Before revision: The Firth of Clyde is sometimes thought to include the estuary as far upriver as Dumbarton, but the Ordnance Survey map shows the change from river to firth occurring off Port Glasgow , while locally the change is held to be .
After revision: The Firth of Clyde is sometimes thought to include the estuary as far upriver as Dumbarton, but the Ordnance Survey map shows the change from river to firth occurring off Port Glasgow .