Setting bounds of a map to cover collection of POIs on Android

Lately, as I browse web for maps related questions on Android, what’s frequently requested is an example of setting bounds of a map (zooming to a proper level and panning) to be able show all of the pins given on the screen.

Most of the maps APIs provide this functionality such as Google Maps API, so developers seem to have problems with implementing theirs. Google Maps API for Android does not provide functionality for setting bounds to a box. Instead, what’s provided is to zoom to a span.

com.google.android.maps.MapController.zoomToSpan(int latSpanE6, int lonSpanE6)

latSpanE6 is the difference in latitudes * 10^6 and similarly lonSpanE6 is the difference longitude * 10^6. You may question how map controllers know where to zoom in just by the differences. For examples, kms between longitudes differ from equator to poles. Fortunately, Google maps projection has them in the same length. This may remind you the infamous South America versus Greenland syndrome. Although Greenland is much much smaller than South America, it doesnt look so with this map projection.

On the below, I implemented a boundary arranger method for MapView. Method takes three arguments: items, hpadding and vpadding. items as you may guess is a list of POIs. Other arguments are a little bit more interesting. hpadding and vpadding is the percentage of padding you would like to leave horizontally and vertically so that pins don’t appear just on the corners. For instance, if you assign 0.1 for hpadding, 10% padding will be given from top and bottom.

BTW, You’ll have to extend the existing MapView and add this method to your own MapView to use this method properly.

public void setMapBoundsToPois(List<GeoPoint> items, double hpadding, double vpadding) {

    MapController mapController = this.getController();
    // If there is only on one result
    // directly animate to that location

    if (items.size() == 1) { // animate to the location
        mapController.animateTo(items.get(0));
    } else {
        // find the lat, lon span
        int minLatitude = Integer.MAX_VALUE;
        int maxLatitude = Integer.MIN_VALUE;
        int minLongitude = Integer.MAX_VALUE;
        int maxLongitude = Integer.MIN_VALUE;

        // Find the boundaries of the item set
        for (GeoPoint item : items) {
            int lat = item.getLatitudeE6(); int lon = item.getLongitudeE6();

            maxLatitude = Math.max(lat, maxLatitude);
            minLatitude = Math.min(lat, minLatitude);
            maxLongitude = Math.max(lon, maxLongitude);
            minLongitude = Math.min(lon, minLongitude);
        }

        // leave some padding from corners
        // such as 0.1 for hpadding and 0.2 for vpadding
        maxLatitude = maxLatitude + (int)((maxLatitude-minLatitude)*hpadding);
        minLatitude = minLatitude - (int)((maxLatitude-minLatitude)*hpadding);

        maxLongitude = maxLongitude + (int)((maxLongitude-minLongitude)*vpadding);
        minLongitude = minLongitude - (int)((maxLongitude-minLongitude)*vpadding);

        // Calculate the lat, lon spans from the given pois and zoom
        mapController.zoomToSpan(Math.abs(maxLatitude - minLatitude), Math
.abs(maxLongitude - minLongitude));

        // Animate to the center of the cluster of points
        mapController.animateTo(new GeoPoint(
              (maxLatitude + minLatitude) / 2, (maxLongitude + minLongitude) / 2));
    }
} // end of the method

5 thoughts on “Setting bounds of a map to cover collection of POIs on Android”

  2. mdakin says:

    // pretty neat.
    //
    // Though last part look a little more concise with this refactoring:
    …
    int hpad = (maxLatitude-minLatitude) * hpadding;
    maxLatitude += hpad;
    minLatitude -= hpad;
    …
    // Also I don’t think you need using Math.abs() as your code already guarantees maxLatitude >= minLatitude

    Reply
  3. Burcu Dogan says:

    Thanks for the feedback, definitely I’ll make a few modifications including your suggestions due to the fact this code is being copy-pasted a lot although it should be used just as a reference.

    Reply
  4. Omer Gurarslan says:

    Great article. But please take into consideration that latitudes in GoogleMaps vary between -180 and 180 and this may cause some problems in your code.

    In an example case where there are two points with latitudes -170 and 170 respectively, the code should be able to determine the more reasonable span length, that is not 340, but 20.

    I think that is closely related to why Google Maps API for Android does not provide functionality for setting bounds to a box but instead, what’s provided is to zoom to a span. This way, the option to choose a smaller span length is given to the programmer.

    Reply
  5. Burcu Dogan says:

    Omer, this code has a few other bugs other than what you’ve pointed and I’ll try to come up with a new version as soon as possible. Btw, this piece is only for reference and should not be adopted to a final product as it is.

    Reply

Leave a comment