© 2017 IEEE. This work investigates the estimation accuracy of a coherent optical transmission system's performance when the number of spans in the system is known and the overall system length is approximately known. However, there is no knowledge of the lengths of the individual spans as may happen when institutional knowledge of deployed fibre is lost. Estimating the performance of an optical signal over systems using this fibre may be inaccurate as a result. In this paper, the estimation accuracy of this scenario is examined considering a system with heterogeneous fibre span lengths for a DWDM system transmitting 80 channels modulating polarization multiplexed quadrature phase shift keying (PM-QPSK) at 32 Gbaud across the optical C-band using the Gaussian noise (GN) model in comparison to a reference system with homogenous fibre spans.