Annie, tacksman just means the holder of a lease, there's no title involved. If a clan chief owned the land the tacks would most indeed most likely go to his close relatives when the old tacks expired.
Where the land was owned by a feudal superior, the tacks were his to grant. This was the cause of much aggro, the last clan battle for example was fought over unpaid rents to the Mackintosh by Keppoch, some of whose clan held tacks and paid rent to the Duke of Gordon.
Presumably your ancestor was working for Telford, the project took years to complete and your ancestor presumably took out a lease on a property for that time. Presumably the tack was registered.
This is not the same as the English system of freehold & leasehold. The National Archives might have something on this, there will certainly be piles of stuff on the canal project itself.
http://www.nas.gov.uk/onlineCatalogue/Bests,
Skoosh.