When the function of the atria is hampered by irregular electrical activation, radiofrequency ablation therapy can be used to restore normal activation and rhythm. Electroanatomical mapping systems provide the clinician with a picture of the activation sequence, from which he/she can determine where to ablate. The purpose of this paper is to present an algorithm for incorporating data from these mapping systems into a numerical simulator of the atria. The method is applied to 3D test problems with synthetic mapping data. Convergence to the correct solution is observed provided that a sufficient number of mapping sites are used. For our simulations, we use the monodomain model coupled with FitzHugh-Nagumo cell dynamics.