What is the test for establishing a Prescriptive Easement?

Further the existence of a prescriptive easement must be shown by a definite and certain line of travel for the statutory period. (Dooling v. Dabel (1947) 82 Cal.App.2d 417, 186 P.2d 183.) "The line of travel over a roadway which is claimed by prescription may not be a shifting course, but must be certain and definite. Slight deviations from the accustomed route will not defeat an easement, but substantial changes which break the continuity of the course of travel will destroy the claim to prescriptive rights.... [Citations.] Although manifestly the distance to which a roadway may be changed without destroying an easement will be determined somewhat by the character of the land over which it passes, together with the value, improvements, and purposes to which the land is adapted." (Matthiessen v. Grand (1928) 92 Cal.App. 504, 510, 268 P. 675.)