We address the measurement and correction of fan distortion in optical coherence tomography (OCT). This effect arises from the scanning system configuration and prevents one, in general, from obtaining quantitative topographic data from OCT. Computer simulations allowed us to quantify the effect and evaluate its dependence on the scanning mirror separation and design of the collimating lens, as well as to estimate the optimal axial position of that lens to minimize the fan distortion. We also developed a numerical algorithm based on 3-D ray propagation for the correction of the residual fan distortion. The effect was studied experimentally using a custom developed time-domain OCT in a Michelson configuration provided with a confocal channel, and the accuracy of the fan distortion correction algorithm tested on samples of known dimensions (flat surfaces and spherical lenses). With a proper calibration of the system with use of an onfocal channel, this algorithm makes it possible for time-domain OCT devices to be used as topographers.