“After all these years, we’ve finally got it right!” Colorado governor Bill Owens declared, shortly before
setting a 3.5-inch standard bronze disc in the 13th step on the west side of the Colorado State Capitol on September 29, 2003.
The North American Vertical Datum (NAVD 88)-based “Mile High” bench mark joins two others in the steps of the Capitol. The first Mile High marker was installed on the 15th step in 1909. After the plaque was stolen several times, the inscription “One Mile Above Sea Level” was engraved into the step in 1947. This elevation is purportedly based on the first transcontinental elevation level lines run from Chesapeake Bay to San Francisco.
The second marker, a brass cap set in 1969 by Colorado State University engineering students, used the then-current National Geodetic Vertical Datum (NGVD 29) for its Mile High basis. The CSU students were very confident of their work as the cap is stamped 5280.000 and has a dimple for the exact elevation.
Our project came about after newspaper and magazine articles began to appear in 2002 that described how the mountains of Colorado were suddenly three to seven feet taller than previously believed. We surveyors were aware of the new NAVD 88 datum long before this time, but it was just starting to become general knowledge as maps were being updated.
The project was a volunteer collaboration between the Colorado Department of Transportation (CDOT), the Professional Land Surveyors of Colorado (PLSC), National Geodetic Survey (NGS) and the Metropolitan State College of Denver (MSCD) Surveying and Mapping Program.
Phone calls and e-mails led to meetings with the State Capitol Complex employees to explain why a new mile high location was needed, what a deep rod monument is, and why we were putting it in their flower bed.
Our plan was to use station McDonnell, an existing High Accuracy Reference Point (HARN) on the grounds of MSCD. This point is fairly close to midway between two existing NGS bench marks at Union
Rail Road Station and the State Capitol. The project would have taken three days of fieldwork if the crew returning from the Capitol to McDonnell hadn’t encountered a major spring time rain and hail storm. So technically the fieldwork took four days: a Saturday morning to set deep rod station HWS-001 on the Capitol grounds, another Saturday to run two level loops: the first, with party chief Lawrence de Vries, PLS, in charge, between station McDonnell and Union Rail Road Station; the second, with party chief Joseph Gunderson, PLS in charge, from McDonnell to the deep rod at the Capitol. It was on the return trip from the Capitol when the intense rainstorm hit. Joe’s crew finished the loop at a later, drier date, on a third Saturday, after Dr. Herbert W. Stoughton, PhD, PE, PLS, CP, did the data reduction of the two level loops, to find the actual location of the “new correct” Mile High on the Capitol steps.
Once the fieldwork was done in July it was more meetings with the Capitol people. What kind of ceremony to have?
When can the Governor fit it into his schedule? August 1, Colorado Day—the day we became a state—came and went. A date was set, kinda—mid September, then changed—late September. Then September 30 at 11 a.m. Ten days later it changed to Monday, September 29 at 11 a.m. A week later—same day at 10 a.m. There it stayed, and we got to hear “We’ve finally got it right!”
But at the time they were set they were all correct. It’s the changes and advances in technology that give us more accurate information. In time, hopefully not in my time, we may have to set a fourth cap, and then we may be able to actually obtain the accuracy envisioned by the CSU students.
Originally published in “Lasting Impressions” by Rhonda Rushing.
Published in Caching Now, December, 2009