ekf_localization_node: what frame for IMU?

edit

Hi there, I am trying to fuse GPS with IMU information with ekf_localization_node. For now I have tied by map and odom frames to be always the same, so I assume that GPS is giving absolute map positions, and report them in the map frame. I am confused about the IMU though: it's heading estimate should be in the map frame as it's absolute, but it's motion estimates should be in the base_link frame because they are relative to the robot. The imu message only has a single frame_id, so which should I set it to? Or do I perhaps need to send two copies of the imu message with two different frames there, and set up ekf_localization_node to only attend to the relevant measurements in each?