Adam Clarke on Joshua 22:10

The borders of Jordan, that are in - Canaan - This verse can never mean that they built the altar on the west side of Jordan, for this was not in their territories; nor could it be a place for the purpose of public worship to their own people, if built on the opposite side of Jordan; besides, the next verse says it was built over against the land of Canaan. It appears that when they came to the river they formed the purpose of building the altar; and when they had crossed it they executed their purpose.

A great altar to see to - A vast mass of earth, stones, etc., elevated to a great height, to serve as a memorial of the transactions that had already taken place. Probably it was intended also to serve as a kind of watchtower, being of a stupendous height, altare infinitae magnitudinis, an altar of an immense size, as the Vulgate terms it.