The use of the data assimilation technique to identify optimal topography is discussed in frames of time-dependent motion governed by nonlinear barotropic ocean model. As-similation of arti cially generated data allows to measure the in uence of various error sources and to classify the impact of noise that is present in observational data and model parameters. The choice of length of the assimilation window in 4DVar is discussed. It is shown that using longer window lengths would provide more accurate ocean topography. The topography de ned using this technique can be further used in other model runs that start from other initial conditions and are situated in other parts of the model's attractor.