California Code, Business and Professions Code - BPC § 8760

(a) When it becomes necessary to take testimony for the identification or establishment of old, lost or obliterated corners.

(b) When a corner or monument is found in a perishable condition, and it appears desirable that evidence concerning it be perpetuated.

(c) When the importance of the survey makes it desirable, to administer an oath to his assistants for the faithful performance of their duty.

A record of oaths shall be preserved as part of the field notes of the survey and a memorandum of them shall be made on the record of survey filed under this article.