“The US already owns a 60 foot easement along the entire border”
That does not cover the 1,250 miles of border that sits in the middle of the Rio Grande - Only California, Arizona and New Mexico.
It is one of several reasons why there is so little pre-existing barrier to replace in Texas.
About 400 private landowners must be settled with in 100 miles funded for the Rio Grande Valley, and about 1,000 in total, for all the places in Texas where the plan calls for barrier.
Add to that:
McNamara stressed that Congress would be key to any claim of eminent domain.
“There’s obviously a debate about whether Congress has authorized the ‘emergency’ use of funds to construct the wall, but in the context of eminent domain, the courts have been clear that uncertainty about whether Congress has authorized eminent domain has to be resolved in favor of the property owner, not in favor of the government,” he said.
https://finance.yahoo.com/news/trump-border-wall-eminent-domain-191651556.html