Why does local plan have global plan and local plan?

[NguyenDuyDuc]

Hi,

After I launch a local planner, I also see a global plan topic and a local plan topic.

Why? Could you explain that?

If local planner already has global plan, why do we need a separated global planner?

[rtellez]

Hi Nguyen,

• What do you mean by launching a local planner?

Let me clarify some things:
In ROS 1 the global path planner and the local path planner are launched at the same time by launching the move base. This contains the 2 planners, so you cannot launch only the local planner.

The global planner is used to compute the path from the current location of the robot to the goal destination (let’s say, another room). The local planner is just used to avoid new obstacles that appear in the vicinity of the robot while it is trying to follow the global plan.

This means, there is a global plan that the robot tries to follow. But sometimes, new obstacles appear in front of the robot (let’s say a person moving around the robot). Then, the local planner will generate a new small plan around that new obstacle that allows the robot to avoid it while following the global plan.

That is why you need two separated planners