meeting in inResults )
            {
#ifdef DEBUG
            NSLog(@"BMLTMapResultsViewController mapMeetingAnnotations - Checking Meeting \"%@\".", [meeting getBMLTName]);
#endif
            CLLocationCoordinate2D  meetingLocation = [meeting getMeetingLocationCoords].coordinate;
            CGPoint meetingPoint = [(MKMapView *)[self view] convertCoordinate:meetingLocation toPointToView:nil];
            CGRect  hitTestRect = CGRectMake(meetingPoint.x - BMLT_Meeting_Distance_Threshold_In_Pixels,
                                             meetingPoint.y - BMLT_Meeting_Distance_Threshold_In_Pixels,
                                             BMLT_Meeting_Distance_Threshold_In_Pixels * 2,
                                             BMLT_Meeting_Distance_Threshold_In_Pixels * 2);

            BMLT_Results_MapPointAnnotation *annotation = nil;
#ifdef DEBUG
            NSLog(@"BMLTMapResultsViewController mapMeetingAnnotations - Meeting \"%@\" Has the Following Hit Test Rect: (%f, %f), (%f, %f).", [meeting getBMLTName], hitTestRect.origin.x, hitTestRect.origin.y, hitTestRect.size.width, hitTestRect.size.height);
#endif

            for ( BMLT_Results_MapPointAnnotation *annotationTemp in points )
                {
                CGPoint annotationPoint = [(MKMapView *)[self view] convertCoordinate:annotationTemp.coordinate toPointToView:nil];
#ifdef DEBUG
                NSLog(@"BMLTMapResultsViewController mapMeetingAnnotations - Comparing the Following Annotation Point: (%f, %f).", annotationPoint.x, annotationPoint.y);
#endif

                if ( !([[annotationTemp getMyMeetings] containsObject:meeting]) && CGRectContainsPoint(hitTestRect, annotationPoint) )
                    {
#ifdef DEBUG
                    for ( BMLT_Meeting *t_meeting in [annotationTemp getMyMeetings] )
                        {
                        NSLog(@"BMLTMapResultsViewController mapMeetingAnnotations - Meeting \"%@\" Is Close to \"%@\".", [meeting getBMLTName], [t_meeting getBMLTName]);
                        }
#endif
                    annotation = annotationTemp;
                    }
                }

            if ( !annotation )
                {
#ifdef DEBUG
                NSLog(@"BMLTMapResultsViewController mapMeetingAnnotations -This meeting gets its own annotation.");
#endif
                NSArray *meetingsAr = [[NSArray alloc] initWithObjects:meeting, nil];  
                annotation = [[BMLT_Results_MapPointAnnotation alloc] initWithCoordinate:[meeting getMeetingLocationCoords].coordinate andMeetings:meetingsAr andIndex:0];
                [annotation setDisplayIndex:displayIndex++];
                [points addObject:annotation];
                }
            else
                {
#ifdef DEBUG
                NSLog(@"BMLTMapResultsViewController mapMeetingAnnotations -This meeting gets lumped in with others.");
#endif
                [annotation addMeeting:meeting];
                }

            if ( annotation )
                {
                if ( !ret )
                    {
                    ret = [[NSMutableArray alloc] init];
                    }

                if ( ![ret containsObject:annotation] )
                    {
                    [ret addObject:annotation];
                    }
                }
            }
        }

    // This is the black marker.
    BMLT_Results_MapPointAnnotation *annotation = [[BMLT_Results_MapPointAnnotation alloc] initWithCoordinate:[[BMLTAppDelegate getBMLTAppDelegate] searchMapMarkerLoc] andMeetings:nil andIndex:0];

    if ( annotation )
        {
        [annotation setTitle:NSLocalizedString(@"BLACK-MARKER-TITLE", nil)];
        [ret addObject:annotation];
        }

    return ret;
}

You can see it in action in the released version of the app.

Meetings in close proximity are gathered into red annotations, which open a list.