Sebastiaan W. van de Graaff was the son of Cornelis J. van de Graaff*, governor of the Cape of Good Hope, and his wife, Hester C. Reynet. He came to the Cape with his parents in January 1785 as a captain engineer in the service of the Dutch East India Company and was put in charge of the complement of outstanding surveyors, cartographers and engineers assembled by his father. He was closely associated with the compilation of six maps of various parts of the colony during his first three years at the Cape. Early in 1786 he and Lieutenant D.M. Barbier* improved and updated the map drawn by C.D. Wentzel* to illustrate A.F. Beutler's* expedition of 1752. That same year he produced a detailed topographical map of the Cape Peninsula and Table Bay. Also in 1786 he and Barbier produced a beautiful 60X140 cm map of the Cape based partly on earlier sources. The next year he and Barbier surveyed a region near Cape Town and adjacent to False Bay, and drew a rough map. With Barbier he measured a baseline of 650 toises (1267 m) on the beach east of Muizenberg, which they used as the starting point for their trigonometric survey of False Bay. A map of the Bay and its coastal districts based on this survey was completed in 1787. That same year he, L.M. Thibault* and Barbier made a partial survey of Hout Bay, on which the latter based a plan of the bay (1787).
Van de Graaff, Thibault and Barbier also jointly prepared a plan of the Castle, showing proposed improvements (1786); a plan of Cape Town, the Castle and fortifications, showing improvements to be made (2 sheets, 1786); and a map showing fortifications at the Cape (4 sheets, 1786-1787). Van de Graaff was married to Adriana C. van Lynden, daughter of the unpopular fiscal J.N.S. Van Lynden.