Question

I’ve been researching this on Google and SO but I’m stuck, I think I’m missing something fundamental. Most examples I’ve seen don’t deal with an arbitrary mapWidth and a single point, just the span of an Overlay.

I have a database of map points, a MapView and a Geocoder. I can search for a postcode in my app, and have an Address returned by my Geocoder.

Using this Address, I can build a GeoPoint and search my DB and get back a list of nearby points. The issue comes from trying to zoomToSpan using a span constructed from from the returned Address point and the distance to the nearest point in the database.

I only want the span to encompass the nearest two points (if available). Here’s the relevant code:

Collections.sort(listingDisplay, mComparator);
    listingDisplayAdapter.notifyDataSetChanged();

    float spanWidth =0;

    if (listingDisplay.size() > 1) {

        spanWidth = (float) (2 * distanceFromPoint(listingDisplay.get(1),
                current));

    } else if (listingDisplay.size() == 1) {

        spanWidth = (float) (2 * distanceFromPoint(listingDisplay.get(0),
                current));

    }

    Log.v(TAG, "SpanWidth: " + spanWidth);

    // Create span
    int minLat = (int) (current.getLatitudeE6() - (spanWidth * 1E6) / 2);
    int maxLat = (int) (current.getLatitudeE6() + (spanWidth * 1E6) / 2);
    int minLong = (int) (current.getLongitudeE6() - (spanWidth * 1E6) / 2);
    int maxLong = (int) (current.getLongitudeE6() + (spanWidth * 1E6) / 2);

        // Zoom against span. This appears to create a very small region that doesn't encompass the points
mapController.setCenter(current);
mapController.zoomToSpan(Math.abs( minLat - maxLat ), Math.abs( minLong - maxLong ));

ListingDisplay contains a list of the closest points, with a comparator, mComparator sorting this list with the closest locations to my returned Address (the GeoPoint called: current) at the top of the list .

I then set the value of spanWidth based on the closest, and try to figure out the span from this.

My question is, how can I construct a span from a given distance and centerpoint?

Answer 1

After a very, very long time, I eventually realized that I wasn’t considering some important information, chiefly among them was the fact that distances on Android are calculated using the WGS84 ellipsoid.

I ended up using the helper methods inside Jan Matuschek’s excellent and simple GeoLocation class, which comes with a very thorough explanation of the concepts involved.

My method essentially boiled down to the following. It can probably be optimized a good deal, down to a simple SQL query, but here it is for my purposes, where listingDisplay is an array of database-retrieved custom LocationNode objects, and the GeoPoint current is created directly from a returned Address of a standard Android Geocoder.

public void setRegionForGeoPoint(GeoPoint current) {

    // Earth radius in KM
    final double EARTH_RADIUS = 6371.01;

    // Dummy span distance in KM for initial search; distance buffer is in M
    final double DISTANCE_BUFFER = 50;
    final double dummyDistance = 100.0;
//Create a list to store distances
    List<Double> distancesList = new ArrayList<Double>();


    // Loop through and modify LocationNodes with distance from user
    for (LocationNode location : listingDisplay) {
        location.setDistance((float) distanceFromUser(location));

        // Dynamically calculate distance from our current point (epicentre)
        distancesList.add(distanceFromPoint(location, current));
    }

    // Sort distances
    Collections.sort(distancesList);

    // Calculate regional span
    float spanWidth = (float) dummyDistance;
    double distance = 0;

    if (distancesList.size() > 0) {
        if (distancesList.size() > 1) {

            distance = distancesList.get(1);
            spanWidth = (float) (distance + DISTANCE_BUFFER);

        } else if (distancesList.size() == 1) {

            distance = distancesList.get(0);
            spanWidth = (float) (distance + DISTANCE_BUFFER);

        }

        //Obtain the spanwidth in metres.
        double spanWidthInKM = (double) spanWidth / 1000;

        // Create span
        GeoLocation[] boundingBoxSpan = currentGeoLocation
                .boundingCoordinates(spanWidthInKM, EARTH_RADIUS);

        //Create min/max values for final span calculation
        int minLatSpan = (int) (boundingBoxSpan[0].getLatitudeInDegrees() * 1E6);
        int maxLatSpan = (int) (boundingBoxSpan[1].getLatitudeInDegrees() * 1E6);
        int minLongSpan = (int) (boundingBoxSpan[0].getLongitudeInDegrees() * 1E6);
        int maxLongSpan = (int) (boundingBoxSpan[1].getLongitudeInDegrees() * 1E6);

        //Finally calculate span
        int latSpanE6 = Math.abs(minLatSpan - maxLatSpan);
        int lonSpanE6 = Math.abs(minLongSpan - maxLongSpan);

        // Set center
        mapController.setCenter(current);

        // Zoom to span
        mapController.zoomToSpan(latSpanE6, lonSpanE6);

    } else {

        //TODO: Handle the case when we have no distance values to use
    }
}


public double distanceFromPoint(LocationNode location, GeoPoint point) {

    // Calculate distance from user via result
    Location locationA = new Location("point A");
    locationA.setLatitude(location.getLatitude());
    locationA.setLongitude(location.getLongitude());

    Location locationB = new Location("point B");
    locationB.setLatitude((double) (point.getLatitudeE6() / 1E6));
    locationB.setLongitude((double) (point.getLongitudeE6() / 1E6));
    double distance = locationA.distanceTo(locationB);
    Log.v(TAG, "Calculated Distance: " + distance);
    return distance;
}