Georeferencing Sanxay using different sources
BAtlas 14 E2 Sanxay , coordinates from georeferenced BAtlas 14 map sheet at scale 1:1 000 000
BAtlas 14 map by map directory "Sanxay", midpoint of location Sanxay found using Google Maps or Google Earth
Satellite imagery where archaeological site (fanum, bath and theater at Herbord, commune Sanxay) is visible and known from literature or any other source
Conclusion
Georeferencing the Barrington Atlas directly will result in an error of up to 3 kilometers if the place is correctly mapped.
If you do not know the exact location of the archaeological site from a reliable source, or if the site is not visible,
the Barrington Atlas offers an acceptable approximation if you can identify and find the place in the directory.
Trying to find the site without any additional information is like looking for a needle in a haystack.
Ultimatelly it depends on what accuracy you want and what purpose your mapping application will serve.