how to find the true pos?

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in a project, we use amcl move_base and odom laser . the odom must have error and the amcl can product the pos , through tf to tell the robot pos.

and when we run it some times .

the odom will product 1meter's error.

when we close the robot.

we must save the pose to file. when I restart the program , we can read the pos to rviz.

we use the /map /odom to cal the pos but ,it always product some error. so i use tf /map /base_link and we get some error too.

anyone can help me ? how to cal the pos?