why the robot position coordinate change a lot when doing rotation?

edit

Dear all, Sorry forget my silly question, here I follow some tutorial code to write a test code to control TB3 burger to move a triangle shape.

Below are part of code that to obtain the current position and then using "close-loop" way to move the burger to specified distance (say 0.3m) and then rotate specified angle (say 120deg).

But I found issue so print-out the detail data to check--I find the problem is as soon as the burger starting rotate. the position.x obtained from curr_pos_.x = msg->pose.pose.position.x will jumped a lot (from ~0.3 to ~0.5!!).. so I wonder if such way to get the "current" position of robot are correct or not?? I search around and find suggestion that odometry is not precise, however it should be have such big error? if it indeed naturally is, should I consider to use some other way like IMU?

Thanks a lot for your kindly suggestion..

BR LM

void velControl::odomCallback(const nav_msgs::Odometry::ConstPtr& msg)  
{  
    geometry_msgs::Quaternion orientation = msg->pose.pose.orientation;  //obtain Quaternion info.
    tf::Matrix3x3 mat(tf::Quaternion(orientation.x, orientation.y, orientation.z, orientation.w));  

    double yaw, pitch, roll;  
    mat.getEulerYPR (yaw, pitch, roll);
    //Get the matrix represented as euler angles around YXZ, roundtrip with setEulerYPR. 
    //Yaw around Z axis; Pitch around Y axis; Roll around X axis
    curr_pos_.x = msg->pose.pose.position.x;  
    curr_pos_.y = msg->pose.pose.position.y;  
    curr_pos_.theta = yaw;  

       velocityControl();

    if(Is_Fininsh_)    //Is_Fininsh_ =true= robot complete one round line+rotaion;
       {   start_pos_.x = curr_pos_.x;  
        start_pos_.y = curr_pos_.y;  
        start_pos_.theta = curr_pos_.theta; 
       }

}

Below are part of the move forward and rotate. I did not paste all as too much code.. concept are similar.

   if ( line_travel < triagle_line1 )  
        {  
            #ifdef VERBOSE
             cout<< curr_pos_.x <<"||"<< start_pos_.x<<"||  ||"<<curr_pos_.y<<"||"<<start_pos_.y<<"||  ||"<< line_travel <<endl;
              #endif
            controlVel_.angular.z = 0.0;
            controlVel_.linear.x  = vel_line_;
            controlVel_.linear.y  = 0;
            controlVel_.linear.z  = 0;  
            Is_Fininsh_ = false;  
        }  
        else  
        {   
            controlVel_.angular.z = 0.0;
            controlVel_.linear.x  = 0.0;
            controlVel_.linear.y  = 0;  
            controlVel_.linear.z  = 0;  
            state_ = 1;  
            Is_Fininsh_ = true;  

            cout<< curr_pos_.x <<"||"<< start_pos_.x<<"||  ||"<<curr_pos_.y<<"||"<<start_pos_.y<<"||  ||"<< line_travel <<endl;
             }  
        break;  

    case 1: 
        ang_travel = abs(angleWrap(curr_pos_.theta - start_pos_.theta));
        if ( ang_travel < TRIANGLE_Ang1 )  
        {   
            #ifdef VERBOSE
            cout<< curr_pos_.x <<"||"<< start_pos_.x<<"||  ||"<<curr_pos_.y<<"||"<<start_pos_.y<<"||  ||"<< line_travel <<endl;
            #endif
            controlVel_.angular.z = vel_angle_;  
            controlVel_.linear.x  = 0;  
            controlVel_.linear.y  = 0;  
            controlVel_.linear.z  = 0;  
            Is_Fininsh_ = false;  
        }  
        else  
        {  
            controlVel_.angular.z = 0.0;  
            controlVel_.linear.x  = 0.0;  
            controlVel_.linear.y  = 0;  
            controlVel_.linear.z  = 0;  
            state_ = 2;  
            Is_Fininsh_ = true;  
            cout<<"[State#1]: Angle1 rotation completed"<<endl;
            cout<< curr_pos_.x <<"||"<< start_pos_.x<<"||  ||"<<curr_pos_.y<<"||"<<start_pos_.y<<"||  ||"<< line_travel <<endl;
            cout<<endl;

        }  
        break;