If one observer makes a measurement at a location, but it does not correspond to the measurement another observer would make or has made at that same location, then there is a problem and the observation can be considered to be subject to uncertainty. It is possible that one observation is correct or that neither observation is correct. It is possible that both observations can be completely correct, or both may be correct to some degree. Unfortunately, within the processes of establishing geographical databases, situations of uncertainty are much more common than situations of certainty (where the observers agree). Indeed, if two observers happen to agree on one observation, there is every chance that a third observer would disagree. Some researchers would consider a situation ...