Baumann is best known for his exploration of the interior of German East Africa (present-day Tanzania, Rwanda and Burundi), and for producing maps of the region. In 1888, with geographer Hans Meyer, he explored the Usambara region, with designs of continuing on to Mount Kilimanjaro. The two explorers' progress was stopped, however, due to ramifications associated with the so-called Abushiri Revolt (1888-89). Within a matter of days Baumann and Meyer were captured and held as prisoners. According to the British Rear-Admiral Edmund Fremantle:'They had been robbed, stripped, and subjected to every indignity, but except that they have lost all, including their instruments and observations, they do not seem much the worse'. Only after a large ransom was paid to rebel leader Abushiri ibn Salim al-Harthi were the two men released.