 Much of this military work would remain secret for many years.In 1913, 1922 and 1924, trials are done with aerial photogrammetry, first with balloons and later with aircraft, but only as of 1930 this is used for production of maps, and in 1940 terrestrial photogrammetry is abandoned.On 21 June 1935, a Federal Act is passed on the production of the new National Map series.