Projection analysis and georeference of the 1: 2M Africa map by Régnauld de Lannoy de Bissy (1891-1902)

The 1:2 million scale Carte d’Afrique by Régnauld de Lannoy de Bissy, consisting of 63 sheets, is a nice summary of humanity’s knowledge of Africa at the end of the 19th century. According to the literature, the map system has the ‘meridional orthographic’ projection, with the central meridian 10 degrees East of Paris (cca. 12 degrees and 20 minutes from Greenwich). No datum information is found. At this scale, it is less important indeed for any georeferencing work. However, defining an orthometric projection with the central meridian of 12 degrees 20 minutes leads to unacceptably high errors of the georeference of the sheets of this map product. We tried another solution. As the contemporary German map product (the 1:4 million scale ‘Habenicht map’) has a ‘Flamsteed’ projection, we tried this for the French series. The Flamsteed projection is called sinusoidal in modern GIS applications. The georeference with the sinusoidal projection was much more convincing, resulting in a conclusion that the actual projection of the French map product is also the ‘Flamsteed’ or sinusoidal projection, with the projection center at the crossing of the Equator and the meridian 12.33333 degrees East of Greenwich. However, georeferencing using this projection also produces errors: The errors have a magnitude of up to some 10 kilometers far from the center, hinting that the size of the Earth assumed was slightly different from the applied ‘Google Sphere’; it shall invoke further scale analysis of the map product. We are encountering errors of a completely different nature in Central Africa, far from the coast. The interior of the continent was only partially surveyed along traverse lines with geodetic accuracy at that time. These errors are therefore part of the knowledge conveyed by the map at the end of the 19th century.

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Recommended citation: Timár, G., Musyimi, P. K., & Appel, S. Projection analysis and georeference of the 1: 2M Africa map by Régnauld de Lannoy de Bissy (1891-1902). In Conference Proceedings, ISSN (Vol. 2459, p. 3893).