Initial error of local postion in the real quadrotor with GPS

I am using mavros to control a real quadrotor to motion planning.
In simulation this works great but on the real drone the /mavros/local_position/pose initializes with values far from [0,0,0].But in real quadrotor with GPS these are values like [-15, 5, -1], witch is a big trouble for mapping.

What reason result in the initial error? And how to solve the problem ? Because I have RTK.

Thank you!