Before entering into any contract for the construction, alteration, repair, or improvement of any state highway the department shall cause the highway to be surveyed throughout the entire length of the proposed construction, alteration, repair, or improvement and cause to be prepared maps, plans, and specifications, together with an estimate of the cost of the proposed work, and such information and directions as will enable a contractor to carry them out. The maps, plans, specifications, and directions shall be approved by the department and a copy thereof filed permanently in the department’s office.