Conversion from screen coordinates to lat/long

s.semeria · by **s.semeria** » Fri Mar 28, 2008 2:41 pm

Hello,

I was wondering if there's a way to convert an X,Y coordinate pairs on the screen to a lat/long pair on a map.
Does anyone have any ideas about the type of projection Android is using to convert lat/long into X,Y pairs?

In my application I have a cluster of moving points on a map and I would like to sometimes recenter the map on the cluster midpoint. I was able to compute the midpoint in X,Y coords, but I suppose there's no a ready made API to find out the point on the map on which the map should centered

Thanks a lot in any case for your help.

Regards,

s.semeria

plusminus · by **plusminus** » Fri Mar 28, 2008 2:50 pm

Hello s.semeria,

taken from the GoogleGroups:

Use the MapView's getHeight and getWidth methods to get the pixel height and width of the MapView.
Use the MapView's getCenterPoint method to get the lat/long of the map's center in microdegrees.
Use the MapView's getLatitudeSpan and getLongitudeSpan methods to get the number of microdegrees covered by the MapView's width and height.
Now you can calculate the lat/long offset from the center based on the pixel's offset from the center.

Regards,
plusminus

s.semeria · by **s.semeria** » Fri Mar 28, 2008 3:32 pm

Hi plusminus,

thanks a lot for your very quick answer.
If I understood correctly, your suggestion is to map a known lat/long offset to a screen X/Y offset.

I don't understand very well why it should work, but I' not very good at math too :wink:

I'll try and let you know, thanks a lot for your help

Regards,

s.semeria

plusminus · by **plusminus** » Fri Mar 28, 2008 3:44 pm

Hello semeria,

from the "real world" you can get the Lat/Lng displayed in the MapView (2.) and the Lat/Lng-Span displayed (3.).
From your screen you can get the Width/Height plus the X/Y of the point clicked.
The rest is It's simple math.

The result should be pretty accurate.

Regards,
plusminus

s.semeria · by **s.semeria** » Tue Apr 01, 2008 12:07 pm

Hello plusminus,

According to your suggestions I tried to compute the lat/long coord pair for a point of x/y coordinate on the screen as follows:

Syntax: [ Download ] [ Hide ]

Using java Syntax Highlighting

 
int lat = (latSpanOnMap / screenHeight) * y + centerOfMap.getLatitudeE6();
 
int long = (longSpanOnMap / screenWidth) * x + centerOfMap.getLongitudeE6();
 
Parsed in 0.030 seconds,  using GeSHi 1.0.8.4

But it seems not to work? Could you help finding the error?
May be these formulas are wrong??

Thanks a lot

Regards

Stefano

bavarol · by **bavarol** » Tue Apr 01, 2008 2:52 pm

Hi you need a class called PixelCalculator
http://code.google.com/android/referenc ... lator.html
There is a method called

Syntax: [ Download ] [ Hide ]

Using java Syntax Highlighting

void    getPointXY(Point point, int[] xyCoords)
 
        Converts a lat/lon pair to an x/y screen-pixel pair.
Parsed in 0.031 seconds,  using GeSHi 1.0.8.4

Rest of your idea is good.

I hope, that was you wanted.

Regards

jan · by **jan** » Mon Apr 07, 2008 6:41 pm

Hi,

I do the conversion like this:
(hope, the variablenames are selfexplanatory)

Syntax: [ Download ] [ Hide ]

Using java Syntax Highlighting

 
//touchevents of mapView are forwarded here
 
public void debugTouchEventReceivedHandler(MotionEvent event)
 
{
 
if(event.getAction()== MotionEvent.ACTION_UP)
 
{       
 
MapView mapView; //your mapView object
 
Point center = mapView.getMapCenter();
 
int eventScreenCoordX = (int) event.getX();
 
int eventScreenCoordY = (int) event.getY();
 
int viewHeight = mapView.getHeight();
 
int viewWidth = mapView.getWidth();
 
int latitudeSpan = mapView.getLatitudeSpan();
 
int longitudeSpan = mapView.getLongitudeSpan();
 
int microDegreePerPixelLatitude = latitudeSpan / viewHeight;
 
int microDegreePerPixelLongitude = longitudeSpan / viewWidth;
 
int centerScreenX = viewWidth / 2;
 
int centerScreenY = viewHeight / 2;
 
int deltaX = centerScreenX - eventScreenCoordX;
 
int deltaY = centerScreenY - eventScreenCoordY;
 
int deltaLatitude = microDegreePerPixelLatitude * deltaY;
 
int deltaLongitude = microDegreePerPixelLongitude * deltaX;
 
int computedLocationLatitude = center.getLatitudeE6()
 
                + deltaLatitude;
 
int computedLocationLongitude = center.getLongitudeE6()
 
                - deltaLongitude;
 
mapView.getController().centerMapTo(
 
                new Point(computedLocationLatitude,
 
                                computedLocationLongitude), true);
 
}
 
}
Parsed in 0.038 seconds,  using GeSHi 1.0.8.4

Greetings,

Jan

Conversion from screen coordinates to lat/long

Conversion from screen coordinates to lat/long

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