In the late 1850s, the administrative divisions into forts was changed into a division into districts (Dutch: "afdelingen"), asserting Dutch sovereignty (or suzerainty) over not only the forts, but also the territory surrounding the forts. District officers were instructed to make surveys of physical, economic, and socio-political situation of the districts. As a consequence of the tariff system set up in the Anglo-Dutch Gold Coast Treaty, a tax and customs office was established in Elmina in 1867. At the same time, a postal office was established as well.