The assessor shall have free access to all land plats and maps during the time of assessment with a view to ascertain what lands are taxable; and upon the return of the assessor’s books to the board of equalization, the said board shall compare the same with the plats and maps of the county; and in all cases where any lands have been omitted by the assessor, they shall be placed in the assessor’s books and assessed as other lands are required to be assessed by this chapter.