How can navsat_transform_node deal with base_link and map not aligned when GPS data is received ?

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Hello, I do not understand how navsat_transform_node can compute a transform between utm and map if the robot has already moved from its starting position when the first GPS message is received. Indeed, the angle between utm and map is not available anymore because :

1. imu produces the angle between utm and base_link
2. base_link and map are not aligned anymore

Can someone explain this ?

Thank you Yvon