Constable copied the painting Hagar and the angel who said looking back this was an important epoch in his life.
I love hearing how other artists were influenced by each other. When you hear that they saw certain works you can start to make connections.
Claudes sketches were not ends in themselves but was interested in the imagination and how he could look inward to use the landscape as a body that expresses a spirit beneath; as Casper David Friedrich puts it: "It is not faithful representation of air, water, rocks, and trees which is the artists task, but the reflection of his soul and emotions in these objects."
I have put a few of these artists out door studies next to a finished painting done in the studio for you to see how they feed into each other.