The question about ros::publisher and ros::subscriber

edit

I want to achieve receiving and transform data using rostopic at the same time or in a low time. I know that ros::subscriber can subscribe rostopic and receive the data included in topic. So I hope to process the data in one topic and send the processing-data to another topic, and others can subscribe the new topic. Such as the code:

void receiveGPS(const sensor_msgs::NavSatFix::ConstPtr &msg)
{

  gps_common::GPSFix gps;

  gps.header.stamp=msg->header.stamp;

  gps.status.status=msg->status.status;

  gps.latitude=msg->latitude;

  gps.longitude=msg->longitude;

  gps.altitude=msg->altitude;

  gps.position_covariance[0]=msg->position_covariance[0];
  gps.position_covariance[4]=msg->position_covariance[4];
  gps.position_covariance[8]=msg->position_covariance[8];
}

int main(int argc, char **argv)
{

// Set up ROS.

  ros::init(argc, argv, "receiveGPS");

  ros::NodeHandle n;

  ros::Subscriber sub = n.subscribe("gps", 2,&receiveGPS);

  ros::spin();

}

I can get gps.latitude from the topic "gps",and I hope to send gps.latitude to another topic and publish it. Could you tell me what should I do next? Thanks!