If you look at an aerial photograph of Madison’s east side, you’ll see it right away: a large, irregular rhombus-shaped island of green between Highway 30 and Milwaukee Street, surrounded by dense residential neighborhoods.  That 65-acre plot of land has been owned by the Voit family for 175 years, and has been the site of a farm, a concrete plant and even a carnival over the years.  The Voit family had stubbornly resisted selling for development, even as the value of the property soared and developers came knocking.  As of January 20, the property went on the market, making it one of the largest — and most valuable — pieces of undeveloped real estate in the city.  One community group, Save the Farm, is hoping to buy the Voit Farm site, and to use it as a prototype for sustainable urban cooperative living.  Colleen Robinson is an environmental educator who sits on Save the Farm’s board of directors.