Fort Sumter

More than 180 years after its construction, and 150 years after the famous shot that started the U.S. Civil War, Fort Sumter is once again at the forefront of a battle. This time around, the enemy is nature, and the loyal troops defending the fort aren’t depending on cannons and masonry walls. Instead, they’re relying on highly-sophisticated 3-D scanning and surveying technologies from Trimble and Duncan-Parnell, as well as decades of experience and extensive data processing. The “soldiers” in this battle are surveying professionals from Clemson University, and their mission is to help preserve one of America’s most significant historic landmarks. The methodology they have chosen for this effort may ultimately establish a new standard for monitoring sensitive historic and archeological sites.

Fort Sumter Parade Grounds

A storied history.
Sitting on a man-made island of granite and sea shells at the mouth of the Charleston, SC harbor, Fort Sumter originally had walls 8 to 12 feet thick and 50 feet high. On April 12, 1861, Confederate batteries fired on the fort from the surrounding shorelines. The next day, the Union garrison occupying the fort surrendered, and the U.S. Civil War was underway. Later, in August 1863, a Federal bombardment began that ultimately reduced the massive walls to rubble. By the time the war ended, Fort Sumter was in ruins.

In 1898 the Spanish-American War produced a renewed interest in the site, and a huge concrete battery was constructed in the center of the old fort, where it remains today. During World War I a small garrison manned the battery’s two twelve-inch rifles, and during World War II, two antiaircraft guns were installed, although by then the fort was primarily a tourist attraction. In 1948, Fort Sumter became a U.S. National Monument administered by the U.S. National Park Service.

Scanning the north wall

A national treasure threatened.
Because of its location and construction, Fort Sumter National Monument is subject to many natural destructive forces. The island is known to be slowly sinking, and the fort’s walls now sit barely above sea level during high tide. The fort’s northern flank faces the Port of Charleston’s main shipping channel, where frequent traffic generates moderate wave action. The tides and wind also work together to keep much of the exterior wall surface perpetually damp. For historians, archeologists and preservationists alike, these threats to Fort Sumter are serious matters.

To better understand the situation, in 2011 a multi-year project was launched with the goal of assessing the structural integrity of the fort, instituting a structural health monitoring program, and accurately measuring the fort in order to establish a benchmark for future comparisons. If measurements in subsequent years were to show that the fort is deteriorating too rapidly, additional preservation measures could be considered before it’s too late.

Scanning a gun casement

Measuring tasks for the project are being led by Clemson University’s Peter Messier, with support from a graduate student and three former students who are now licensed practitioners, and several of the university’s departments. Scanning and survey instruments are all products from Trimble Navigation, Ltd., while scanning assistance and comprehensive support is being provided by Duncan-Parnell.

Project scope.
From the start, the challenge was daunting: how to measure every aspect of a 180-year-old fort (on a man-made island in an active harbor), to determine whether it is moving from one year to the next, all with millimeter-level accuracy. In the summer of 2011, the project team developed an ambitious plan that would include:

• Establishing control and monitoring points
• Conducting numerous precise measurements
• High-resolution 3-D scanning of the entire fort
• 3-D modeling of six gun casements

Peter and his students would be responsible for establishing all control/monitoring points, conducting all measurements and many other details. Duncan-Parnell’s Dan Bonenfant would be responsible overseeing the scanning operation and processing data. And additional Clemson engineering resources would develop the 3-D model.

Benchmark for measuring vertical movement of the island and fort

Preparation work.
With a plan in place, the team started by locating SC State Plane Coordinates to determine an absolute position for Fort Sumter. Static GPS measurements were taken on two different days at different times to help ensure accuracy. Five primary control points were established, including three on the surrounding shorelines and two on the fort itself. These will provide a basis for monitoring long-term movement of the island, and allow the team to isolate the fort’s control points from the island.

In order to determine absolute elevation over time, a deep benchmark was established outside the fort’s walls by the South Carolina Geodetic Survey. A stainless steel rod was driven to refusal (approximately 70’), making it independent of any vertical movement of the fort or island. So even if the island exhibits sinking, the rod will remain at the same elevation.

Removable target on the fort's wall

In order to track any movement of the fort, 96 permanent monitoring points were established along the top of the walls and inside some of the gun casements. These will provide precise definition and stability for measuring the fort’s components over time. Stainless-steel threaded rods were permanently installed for the wall monitoring points. When measurements are conducted, custom-built targets are mounted on each rod. For protection, these rods are topped with stainless-steel acorn nuts when not in use.

Other preparation work included taking core samples from a handful of gun casements. By determining the exact make-up of the wall material, the team will be better able to predict movement once the 3-D model is completed in the months ahead.

The measuring process commences.
As summer drew to a close, baseline measurements of Fort Sumter began. The team started by running a 3-D traverse circuit along and very near the outside perimeter of the fort, and another through the fort’s interior. Moving forward, this would provide both horizontal and vertical control for terrestrial scanning stations, and tie all scanning into one unified coordinate system. Due to a desired accuracy tolerance of +/- 1 to 2 millimeters, this process was very time consuming to set up and measure.

Very difficult terrain to navigate and scan from

Next up was one of the more challenging aspects of the project: scanning the fort’s northern and northeastern exterior walls. These walls are particularly difficult to scan due to the large boulders at the base of the walls, and a 3 to 4 hour window of opportunity for scanning centered around low tide. At high tide, the area is inaccessible, and at all times, it is dangerous work. The team relied on rock climbing gear for protection, with spotting provided overhead from the top of the fort’s walls.

Trimble CX in action

A Trimble® CX™ 3D Scanner was used to scan sections of the walls at extremely high resolutions. During post-processing, these scans will be overlaid to create what amounts to a seamless image of the fort. Although lower resolution scans may have been suitable for 3-D modeling purposes, the higher-res scans will provide comprehensive data for future analyses. They also provide an astonishing amount of surface detail, down to tiny cracks in individual bricks. This same scanning approach was employed inside the fort as the gun casements were scanned.

With the approach of Thanksgiving and winter weather, the team shifted its focus to the remaining two primary measuring exercises for the project:

• Precise level circuit to the wall monitoring points
• Precise radial survey to the same points

Both of these measuring procedures were conducting using the industry’s most advanced robotic total station, the Trimble S8, and the Trimble DiNi® precise digital level with invar staff.

Trimble S8, with Sullivan's Island in the distance

The level circuit provides precise elevations for each of the monitoring points that were installed along the top of the fort’s walls. When compared to the results of future measurements, it will be possible to determine if the fort is experiencing any vertical structural deformation over time. The level circuit originated at, and closed back to, the deep benchmark buried earlier. It required multiple runs due to various constraints, as well as data reduction, analysis and computations.

The radial survey provides a method of analyzing any horizontal structural deformation of the walls over time. Precise horizontal positions (x, y) were determined based on back-sights to the three precise shore points located 1.1 to 1.3 miles away. Because of obstructions, two eccentric observation stations on the roof of the battery were required. Special procedures, data reduction, analysis and computations were also required.

Composite of several hi-res scans

The team presses on.
The Fort Sumter measuring and monitoring project is well underway, but there is still much to be done. As of this writing, the initial round of monitoring measurements have been taken, and a sizeable number of 3-D scans have been captured. Still ahead lie the remaining scans, data processing, development of the 3-D model, and more. Additional progress reports will be published in the future so please stay tuned.