location.distanceTo problem

rossinglish · April 13th, 2011, 08:19 AM

Hello Reto,
First let me complement you on a Great Book!

I have a strange problem using Location objects.

1st I set my current location with the following code: This works fine.

Code:

currentLocation = locationManager.getLastKnownLocation(provider);

Next, I modified a portion of your Earthquake example to as follows:

Code:

private void loadQuakesFromProvider() {
.......................
          Long datems = c.getLong(EarthquakeProvider.DATE_COLUMN);
          String details = c.getString(EarthquakeProvider.DETAILS_COLUMN);
          Float lat = c.getFloat(EarthquakeProvider.LATITUDE_COLUMN) ;
          Float lng = c.getFloat(EarthquakeProvider.LONGITUDE_COLUMN);
          String mag = c.getString(EarthquakeProvider.MAGNITUDE_COLUMN);
          String dep = c.getString(EarthquakeProvider.DEPTH_COLUMN);
          String link = c.getString(EarthquakeProvider.LINK_COLUMN);

          Location location = new Location("dummy");
          location.setLongitude(lng);
          location.setLatitude(lat);

          Date date = new Date(datems);
          
           if(currentLocation != null){
        	  int dist = (int)currentLocation.distanceTo(location);

        	String distance = String.valueOf(dist/1000);
          }

...............

The problem is that the returned distance is always way off and wrong.
In the Earthquake example I want to add the distance of each earthquake to the listview, which it does but with the wrong values.

Can anyone see where I have made a mistake, it really is frustrating

Thanks in advance
Ross

rossinglish · April 14th, 2011, 07:40 AM

OK, that was fun

but at least now it calculates the correct approximate distances with the location.distanceTo function.

What I found:

EarthquakeProvider.java

Notice the following declarations:

Code:

 
// Column indexes 
public static final int LONGITUDE_COLUMN = 3;
public static final int LATITUDE_COLUMN = 4;

This little reversal in the index for the LONGITUDE_COLUMN and LATITUDE_COLUMN was what was throwing my distance calculation off.

Earthquake.java

private void loadQuakesFromProvider(){

Code:

 
Float lat = c.getFloat(EarthquakeProvider.LATITUDE_COLUMN) ;
          Float lng = c.getFloat(EarthquakeProvider.LONGITUDE_COLUMN);
          String mag = c.getString(EarthquakeProvider.MAGNITUDE_COLUMN);
          String dep = c.getString(EarthquakeProvider.DEPTH_COLUMN);
          String link = c.getString(EarthquakeProvider.LINK_COLUMN);

          Location location = new Location("dummy");
          location.setLongitude(lng);
          location.setLatitude(lat);
         if(currentLocation != null){
        	  int dist = (int)currentLocation.distanceTo(location);

        	  distance = String.valueOf(dist/1000);
          }

My location object had the lat & lng values reversed!! because the index values had been reversed as pointed out earlier.

The question for me was why was it displaying the correct map overlay coordinates ?????

Then I looked closer at:

EarthquakeOverlay.java

Code:

 


 private void refreshQuakeLocations() {
    if (earthquakes.moveToFirst())
      do { 
    
        Double lat = earthquakes.getFloat(EarthquakeProvider.LATITUDE_COLUMN) * 1E6;
        Double lng = earthquakes.getFloat(EarthquakeProvider.LONGITUDE_COLUMN) * 1E6;
        String mag = earthquakes.getString(EarthquakeProvider.MAGNITUDE_COLUMN);
        String dep = earthquakes.getString(EarthquakeProvider.DEPTH_COLUMN);
        GeoPoint geoPoint = new GeoPoint(lng.intValue(), 
                                         lat.intValue());

Notice that in new GeoPoint() the lat & lng parameters have been reversed.

It should be GeoPoint(int latitudeE6, int longitudeE6).

This is why the GeoPoints were showing up correctly on the map!!

So I changed the column indexes and corrected the GeoPoint input parameters and it works like a charm

I hope this helps someone else trying to include the distanceTo function in this example.

Happy Programming
Ross