return pixel coordinate (px,py) for position (lat2,lon2)
		in an area image. Note that the results are relative to top,left
		and may be outside the image

def coord_to_pixel(self, lat, lon, width, ground_width, lat2, lon2):
		'''return pixel coordinate (px,py) for position (lat2,lon2)
		in an area image. Note that the results are relative to top,left
		and may be outside the image'''
		pixel_width = ground_width / float(width)

		if lat is None or lon is None or lat2 is None or lon2 is None:
			return (0,0)

		dx = mp_util.gps_distance(lat, lon, lat, lon2)
		if lon2 < lon:
			dx = -dx
		dy = mp_util.gps_distance(lat, lon, lat2, lon)
		if lat2 > lat:
			dy = -dy

		dx /= pixel_width
		dy /= pixel_width
		return (int(dx), int(dy))