How can i create new worlds in gazebo_sitl and mavros

Hello everyone. I’m working with mavros and gazebo sitl. I’d like to know how can i create or install new worlds in order to use them after. When i create a new world in /Tools/sitl_gazebo/worlds and after i use roslaunch px4 posix_sitl.launch (i have modified the default world name to my new world) system cannot open it. Thanks so much for your attention