The accepted route of the sarsens is based on Atkinson's 1956 Sarsen Route From His Book "Stonehenge"
I think research has shown that that route is not the most likely and a route through Marden Henge is more probable.
The extent of Cannings Marsh is one of the key features. Other factors are explored elsewhere on this blog.
This is the latest revision of the route which fits the geography best.
Click map for larger version
Update - Thanks to Simon Banton for correction on southern section