What do three frames(baselink\odom\map) mean?
I've read over robot_localization documents , REP105 and the ROSCon 2015 presenation but still got confused by these frames. Here's my question:
1.About base_link frame: It's said to be attached to the robot.Does it mean it always keeps x axis pointing forward like ENU coordinates and move with the robot?
2.About odom frame: Why is it continuous ? The input data of imu and odometry are offered in certain frequency,isn't it discrete data? And why does it drift over time?Is it because the position is calculated by twiste data rather than given by GPS?
3.About map frame: As can be seen from the rviz, the map frame seems to go with odom frame.What's the diffrence between them?
Last but not least,my GPS,IMU,Odometry all pub discrete data but vary in frequency. What I learn from the robot_localization package is that IMU and Odometry are continuous.Which is right? I do appreciate it if someone can enlighten it to me.