A method is provided for estimating the positional uncertainty in drilling a well such as an oil well. A first set of values is supplied representing a first three-dimensional uncertainty of the actual position of a drill bit with respect to the estimated position. A second set of values is supplied representing a second three-dimensional uncertainty of the actual position of a geological feature with respect to the estimated position thereof. For example, the first set of values relates to positional uncertainties because of the drilling procedure whereas the second set of uncertainties is associated with the obtaining and interpretation of seismic data. The first and second sets of values are combined to form a third set of values which represents a third uncertainty of the position of the drill bit with respect to the geological feature. A probability is then calculated from the third uncertainty and gives the probability that the drill bit will reach a predetermined position relative to the geological feature.