visad/src/main/java/ucar/nc2/iosp/mcidas/McIDASAreaProjection.java
public boolean crossSeam(ProjectionPoint pt1, ProjectionPoint pt2) {
    // either point is infinite
    if (ProjectionPointImpl.isInfinite(pt1) || ProjectionPointImpl.isInfinite(pt2)) {
      return true;
    }
    if (Double.isNaN(pt1.getX()) || Double.isNaN(pt1.getY()) || Double.isNaN(pt2.getX()) || Double.isNaN(pt2.getY())) {
      return true;
    }
    // opposite signed X values, larger then 5000 km
    return (pt1.getX() * pt2.getX() < 0)
            && (Math.abs(pt1.getX() - pt2.getX()) > 5000.0);
  }
public boolean crossSeam(ProjectionPoint pt1, ProjectionPoint pt2) {
    // either point is infinite
    if (ProjectionPointImpl.isInfinite(pt1) || ProjectionPointImpl.isInfinite(pt2)) {
      return true;
    }
    if (Double.isNaN(pt1.getX()) || Double.isNaN(pt1.getY()) || Double.isNaN(pt2.getX()) || Double.isNaN(pt2.getY())) {
      return true;
    }
    // opposite signed X values, larger then 5000 km
    return (pt1.getX() * pt2.getX() < 0)
            && (Math.abs(pt1.getX() - pt2.getX()) > 5000.0);
  }
This returns true when the line between pt1 and pt2 crosses the seam.
When the cone is flattened, the "seam" is lon0 +- 180.

@param pt1 point 1
@param pt2 point 2
@return true when the line between pt1 and pt2 crosses the seam.