From the Brooklyn Historical Society:
This month’s featured map shows a plan for the Parade Ground, laid out just south of Prospect Park. Parade grounds served a significant purpose in the 19th century by providing large expanses of land where the military could conduct drills and exercises. Originally, the park’s designers Frederick Law Olmsted and Calvert Vaux proposed that the park’s parade ground be located in East New York, but they later settled on an area south of the park. Completed in 1869, about two years after the park opened to the public, the Parade Ground served the military’s needs while protecting the grasses of the Long Meadow from the stress of repeated drills. As early as 1881 the Grounds began to be used for field sports when not being used drills and parades. By 1905 the Parade Grounds consisted of twenty-five baseball diamonds, only half of which were regulation size and during the winter the area hosted rugby and four football fields.