Apparently the Surveyor General James Tilton who signed this has an interesting story too.

A couple years after this survey in 1860 his 12 year old slave Charles Mitchell was contacted by free black people in Victoria (still under British control), after which he stowed away on a ship. When discovered, he was arrested but two days later was granted freedom. James Tilton protested and was featured in newspapers as far away as San Francisco.

Tilton later worked for the Northern Pacific Railroad Co. and was supposedly the first to summit Denny Mountain at Snoqualmie Pass. Perhaps the first Euro-American to do so, since people have been in the region since at least the early Holocene epoch.