Geomancy is one of the clearest cases of a divinatory technique with a traceable route of transmission, and the route runs through the Islamic world.
Ilm al-raml
The Arabic name is ilm al-raml, the science of the sand, or khatt al-raml, the line of the sand, referring to the marks made in earth from which the figures are generated.
It was established in the Islamic world by around the ninth to tenth centuries and became a widely practised art with its own substantial literature. Its practitioners generally attributed its origin to Idris, identified with the biblical Enoch and with Hermes, or to the prophet Daniel, the standard move for giving a technique prophetic authority.
Its actual origins are not settled. A North African or sub-Saharan African source has been argued for, and the technique has close relatives in West African divination.
The evidence
The system was not marginal. Emilie Savage-Smith and Marion Smith's study is built around a brass geomantic instrument made by Muhammad ibn Khutlukh al-Mawsili and dated to 1241-42, a precision-engineered device with sliding dials for generating and reading the figures.
Nobody manufactures a brass calculating instrument for a fringe amusement. This was an established professional practice with a technical apparatus.
Into Latin
The transmission occurred in the twelfth century, in the same movement of translation from Arabic into Latin that carried astronomy, algebra, medicine and Aristotle into Europe, largely through Spain.
Hugh of Santalla, working in the Ebro valley in the first half of the twelfth century, produced the Ars Geomantiae, among the earliest Latin treatments. Other translations followed, and the art acquired the Latin figure names still in use.
Therese Charmasson's study of the Latin manuscript tradition is the standard scholarly account of what happened next: geomancy became a substantial medieval European practice with its own manuscript families, its own disputes about technique, and its own defenders and critics within the universities.
Why Populus
Because this figure (all four rows doubled, wholly passive, the crowd), is the one that most clearly shows the system's character. It is not a picture of anything. It is a position in a sixteen-value code, and the reading assigned to it descends through Arabic and then Latin sources that can be named and dated.