Theodor (sometimes Theodore) F.N. (sometimes N.F.) Risler arrived in South Africa from Germany. He entered the civil service of the Cape Colony in January 1859 as a draughtsman in the office of the surveyor-general and in 1861 qualified as a land surveyor. During the next ten years he surveyed and mapped various properties in Port Elizabeth (1863), George district (1864), Clanwilliam district (1865), Ceres (1869), the village of Braunschweig, King William's Town district (1870), and Fort Beaufort district (1871). In 1881 he compiled a survey map of part of the district of Quithing [not identified] in Basutoland (now Lesotho). During 1867 he was acting examiner of diagrams and examiner in the practice of land surveying in the office of the surveyor-general for a few months. He was also an examiner in German for the Board of Public Examiners (before 1873), a commissioner on the Board of Immigration (1877), served on the Land Board of Moirosi's country, Lesotho in 1880, and during 1882-1883 was a special commissioner to Europe in connection with immigration. In May 1876 he was promoted to registry surveyor in the Deeds Registry Office, where he remained to his death in 1892.
Risler compiled Tables containing the natural sines and cosines, to six decimal figures, of all angles between 0? and 90? to every ten seconds of the quadrant, with the proportional parts for single seconds, and some numbers useful in trigonometrical calculations, specially arranged for the use of land-surveyors at the Cape of Good Hope (Cape Town, 1871, 52p). He was an early member of the South African Philosophical Society (founded in 1877) for a few years.