Gila and Salt River Meridian

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The Gila and Salt River Meridian intersects the base line on the south side of Gila River, opposite the mouth of Salt River, in latitude 33° 22' 40" north, longitude 112° 17' 25" west from Greenwich, and governs the surveys in the territory of Arizona. The initial point is located on "Monument Hill", an easily visible hill just south of the confluence of the Gila and Salt Rivers, near Avondale, Arizona, about 40 miles (64 kilometers) southwest of Phoenix. The original marker has been lost for many years, but a brass plaque was mounted in rock in 1931 (due to damage of the plaque, this could be 1961), then a large concrete, brass and aluminum monument was constructed about 14.8 feet (center-to-center) from the 1931 marker. That monument was, in turn, modified in 2006. The location was added to the National Register of Historic Places (listing #02001137) as of 10/15/02, but apparently the plaque for that listing was constructed on the summit at the same time as the 2006 modification of the nearby monument.

For the 1984/2006 marker, hand held GPS units give a reading of 33°22.631' N Latitude and 112° 18.366' W Longitude due to the specific Geoid used. The angle between the 1984/2006 marker and the 1931 marker is at a compass heading of 292°, aka 22° north of due west. The location is immediately east of Phoenix International Raceway and the western side of Monument Hill is terraced for low-priced race viewing. See External links for further details.