@Nullable
    public GeoItem toGeoItem(final GeoStyle style, final int zLevel) {
        if (!isValid(this)) {
            return null;
        }
        final List<Geopoint> points = Arrays.asList(getBottomRight(), bottomLeft, getTopLeft(), topRight);
        return GeoPrimitive.createPolygon(points, style).buildUpon().setZLevel(zLevel).build();
    }
Creates a Viewport with given center which covers the area around it with given radius
main/src/main/java/cgeo/geocaching/location/Viewport.java