Largest known Roman mosaic, beneath Baths of Trajan, opens to the public

Related — this isn't the only map the Romans made that has survived.

https://formaurbis.stanford.edu/

I wonder who would write this type of article without including the images.

https://www.gettyimages.com/photos/baths-of-trajan

I've always wondered how mosaics were made, like, how could they know where to put which stone? Did they prepare a sketch on papyrus first and then put stones to match it? I'm having a lot of difficulty conceptualizing how it could be done.

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