How much of the West could Lewis and Clark actually map?

[last1out]

They went up one river, through a gap in the mountains and down another. They could probably see about 10 miles on ether side of the trail at best.
So how did that help map the West? Wouldn't it take about a hundred such trips just to find the things you want to map, like major lakes and forests.

[Lure]

I never thought of them as cartographers,but trailblazers.They were exploring the way to the pacific thru lands purchased from France in the Loiusiana purchase,which didn't include the whole of the southwest and California.IIRC the southern boundaries were the Missouri river in the plains states area.

They got a lot of their information from different Indian tribes and even furtrappers,who knew localized conditions along the way.

Their trip made it possible for future settlers and commercial ventures to further refine the mapping process.

[Freddy the Pig]

As Lure says, the goal wasn't so much to MAP the northwest but to get an idea of its size and identify the major geographic features--which turned out to be the Rocky Mountains and Missouri and Columbia Rivers. The expedition split up at several points and so covered more than just one route. And, of course, the local Indians were able to describe nearby land which the party did not directly traverse.

See Bernard DeVoto's "The Course of Empire" for a good history of how geographic knowledge advanced over time, and the contributions made thereto by the Lewis and Clark expedition.