Swallowhead Spring, Hoare's Different Position

From The Ancient History of Wiltshire, Volume 2 Sir Richard Colt Hoare (Click to enlarge)

Probably just an error but I found it interesting that he marks Swallowhead Spring in  a different position to where even the earliest Ordnance Survey map places it, on the elbow where the nascent Kennet turns east.

The spring he marks has an interesting history of changes and infilling, and probably has a much richer buried history than any the other one.