The county planning commission shall have power to make, adopt and publish an official master plan of the county for the purpose of bringing about coordinated physical development in accordance with the present and future needs. The official master plan shall be developed so as to conserve the natural resources of the county, to insure efficient expenditure of public funds and to promote the health, safety, convenience, prosperity and general welfare of the inhabitants. Such official master plan may include, among other things, studies and recommendations relative to the location, character and extent of highways, railroads, bus, streetcar and other transportation routes, bridges, public buildings, schools, parks, parkways, forests, wildlife refuges, dams, and projects affecting conservation of natural resources. The county planning commission may adopt the official master plan in whole or in part and may subsequently amend or extend the adopted plan or portion thereof. Before the adoption, amendment or extension of the plan or portion thereof, the commission shall hold at least one public hearing thereon, fifteen days’ notice of the time and place of which shall be published in at least one newspaper having general circulation within the county, and notice of such hearing shall also be posted at least fifteen days in advance thereof in one or more public areas of the courthouse of the county. Such hearing may be adjourned from time to time. The adoption of the plan, or part thereof, shall be by resolution carried by not less than a majority vote of the full membership of the county planning commission. After the adoption of the official master plan, or part thereof, an attested copy shall be certified to the county commission, to the recorder of deeds and to the clerk of each incorporated area covered by the plan or part thereof.