  As the 50th anniversary of the Cerro Tololo Inter-American Observatory (CTIO)
draws near, the author was surprised to learn that the published latitude and
longitude for CTIO in the Astronomical Almanac and iraf observatory database
appears to differ from modern GPS-measured geodetic positions by nearly a
kilometer. Surely, the position for CTIO could not be in error after five
decades? The source of the discrepancy appears to be due to the ~30" difference
between the astronomical and geodetic positions -- a systematic effect due to
vertical deflection first reported by Harrington, Mintz Blanco, & Blanco
(1972). Since the astronomical position is not necessarily the desired quantity
for some calculations, and since the number of facilities on Cerro Tololo and
neighboring Cerro Pachon has grown considerably over the years, I decided to
measure accurate geodetic positions for all of the observatories and some
select landmarks on the two peaks using GPS and Google Earth. Both sets of
measurements were inter-compared, and externally compared to a high accuracy
geodetic position for a NASA Space Geodesy Program survey monument on Tololo. I
conclude that Google Earth can currently be used to determine absolute geodetic
positions (i.e. compared to GPS) accurate to roughly +-0.15" (+-5 m) in
latitude and longitude without correction, or approximately +-0".10 (+-3 m)
with correction. I tabulate final geodetic and geocentric positions on the
WGS-84 coordinate system for all astronomical observatories on Cerro Tololo and
Cerro Pachon with accuracy +-0".1 (+-3 m). One surprise is that an oft-cited
position for LSST is in error by 9.4 km and the quoted elevation is in error by
500 m.
