Any corporation, association or person within the terms of this act desiring to construct transmission mains for the transportation or conveying of natural gas from its source to the locality or localities where utilized, shall submit to the commission, accompanied by due application, a map or plat of such proposed line or lines which it desires to construct, showing the dimensions and character of such proposed pipe line or lines, its compression stations, control valves, and connections, and shall first receive the approval of the commission of such map, route and type of construction before proceeding with the actual construction of such transmission lines, and it shall be the duty of the commission to examine and inquire into the necessity and practicability of such transmission line or lines and to determine that such line or lines will when constructed and in operation serve the convenience and necessities of the public before approval of such map and proposed transmission line or lines: Provided, That persons, associations or corporations having already acquired the rights of common purchasers and common carriers at the time the provisions of this act became effective shall be required to file the map or plat provided for in this section only.