Rober Moffat was the eldest surviving son of the missionary Robert Moffat and his wife Mary, born Smith. Robert junior requested to be examined for admission as a land surveyor in the Cape Colony in 1848 and duly qualified, but also became a trader. During the next few years he compiled a "Map of south eastern Africa, 1848-1851" from available information relating to the Orange River Sovereignty (now the Free State) and regions further north. The map emphasized the interests of African chiefs, while almost completely ignoring the presence of the white settlers who had occupied parts of the territory since 1836. Early in 1850 he offered to act as surveyor-general of the Orange River Sovereignty. When he was not appointed to the post he resigned in February that year and left the area. A few months later J.H. Ford* was appointed superintending land surveyor of the territory.
Moffat was a Fellow of the Royal Geographical Society. In 1854 he travelled from Colesberg through southern Bushmanland to Namaqualand, where he became managing agent and surveyor for a copper company. He described the general features of the country in "Journey from Colesberg to Steinkopf in 1854-1855", in the Journal of the Royal Geographical Society (1858). Two years later he returned to the Freee State, but first undertook a survey, with financial support from the government, to fix the position of part of the Orange River, forming the northern border of the Cape Colony. The survey started on 13 August 1856 at Gams (near present Aggenys), the position of which he fixed by triangulation from the northernmost beacons of Thomas Maclear's* arc of meridian survey. Latitudes were determined with a sextant, but the determination of longitudes proved impossible owing to the failure of his watch. The journey ended at Kuruman on 14 November. The results of the survey were incorporated in a map of the region on a scale of two miles to the inch (c. 1:127 000). An account of the country he traversed was published in the same volume as his previous paper, titled "Journey from Little Namaqualand eastward, along the Orange River, the northern frontier of the Colony, etc., in August 1856." He took some photographs of the Orange River below the Augrabies Falls and these, together with a sketch of the falls which he made, were presented to the South African Museum by Maclear. His journey was also described in a Parliamentary Report entitled Report of a survey of a portion of the Orange River eastward of Little Namaqualand (Cape Town, 1858, 12p). The report included his journal of the journey, a description of the physical geography of the region, and some information about its native inhabitants.
Moffat's papers contain some interesting geological notes. For example, where the railway station De Kop now is, near the Kromrivier in Bushmanland, he found an abraded and polished uneven floor of granite beneath a conglomerate (later recognised as tillite) which was overlain by shales and sandstones (of the Dwyka Formation), while he also noticed an abundance of basalt (Karoo dolerite). Similarly, at a point some 210 km south-south-east of Augrabies Falls, he found a similar conglomerate resting on abraded and polished granite. This is probably the first description of the Dwyka tillite resting on a glaciated surface, though he did not suggest a glacial origin. He thought that the salt found in the pans of the region was deposited when water flowing from the shales evaporated - a more sensible explanation than that offered by the contemporary geologist Andrew Wyley*, who ascribed it to a recent retreat of the sea. Travelling east from Skeurberg (some 65 km south-west of Olifantshoek) to beyond Motito (now Bothithong, some 60 km north-east of Kuruman), he correctly described the downward succession of the sandstones (Matsap Formation), ribboned schists (Griquatown Group), limestone (Cambell Rand Series), and sandstone (Black Reef Formation) However, he did not name the formations or investigate their distribution, as G.W. Stow* did 15 years later.
Moffat was interested in reptiles, for in the annual report of the South African Museum for 1857 he was described as an assiduous collector of reptiles for the museum. He was married to Ellen Moffat, born Platt, with whom he had five children.