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RE: Have you ever seen one of these.
the contour lines were likely just super-imposed later from other accurate sources.
They used these ancient ones a lot for identifying Kimberlites over vast distances under deep covers of desert sand.
Thank you @gavvet for the update and explanation. I actually wondered, after I had commented rather hastily, whether they were using some sort of long wave technology for some purpose, rather like submarines do.
I am no geologist but I am involved in mining is Sth America.
Thank you again for your high standards! Namaste!