Hello everyone,

I'm working with some researchers who are looking at US Army Map Service (AMS) maps of Africa, specifically in Chad. At the bottom of the map in the Reference section there is a key that lists "Towns of Importance" with rankings 1st through 4th and their symbols. Here is an example, AMS Fort Lamy Chad 1942:

The researchers want to know if there was a *specific* convention for establishing the level of importance, such as population thresholds. If not, then generally what convention was used. I found a guidebook on Geographical Names for Military Maps the AMS published in 1945:

But it makes no mention of these conventions. These maps from Chad were copied and translated from the French Army mapping service, Service Geographique de L'Armee, so perhaps it's their convention. Searching the web, HathiTrust, and JSTOR for terms in French, I couldn't find anything useful except for this: LES ECRITURES SUR LES CARTES TOPOGRAPHIQUES ("ENTRIES ON TOPOGRAPHIC MAPS") 1934, which seems promising. But it's not available digitally and very few libraries own copies. I found some for sale, including this one on eBay that has some pages scanned (but none that give me the info I need):

My question! Does anyone have a reference to a source that identifies how these importance levels for towns and settlements were established?

Thanks and best - Frank