Land along the border is largely privately held. To build a wall, private land has to be aquired by the government. Here’s one view:
Land transfered by the King of Spain was aquired to build the current partial border wall? Yes, land that had been held by the same family since the initital land grant from the King of Spain in 1767.
Surely the feds own some of the border lands? Yes, about 33% of the land is either federal or tribal lands
Where is most of the 67% of private land? TEXAS
CATO via Daily Beast | The Great Wall Of Trump Would Be the Ultimate Eminent Domain Horror Show
Washington Post | Donals Trump’s Great Wall of eminent domain