It shall be the duty of such commissioners, subject to the approval of the board of supervisors, to determine and fix what road or roads shall be constructed, or constructed and maintained, or maintained in the district out of the proceeds of the sale of bonds and the levy of taxes. It shall be their duty to let all contracts for the construction, or for the construction and maintenance, or for the maintenance of such roads in the manner provided by law for the letting of contracts for public work by the board of supervisors. It shall be their duty to employ a competent engineer to survey and lay out the road or roads in the district as they shall determine upon, whose duty it shall be to make an estimate of the cost of constructing and maintaining such highway or highways for each separate mile covered by such survey and to report the survey and estimate to said commissioners before contracts are let for the construction or for the construction and maintenance of such highway or highways. Said commissioners shall have the power to adopt or reject such survey and estimate and in the latter event to have another made. When adopted, it shall be their duty to report the same to the board of supervisors, whose duty it shall be to order the clerk of said board to file the same among the records of the office, spread the same on the minutes of the board, and make an order adopting such survey and estimate so reported and adopted by the commissioners. All of said acts of said commissioners shall be subject to ratification or rejection by the board of supervisors.