Upon the filing of an application to purchase any lands subject to the operation of this chapter, together with the required proof, the Secretary of the Interior shall cause the lands described in said application to be appraised, said appraisal to be on the basis of the value of such lands at the date of appraisal, exclusive of any increased value resulting from the development or improvement of the lands by the applicant or his predecessors in interest, and in such appraisal the Secretary shall consider and give full effect to the equities of any such applicant. (Dec. 22, 1928, ch. 47, § 2, 45 Stat. 1070 .)