A
permanent record of physical location. Placed all over the
USA by the
USGS (United States Geological Survey), these little
numbered brass plates are used as
standards by which other geologic features' locations are
measured. The marked
center of a plate is a '
corner' by which locations can be determined through
triangulation. This method is increasingly being
deprecated in favor of
GPS technology.
You can often find your city's
official benchmark set into the
concrete in front of the local
City Hall.