Section 1. A board of officers, hereinafter called the board, upon whom authority to take real estate by eminent domain on behalf of the commonwealth or of any county, city, town or district thereof, hereinafter described as the body politic, has been conferred by law, for the purpose of effecting a public improvement, may, at its election, instead of proceeding in accordance with chapter seventy-nine, or in accordance with chapters seventy-nine and eighty if betterments are to be assessed, institute proceedings for the taking of real estate or any interest therein, and for the assessment of betterments, in accordance with this chapter. So much of sections two to fifteen, inclusive, as relates to the assessment of betterments shall apply only to proceedings thereunder wherein the board votes under section two to assess betterments and so states in the order adopted thereunder.