send some terrain data

def send_terrain_data_bit(self, bit):
        '''send some terrain data'''
        lat = self.current_request.lat * 1.0e-7
        lon = self.current_request.lon * 1.0e-7
        bit_spacing = self.current_request.grid_spacing * 4
        (lat, lon) = mp_util.gps_offset(lat, lon,
                                        east=bit_spacing * (bit % 8),
                                        north=bit_spacing * (bit // 8))
        data = []
        for i in range(4*4):
            y = i % 4
            x = i // 4
            (lat2,lon2) = mp_util.gps_offset(lat, lon,
                                             east=self.current_request.grid_spacing * y,
                                             north=self.current_request.grid_spacing * x)
            alt = self.ElevationModel.GetElevation(lat2, lon2)
            if alt is None:
                if self.terrain_settings.debug:
                    print("no alt ", lat2, lon2)
                return
            data.append(int(alt))
        self.master.mav.terrain_data_send(self.current_request.lat,
                                          self.current_request.lon,
                                          self.current_request.grid_spacing,
                                          bit,
                                          data)
        self.blocks_sent += 1
        self.last_send_time = time.time()
        self.sent_mask |= 1<<bit
        if self.terrain_settings.debug and bit == 55:
            lat = self.current_request.lat * 1.0e-7
            lon = self.current_request.lon * 1.0e-7
            print("--lat=%f --lon=%f %.1f" % (
                lat, lon, self.ElevationModel.GetElevation(lat, lon)))
            (lat2,lon2) = mp_util.gps_offset(lat, lon,
                                             east=32*self.current_request.grid_spacing,
                                             north=28*self.current_request.grid_spacing)
            print("--lat=%f --lon=%f %.1f" % (
                lat2, lon2, self.ElevationModel.GetElevation(lat2, lon2)))