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This month, the National Geodetic Survey (NGS) conducted a series of geodetic surveys of the peak of the Washington Monument. These surveys will allow NOAA to establish a new definitive height for the monument and allow comparisons with future surveys to detect any changes in height. Questions? We thought you might have a few, so we put together this handy Q&A.
The official height of the monument since 2001 (but not including the new 2013 survey) is 555 feet 5 1/8 inches. This height refers to a very specific point on the foundation of the monument, and not to the ground around the monument. As such, this question is different from asking "is the monument sinking?"The results of this new survey, including any change of the official height, will be vetted by the National Park Service and announced early in 2014.
Because somebody used surveying to indirectly measure it! We like to think that George Washington, having worked as a surveyor in his early career, would be proud.
In NGS archives, there are no known surveys of the height of the monument prior to 1999, only latitude and longitude. Even then, most surveys of the peak were done from the ground, sighting to the peak. Rarely in NGS history was an instrument placed atop the peak, and even then (prior to 1999) such surveys were only for latitude and longitude determination. It is therefore unclear who performed surveys to determine the height of the monument, but they do not appear to have been done by NGS.
In 1998, according to information on historic National Park Service web pages, the monument was "555 feet tall." That was changed to "555 feet, 5 1/2 inches" by August 18, 2000. On November 9, 2001 the height was listed as "555 feet, 5 1/8 inches" and that height has stood as official for 12 years. The source of the two drops in official height over three years are somewhat obscured in history, but are likely related to the last time the peak was accessed by NGS, in 1999. At that time, the height of the monument was measured using GPS techniques. However, the accuracy of that measurement is slightly suspect because the scaffolding around the peak caused difficulty in using GPS at the time. That survey was not completely processed until the middle of 2001, which may be the source of one or both drops in the official height from 1998 through 2001.
In order to avoid the difficulty of relying solely on GPS this time around, the 2013 survey used an optical surveying technique called "reciprocal vertical angles" combined with GPS. The expected accuracy of the height of the monument from this new survey is expected to be between 1-2 mm. The final results will be announced early in 2014.
This depends on what is meant by the "height" of the monument. If the height refers to the ground around the monument itself, then any sinking of the monument into the ground would be seen as a "height change." However, a 2012 survey of heights around the National Mall area, including points at the base of the Washington Monument, indicated, at most, 1 mm of sinking since the last check (prior to the 2011 Earthquake). The 2012 survey, however, is being re-processed to confirm the results as part of the 2013 survey of the peak.
If, by height, one is referring to the distance between a point on the foundation of the monument and the peak (which is what the official height of 555 feet 5 1/8 inches refers to), then the only documented height change in NGS records come from a 1934 note when the Coast and Geodetic Survey accessed the peak for the purposes of measuring its latitude and longitude. In that note, the team found that "the station is the apex of the Washington Monument, and is marked by the center of the aluminum tip that surmounts the monument. This tip has apparently been burned by lightning as the top is about 1/2 inch square." Given that the peak has a face angle of about 17.25 degrees, a 1/2 inch square apex would mean that between 3/4 and 7/8 of an inch had been removed from lightning burns (or other erosion) between the time of the original construction in 1885 and the 1934 inspection.
NGS is in the business of measuring. While we can answer the question "how much change has occurred?," questions such as "why has a change occurred?" are best left to structural engineers or geologists. And again, this depends on the definition of "height." If we are taking about the foundation-to-peak height, then erosion of the peak is the only reason documented so far in NGS archives. If any other sources are occurring, such questions need to be addressed by a structural engineer. NGS does not have information on that. If we are talking about the ground-to-peak height, then the sinking of the entire monument (foundation and all) into the ground is a possible explanation, especially considering the originally swampy nature of the entire Washington D.C. area. But, as mentioned, a 2012 survey to the base of the monument showed, at most, 1 mm of sinking, when compared to a survey performed prior to the 2011 earthquake.
It is difficult to say for certain if the 2011 East Coast earthquake changed the height of the monument, as there was not a survey of the peak before 2013 with enough accuracy to show changes smaller than about 1-2 inches. And since a height change of that magnitude is not anticipated, it is difficult to say what the actual impact of the earthquake was on the height. A 2012 leveling survey to the base of the monument showed a possible sinking of the monument into the ground of 1 mm since the last pre-earthquake survey. But this value was so close to the expected noise ("measurement error") sources of the survey that it is not statistically significant. The new survey to the peak will help validate the 2012 finding. Ultimately, the 2013 survey will serve as the first high accuracy (within a range of 1-2 mm) height measurement of the peak so that future surveys will be able to detect change at that small level.
The peak of the monument has been used as a visible surveying point for over 100 years. Anyone can theoretically perform a measurement of the peak using proper surveying techniques from the ground. And while NGS has surveyed on and around the monument for decades, the only official statements about the official height of the monument are issued by the National Park Service.
NGS performs surveys of the entire National Mall area every few years, usually at the request of, or in collaboration with, the National Park Service, as part of the overall maintenance and restoration of the area. Measuring an "official height" of the Washington Monument, however, is not usually part of this—but measurements of vertical movement at the base of the monument are part of such surveys.
Access to the peak of the Washington Monument is rare. Since direct access to the peak is required for the "reciprocal vertical angles" method of height determination (which allow for an expected 1-2 mm accuracy of the height), NGS has no plans to remeasure the height of the peak again soon. Should an opportunity to occupy the peak with survey instruments occur in the future, NGS will work with the National Park Service to determine if such a survey can be performed.
Yes! Heights affect individuals every day in ways that are often overlooked. Here's one example we can all relate to: the height of land is a critical determiner of how, when, and where flooding will occur.