Union Square
Union Square was a  located on Earth in the city of San Francisco, California. Powell Street was one of the four streets which bordered this square.

On the private business card of Dixon Hill, his address was given as being at a three-story building overlooking Union Square. Mr. Hill's card read: "Dixon Hill, Private Investigator, 312 Powell, Union Square, San Francisco, Calif., Prospect 4623." ()

External link 
 
Earth geography
The Dixon Hill Series