Accurate Geodetic Coordinates for Observatories on Cerro Tololo and Cerro Pachon

TL;DR

This paper accurately measures and compares geodetic positions of observatories on Cerro Tololo and Cerro Pachon, revealing discrepancies in published data and demonstrating Google Earth's precision for geodetic positioning.

Contribution

The study provides a comprehensive set of accurate geodetic coordinates for observatories on Cerro Tololo and Cerro Pachon, and evaluates Google Earth's effectiveness for geodetic measurements.

Findings

Google Earth can determine geodetic positions with about 5 m accuracy without correction.

The published LSST position is off by 9.4 km, and its elevation by 500 m.

GPS measurements and Google Earth are consistent within a few meters.

Abstract

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.