In March 1884 Hammar joined Dr J.A. Aurel Schulz* on an expedition to the Victoria Falls and westwards to the Okavango River and the Kalahari. Equiped with a sextant, artificial horizon, magnetic compass, aneroid barometer and chronometer he was able to map the expedition's route with reasonable accuracy. They travelled up the Linyanti River and cut across to the Okavango River above the Popa Falls, then down the river to Lake Ngami, completing the detailed mapping of the course of the Okavango-Taoghe Rivers from the Popa Falls in Namibia into Botswana. They also demonstrated the connection between the Okavango and the Chobe Rivers. The expedition returned to Natal in January 1885. Hammar was Schulz's co-author for the resulting book, The new Africa: a journey up the Chobe and down the Okavanga rivers (London, 1897). His contributions included sketches made during the journey and his very detailed map, reproduced on a scale of 1:2 000 000.

By 1889 Hammar had moved from his farm to Pietermaritzburg, where he was appointed in November 1890 as government surveyor in the office of the surveyor-general of Natal. During the next 25 years he was the driving force behind the secondary triangulation of Natal, which he pursued with scant support whenever his other work allowed. The task had been started by Lieutenant H.D. Laffan* and A. Mair*. Hammar first triangulated parts of Victoria County (north of Durban) and then parts of northern Natal, the latter mainly for military purposes. During 1893-1894 he surveyed the border between Zululand and the Transvaal. He used his triangulation as basis for the first truly topographic maps of several parts of Natal, including the Lower Tugela Division, Victoria County (1899); the Inanda Division, Victoria County (1900); and the Umgeni and Pietermaritzburg Divisions (1903). These maps, which were not published, were on a scale of 1 inch to the mile (1:63 360).

After the Anglo-Boer War (1899-1902) he was no longer in the employment of the Natal government, but he was re-appointed as surveyor in the office of the surveyor-general in Pietermaritzburg in July 1908. From March 1910 he was engaged on a survey of the Natal and Zululand coast, up to and including St Lucia Bay, in conjunction with a hydrographic survey carried out by HMS Mutine. This project included connecting hydrographic shore stations to the secondary triangulation, which he did with great care. Also included in the work were a traverse survey of the coast and a topographic survey of the coastal strip. Hammar was an excellent topographer and compiled six maps of the coast, on a scale of 1 inch to the mile (1:63 360). When the work was completed in 1912 he was appointed director of Trigonometrical Survey under the surveyor-general in Pietermaritzburg, a post which he held to his retirement in 1922.

Much of the computational work to adjust the secondary network was carried out by O.A.E. Schreiber*, based on a local Cassini-Soldner coordinate system. Hammar's Report on the secondary triangulation of the province of Natal and Zululand... was submitted to the surveyor-general in March 1914. Unfortunately the triangulation on which he worked so hard could not be incorporated into the post-1920 trigonometrical survey of the Union of South Africa, as the latter was based on the Gauss Conform system. However, many of his stations were later re-measured, and his results played an important role in stabilizing the cadastral work in Natal. After his retirement he continued with survey work for the Trigonometrical Survey of South Africa in Zululand, and for the Coast and Hydrographic Survey along the Natal south coast.

Hammar became a member of the South African Association for the Advancement of Science in 1907 and was still a member by 1918. He was a competent landscape painter, but being a very modest man he refused to exhibit his work. However, in 1890 his wife exhibited four of his oil paintings in Pietermaritzburg, using the pseudonym "Mr Dauber". His later works, dating from 1902 to 1905, included paintings of the Howick Falls, Kranskop, Inchanga, and the Umzimkulu River near Port Shepstone. The railway station Hammarsdale, halfway between Durban and Pietermaritzburg, was named in his honour.