In a method of implementing a positioning system a buffer is configured to store a buffer record for each of a plurality of position fixes determined by the receiver and a database is configured to store a database record for each of the position fixes. Buffer records are accumulated, with each buffer record corresponding to one of the position fixes determined by the receiver. A first threshold position near an area in which a degradation event occurs is identified and recorded in a first database record. A second threshold position near the area is identified in a second database record. The second threshold position is indicated by one of the position fixes outside of the area and is proximate to a position at which the degradation event ends.