Along and Across

Along adverb - Toward or at a point lying in advance in space or time.

Across and along are semantically related in down topic. In some cases you can use "Across" instead a preposition "Along", when it comes to topics like on, via, of.

Across adverb - From one side to the other of an intervening space.

Along and across are semantically related in on topic. You can use "Along" instead a preposition "Across", if it concerns topics such as via, of.