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Buchwald based his decision on the 1848 Treaty of Guadalupe Hidalgo, which settled the Mexican-American War, and required that the United States recognize Mexican land grants as long as the owner filed a claim. The original owner of the coastal property filed such a claim. The U.S. government challenged his land patent, but the U.S. Supreme Court confirmed his ownership in 1859 -- 14 years before the California Constitution was first drafted.

I can't make head nor tail of this.[1]

EDIT: OK, this[2] is clearer:

The judge's ruling skirts the fundamental conflict between the rights of private property owners and the rights of Californians to access the shoreline. Instead, Buchwald rooted his decision in the land's history during the mid-19th century. Since there was no public easement attached to the property at the time the United States acquired California from Mexico, the judge reasoned, the question of whether the California Constitution now guarantees access to the beach is immaterial.

The original owner of the property was Jose Maria Alviso, who received a provisional land grant from the Mexican government in the late 1830s. He later transferred the property to his brother, Jose Antonio Alviso, whose rights to the property were upheld under the 1848 Treaty of Guadalupe Hidalgo, which settled the Mexican-American War. The U.S. government challenged Alviso's land patent, but the Supreme Court confirmed Alviso's ownership in 1859.