How to add a map scale in MapView on Android?

Question

I\'m struggling with adding a map scale that displays current length on screen depending on current zoom level. I\'m having a feeling that it might exist some predefined cla

Answer 1

As far as I know there isn't a predifined class to do that.

One possibility is to create an overlay that checks current longitude span, as it changes accordingly to latitude, and then draw the scale at the correct size.

Bellow you can find an example on how to do that:

ScaleBar Overlay

public class CopyOfScaleBarOverlay extends Overlay {    
private static final String STR_M = "m"; 
private static final String STR_KM = "km"; 

//Constants
private static float scaleBarProportion = 0.25f;
private float cMarginLeft=4;
private float cLineTopSize=8;
private float cMarginTop=6;
private float cMarginBottom=2;
private float cTextSize=12;
private float distanceFromBottom=100;


//instantiation
private Context context;

private Paint paintLine, paintText, paintRectangle;
private Location l0;
private Location l1;
private float ds;
private int width, height, pi;
private float marginLeft, marginTop, marginBottom, lineTopSize;
private String unit;



public CopyOfScaleBarOverlay(Context context){
    super();
    this.context=context;

    paintText= new TextPaint();
    paintText.setARGB(180, 0, 0, 0);
    paintText.setAntiAlias(true);
    paintText.setTextAlign(Align.CENTER);

    paintRectangle = new Paint();
    paintRectangle.setARGB(80,255,255,255);
    paintRectangle.setAntiAlias(true);

    paintLine = new Paint();
    paintLine.setARGB(180, 0, 0, 0);
    paintLine.setAntiAlias(true);

    l0 = new Location("none");
    l1 = new Location("none");

    ds=this.context.getApplicationContext().getResources().getDisplayMetrics().density;
    width=this.context.getApplicationContext().getResources().getDisplayMetrics().widthPixels;
    height=this.context.getApplicationContext().getResources().getDisplayMetrics().heightPixels;
    pi = (int) (height - distanceFromBottom *ds);

    marginLeft=cMarginLeft*ds;
    lineTopSize=cLineTopSize*ds;
    marginTop=cMarginTop*ds;
    marginBottom=cMarginBottom*ds;


}

@Override
public void draw(Canvas canvas, MapView mapview, boolean shadow) {
    super.draw(canvas, mapview, shadow);
    if(mapview.getZoomLevel() > 1){

        //Calculate scale bar size and units
        GeoPoint g0 = mapview.getProjection().fromPixels(0, height/2);
        GeoPoint g1 = mapview.getProjection().fromPixels(width, height/2);
        l0.setLatitude(g0.getLatitudeE6()/1E6);
        l0.setLongitude(g0.getLongitudeE6()/1E6);
        l1.setLatitude(g1.getLatitudeE6()/1E6);
        l1.setLongitude(g1.getLongitudeE6()/1E6);
        float d01=l0.distanceTo(l1);
        float d02=d01*scaleBarProportion;
        // multiply d02 by a unit conversion factor if needed
        float cd02;
        if(d02 > 1000){
            unit = STR_KM;
            cd02 = d02 / 1000;
        } else{
            unit = STR_M;
            cd02 = d02;
        }
        int i=1;
        do{
            i *=10;
        }while (i <= cd02);
        i/=10;
        float dcd02=(int)(cd02/i)*i;
        float bs=dcd02*width/d01*d02/cd02;

        String text=String.format("%.0f %s", dcd02, unit);
        paintText.setTextSize(cTextSize * ds);
        float text_x_size=paintText.measureText(text);
        float x_size = bs + text_x_size/2 + 2*marginLeft;

        //Draw rectangle
        canvas.drawRect(0,pi,x_size,pi+marginTop+paintText.getFontSpacing()+marginBottom, paintRectangle);

        //Draw line
        canvas.drawLine(marginLeft, pi+marginTop, marginLeft + bs, pi+marginTop, paintLine);
        //Draw line tops
        canvas.drawLine(marginLeft, pi+marginTop - lineTopSize/2, marginLeft, pi+marginTop + lineTopSize/2, paintLine);
        canvas.drawLine(marginLeft +bs, pi+marginTop - lineTopSize/2, marginLeft+bs, pi+marginTop + lineTopSize/2, paintLine);
        //Draw line midle
        canvas.drawLine(marginLeft + bs/2, pi+marginTop - lineTopSize/3, marginLeft + bs/2, pi+marginTop + lineTopSize/3, paintLine);
        //Draw line quarters
        canvas.drawLine(marginLeft + bs/4, pi+marginTop - lineTopSize/4, marginLeft + bs/4, pi+marginTop + lineTopSize/4, paintLine);
        canvas.drawLine(marginLeft + 3*bs/4, pi+marginTop - lineTopSize/4, marginLeft + 3*bs/4, pi+marginTop + lineTopSize/4, paintLine);

        //Draw text
        canvas.drawText(text, marginLeft +bs, pi+marginTop+paintText.getFontSpacing(), paintText);
    }
}

}

To use

On your activity that extendes MapActivity, add the following:

mapView.getOverlays().add(new CopyOfScaleBarOverlay(this));

Note

The example is using metric units. To use a different unit system, multiply d02 in the code above by a unit conversion factor and adjust strings with the unit name.

Enjoy it.