A robust line impedance identification method is presented in this paper. It determines the characteristic impedance of on-wafer thru-line-reflect (TLR) standards measured after an initial off-wafer line-reflect-match or TLR calibration. The only assumption made is that the obtained trans-wafer error boxes are a cascade of a symmetric probe-related disturbance and a change in reference impedance. The proposed method yields an unbiased estimate of the complex characteristic impedance. Results from coplanar lines on a medium resistivity silicon substrate support the made assumption.