Binocular vision disorders such as heterophoria and hyperphoria are relatively common. This paper shows, that if a cave survey team does not take careful account of the possibility of one or more of its members suffering from such a disorder, very serious errors can occur. Both theoretical as well as empirical evidence is presented. The likely magnitude of these errors implies that BCRA grade 5 cannot be achieved unless surveyors check for, or if necessary correct for, binocular vision disorders. Various techniques to eliminate such errors are explained and compared. Finally a recommendation is made concerning an additional clause to the notes which accompany the BCRA cave surveying grade definitions. To achieve BCRA grade 5, the possibility of binocular vision disorders must be taken into account.