Several times Petermann indicated that he disagreed with the reports of explorers, probably basing his arguments on reports and literature at his disposal or on sound geographic reasoning. In a map, concerning among other things lake Uniamesi, he tends to disagree with Jakob Erhardt, one of the missionaries of the Church Mission Society of London, concerning its situation and extension. Erhardt was erroneous in that he situated the lower tip of the lake around 13° South 36° East and made it bend westward to 28° East, with a probable extension to 24° East. Furthermore, he thought that Lake Victoria, Lake Tanganyika and Lake Nyassa were one inland sea extending to 6° North. A lack of data led Petermann to agree with his erroneous shape of the lake or lakes, but he was not convinced of its wide extension to the west and north. As for the sources of the Nile he was not so much mistaken, as they were indicated by a text near the equator between 30° and 36° East, but this was not so hard to induce. When one views the ethnographic and trade information in the map and interprets them from a relative topological point of view we can see this as the strength of the missionaries' reports. As their mission was focused on people and not on the natural environment this kind of information was of great importance to them. But geometrically their information could not be trusted, in part because most of their information was based on verbal reports of the native tribes.