What larger idea about the traveler’s journey is mainly expressed by these images of nature in the poem

i claim you tobemy lord

the aesthetic impact is that moment when you are relating spatially to the target. the "ai" occurs when you have an impression that includes both you and the target and is usually a spatial relationship.

the third paragraph is your answer,

"but, in a larger sense, we can not dedicate — we can not consecrate — we can not hallow — this ground. the brave men, living and dead, who struggled here, have consecrated it, far above our poor power to add or detract. the world will little note, nor long remember what we say here, but it can never forget what they did here. it is for us the living, rather, to be dedicated here to the unfinished work which they who fought here have thus far so nobly advanced. it is rather for us to be here dedicated to the great task remaining before us— that from these honored dead we take increased devotion to that cause for which they gave the last full measure of devotion—that we here highly resolve that these dead shall not have died in vain—that this nation, under god, shall have a new birth of freedom—and that government of the people, by the people, for the people, shall not perish from the earth. "