In all the navigation tutorials the robot requires a pre-built map.Can i do the navigation in an unknown environment without a pre defined map,so that it moves without collision

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Yes, in 2 ways.

1. You can build a map while your navigating, there's no requiring the map is prebuilt, just if you have one its available. You can navigate fine in SLAM because its still providing the map -> odom transform & also giving you the map image if that's helpful for navigating in your case

2. Nothing in navigation strictly requires a map, you can navigate without a map, just can't use the static layers in your costmap meaningfully