Surveyors Tackle The Washington Monument
Soon after, the National Geodetic Survey (NGS) performed a leveling analysis of benchmarks on and around the historic structure, but found no significant settlement had occurred as a result of that earthquake.
A repair effort was started in 2013 and, for the first time since 1999, scaffolding enclosed the entire Washington Monument. Just like in 1999, and in 1934 before that, this meant that the rare opportunity to survey the peak of the Washington Monument itself was available. Working with the National Park Service (NPS), NGS formulated a plan to survey the peak of the Washington Monument while the scaffolding made access possible.
This article describes the geodetic survey conducted at the peak of the monument during the months October 2013 through April 2014. (The top/center of the peak was the surveyed point, but it is part of the monument itself. As such, one might be justified in calling the Washington Monument the largest geodetic control mark in the world!). The purpose of the survey was two-fold:
- To survey the location of the Washington Monument peak in the NSRS (latitude, longitude, ellipsoid height and orthometric height) more accurately than has ever been done before
- If possible, to compare with the previous surveys and determine if any change was detectable over time
Geodetic surveys are not normally concerned with the heights of buildings. Nonetheless, as the height of the Washington Monument has been a publicly and prominently reported value since 1885, an attempt to validate the architectural height was planned, since the amount of additional labor to collect the extra measurements necessary was minimal in comparison to the overall scope of the project.
Complete details on the survey can be found in the recently-released “NOAA Technical Report NOS NGS 51” (NGS 2015).
In order to put the 2013-2014 survey into context, an overview of all geodetic surveys near the Washington Monument over the years is pertinent.
1880-1900: Most surveys of the Washington Monument during this era are in paper files in the National Archives. However, investigations at NGS in 1999 and 2013 yielded two references to the architectural height:
- In 1885, just one year after the completion of the Washington Monument, in the handwritten annual report of Lt. Col. Casey, he states that the monument is 555 feet, 51⁄8 inches tall. No mention is made as to what ground point or points this measurement refers.
- A note in the 1896 “Report of the Superintendent” of the Coast and Geodetic Survey (C&GS, predecessor to NGS) describes a brass bolt near the southwest corner of the Washington Monument, stating that “the aluminum point of the pyramidion is said to be 555 feet, 4½ inches above this bolt,” although it makes no mention of who said the pyramidion was that high above the bolt.
1901-1933: C&GS performed nine leveling surveys to marks on or around the Washington Monument during these years. Most (but not all) of these marks have been destroyed. For example, marks B, C, D and E are described in the 1907 field book as: “In the driveway and subject to the wear and battering due to the passing carriages.” Well, carriages might be a thing of the past, but so is the driveway in which these marks were set. When it was removed, these marks likely went with it, as these four marks were never used by C&GS again after 1912.
As the 2013-2014 survey was being planned, the assumption was that only two marks (“A” and “I”) had survived since the early 1900s. Dave Doyle reported in 2012: “When C&GS revisited the Monument in 19211, the marks from 1901 had all been destroyed.” Contrary to this statement, there is significant evidence that at least one of the 1901 marks did survive.
The designations of marks used in 1901 were A, B, C, D, E, F, G, H and B 1. There is compelling, but not yet definitive, evidence that mark B 1 was not destroyed, but was actually rediscovered by NGS in 1999 and (not recognized as an historic mark with an existing designation) was treated as a new mark and given a new designation (W M BASE SW). Three marks from the 1907 leveling survey (with designations M, N and O) may also be in this same situation (e.g., lost, later re-found and then renamed). Once further information has been collected, NGS will issue a report about these marks.
1934: Scaffolding was erected around the Washington Monument for a restoration project. A C&GS crew ascended to the peak and performed a triangulation survey to visible marks. Evidence in NGS archives supports the conclusion that no attempt to measure the height of the monument was done as part of this survey (although there is some conflicting anecdotal evidence about this).
1935-1998: The primary work of C&GS (which became NGS during this era) around the Washington Monument was monitoring of leveled benchmarks in the National Mall area.
1999: The entire Washington Monument was again encased in scaffolding. NGS surveyed the peak with GPS, but no final report was publicly disseminated. Details about the survey and its preliminary results are in the Doyle report/article of 2000, with further details and an attempt to determine the final results in NGS (2015).
2000-2012: More leveling near the Washington Monument occurred in these years, both pre and post-earthquake.
Once the announcement that scaffolding would again surround the Washington Monument was made, NGS reached out to NPS to discuss collaborating on a new survey. The response was enthusiastic, and preparations began.
A review of the available data from the two other NGS surveys performed at the peak (1934 and 1999) was performed. Almost all of the data from 1934 was in a non-digitized form.
With no formal report from 1999 available, it was necessary to perform some geodetic forensics to make educated guesses as to what had been done then. Thankfully, some very useful information was available through conversations with the 1999 survey project manager, as we noted in personal communication with Doyle in 2014:
- The architectural height was computed, and was based on “the CASEY marks”
- Leveling to the floor was done, but nothing was ever done with that data
- A report was completed but not publicly disseminated
The Breidenbach Adapter
Safety training and a reconnaissance trip to the peak were performed Oct. 28, 2013. Measurements were taken of the peak and the surrounding environment.
NGS was able to corroborate the 1934 recovery note from the NGS integrated data base (there was no recovery note from the 1999 survey) stating: “THIS TIP HAS APPARENTLY BEEN BURNED BY LIGHTNING, AS THE TOP IS 1⁄2 INCH SQUARE.” Measurements of the tip proved that its rounding shortened the height of the Washington Monument by 3⁄8 of an inch.
A schedule for all of the survey components was created. One of the key elements to success was the quick design and build of a new adapter to hold all of the instruments over the peak. The 1999 adapter was built by the National Institute of Standards and Technology, and then returned to that organization after that survey, but by 2014 it was unable to be located. Don Breidenbach of NGS designed a new adapter and, with the assistance of Steve Breidenbach, it was ready within days. Known as “the Breidenbach Adapter,” this critical piece of equipment is stored at NGS’s facility in Corbin, Va. for use on any future survey of the peak.
As the survey got underway, NGS reached out to the Council on Tall Buildings and Urban Habitat (CTBUH) for guidance on determining the architectural height of the Washington Monument. CTBUH “developed the international standards for measuring tall building height and is recognized as the arbiter for bestowing such designations as ‘The World’s Tallest Building.’” By the organization’s standard, the height of a building is calculated as:
… measured from the level of the lowest, significant, open-air, pedestrian entrance to the architectural top of the building, including spires, but not including antennae, signage, flagpoles or other functional-technical equipment.
Once access to the interior unprotected floor of the Washington Monument was available in April 2014, CTBUH was able to provide guidance to NGS as to what point, exactly, the architectural height was to be measured. That point has the designation “W M FLOOR 3” and PID “DP2634” in the NGS database.
Although GPS was used in 1999 with moderate success, the scaffolding which surrounds the peak itself was a major source of multipath during that survey. This same issue was expected in 2013.
Initially, NGS proposed mounting a GPS antenna at the peak during the time when the scaffolding was being removed, so as to have a clear open-sky view. However, discussions with the NPS and both Perini Management Services and Grunley Construction concluded the logistics of timing such an occupation were insurmountable. GPS was thus considered a backup to the primary positioning strategy of triangulation and trilateration, although steps to make the GPS as successful as possible were taken.
First, GPS data was collected at the peak continuously Nov. 1-5. Then, on Nov. 7, a differential GPS survey took place, connecting the peak to seven passive control marks around the Washington Monument.
On Nov. 5-6, 2013, a terrestrial survey was conducted using a Leica TDM 5000 total station, involving the Washington Monument peak and five visible ground control points. Measured values included:
- horizontal angles at the peak to nearby ground points
- horizontal angles from ground points to other ground points and to the peak
- slope distances and vertical angles between ground stations
- most importantly for height determination, near-simultaneous reciprocal vertical angles and slope distances between the peak and select ground control points
Moving ahead, on five days in April 2014, geodetic leveling was performed to every ground control point used in the GPS and terrestrial surveys, as well as a variety of benchmarks on and around the base of the Washington Monument itself and select interior floor locations as part of the discussions with CTBUH. Federal Geodetic Control Subcommittee first order class 2 specifications were followed.
The geodetic leveling was adjusted first, but in two ways. First, new NAVD 88 heights were computed (or old ones verified) based on published control around the National Mall area. Second, the same data was used, but adjusted by fixing only the height of W M FLOOR 3 to be exactly 0.000 meters (+/- 0.00 mm). This allowed for the propagation of architectural heights out to the geodetic control around the monument, and eventually up to the peak.
Next, the angles and distances were adjusted. Again, two adjustments were performed. The first held latitudes, longitudes in NAD 83(2011), orthometric heights in NAVD 88 and geoid undulations in GEOID12A fixed at control points surrounding the Washington Monument, so that the latitude, longitude and ellipsoid height of the peak in NAD 83(2011) and orthometric height of the peak in NAVD 88 could be computed. The second performed a similar task, only replacing NAVD 88 orthometric heights with the architectural heights relative to W M FLOOR 3.
The final coordinates of the peak of the Washington Monument in the NSRS came from the first set of adjustments, while the architectural height itself came from the second set.
One additional computation was performed. In order to compare this survey to the heights computed in 1885 and 1999, an architectural height was computed using the 2014 data, but holding the average height of the four Casey marks as 0.000. This architectural height will be called a “Casey Height,” and it was encouraging to see that the 2014 survey appears to have validated the 1885 height to just ¾ of an inch. The complete results are:
* These values are educated guesses, based on evidence left behind at NGS. Multiple conflicting preliminary numbers were reported publicly, but no final numbers were.
The difference between the Casey Heights in 1885 and 2014 is just over 9⁄8 inches. However, 3⁄8 of an inch is explainable due to height loss of the tip due to rounding. As such, the 2014 survey validates Casey’s originally reported height to about 3⁄4 of an inch, which is easily explainable as a combination of measurement errors in 1885 and 2014, and possibly misunderstanding exactly what marks served as the zero height in 1885.
Detecting change of any sort requires two measurements of the same quantity, separated in time, and an accuracy of both measurements whose propagation through time does not exceed the change being detected. Certain obstacles prevented NGS from detecting change based on differences between the 1934 or 1999 surveys and the 2014 survey.
The first obstacle was the non-digitized nature of most of the 1934 data. After some initial work, the time necessary to inspect these records in order to enforce a comparison with the angles measured in 2014 was eventually viewed as beyond the scope of this survey. The second obstacle was that the 1999 survey relied solely on GPS in a high-multipath environment. The original GPS files were available, and were reprocessed using identical software as was used to process the 2013 data, but the quality of the two GPS solutions was not sufficient to detect the sort of mm-level changes being sought.
Even in the vertical, where some effect of the subsidence seen in NGS 2012 was expected, a comparison of the 1999 GPS-derived ellipsoid height and 2014 GPS-derived ellipsoid height showed a difference too large to be accounted for by subsidence alone. Without further information, it is impossible to state whether those differences are due to structural changes or merely artifacts of differences in GPS collection methods and/or multipath.
Therefore, it is expected this 2014 survey will stand as a baseline for future surveys so that change detection will be possible, the next time the peak is accessible.
The peak of the Washington Monument was occupied and surveyed to a degree of accuracy never before achieved. The latitude, longitude, ellipsoid height and orthometric height were computed in the NSRS to millimeter accuracy.
The reported architectural height from 1885 has been validated to ¾ of an inch within the limits of (a) measurement error and (b) assumptions about what “zero height” was chosen in 1885. A new architectural height for the Washington Monument, consistent with the international standard for building heights, has been computed as well. This standard allows the Washington Monument architectural height to be compared with other building heights in a consistent way.
Because the new standard differs from the assumed method used in 1885, the architectural height of the Washington Monument, relative to the international standard, is nearly 10 inches shorter than the 1885 architectural height. This is due mostly to an 843⁄64 inch difference in the chosen “zero height location.”
- Doyle, D (2000) The Washington Monument Height Modernization Project. Professional Surveyor.
- Doyle, D (2012) Movement of the Mall. Professional Surveyor.
- National Geodetic Survey (2012) Project Report: Leveling of the Washington Monument and the Washington Mall, Internal Report, Aug. 16, 2012. http://www.ngs.noaa.gov/PUBS_LIB/WashingtonMallLevelingReport.pdf
- National Geodetic Survey (2015) Final Report, 2013-2014 Survey of the Washington Monument, NOAA Technical Report NOS NGS 51. http://www.ngs.noaa.gov/PUBS_LIB/NOAA_TR_NOS_NGS_51_2015_02_16.pdf
- There is no evidence at NGS of a 1921 survey of the Washington Monument by C&GS. It seems probable, based on context, that the author meant the 1923 survey of the Washington Monument, when mark “C I” was established.
The Washington Monument survey would not have been possible without the assistance and cooperation of many individuals outside of NGS. Thanks go to Michael Morelli, James Perry, Cherie Shepherd and Stephen Lorennzetti at the National Park Service for all of the work done to bring our two agencies together in this endeavor. Special thanks go to Nancy Carretta of Hill International, Steven Monroe of Grunley Construction and Robert Collie of Perini Management Services. Those three worked on site with us at every visit and were a source of incredible information about the work going on around the monument. Finally, thanks to Dave Doyle, who was able to provide various details about the 1999 survey.
This paper is dedicated to the memory of my good friend and colleague, Mr. Mark Eckl, who passed Aug. 20, 2014. Among his many duties and accomplishments at NGS, Mark was the project manager for the 2014 Washington Monument survey. Without his leadership, this survey wouldn’t have taken place. He will be sorely missed.