Behind a chain link fence on Montana Street in Pittsburgh's Perry North neighborhood lies a brick maintenance building, a looming radio tower and a collection of discarded satellites.
Trees surround the hill, their branches occasionally giving way to a glimpse of the U.S. Steel Tower and Highmark building. The scene holds the quiet distinction as the city's highest point of elevation: 1,371 feet above sea level. By contrast, at 841 feet (64 stories), the U.S. Steel Tower is Pittsburgh's tallest man-made structure.
Altitude influences where Good Question! listener Adam Longwill sets up his camera to capture the city. He said he prefers to shoot the skyline in unconventional locations, and assumed the highest elevation might be near a radio tower.
“But what’s the lowest spot a person can go in Pittsburgh?” he asked.
Do you know where this is? The altitude sticker is a little off, but this is a significant spot. Tune in soon to hear about it for one of our upcoming @905wesa Good Question! features. pic.twitter.com/9p5UQIY3vs
— Katie Blackley (@kate_blackley) April 11, 2018
Within Pittsburgh’s city limits, Longwill wanted to identify two sites: the lowest man-made and lowest natural locations.
Elevation maps are drawn using light, detection and ranging, or LIDAR. Scott Hoffman, GIS specialist with the U.S. Geological Survey, said the elevation is gathered by strapping tiny sensors to an aircraft to survey the land below. The sensors filter out non-ground points, including bridges, light poles and buildings, to create a digital model.
“It collects highly accurate elevation data,” Hoffman said. “Within an elevation accuracy of 18.5 centimeters.”
Follow the water
Inside the Allegheny County Sanitary Authority (ALCOSAN) lies the lowest man-made point in the city. According to ALCOSAN Director of Operations and Management Douglas Jackson, the wells that pump wastewater are 112 feet below the surface of the facility, making it less than 600 feet above sea level.
The lowest natural point in Pittsburgh is more hypothetical. Away from commercial development and down the Ohio River at the very edge of the city, Jack’s Run flows into the Ohio River in Brighton Heights. Charlie Jones, senior lecturer with the University of Pittsburgh Department of Geology and Environmental Science, said locating the nadir was relatively easy, but came with a engineering caveat.
“Water always flows downhill,” Jones said. “But the lowest natural point is now flooded by the dams.”
Before the U.S. Army Corps of Engineers built the region’s lock and dam system, Jones said water levels would have fluctuated based on topography.
The lowest natural point would have been at the edge of the city limits, where the Ohio River flows southwest, because water would have been entering it from the higher elevations of the Allegheny and Monongahela rivers. Now, the water level is kept at 710 feet above sea level for navigational purposes.
“You’d have to break the dams in order to get to making it a lower point than anywhere upstream,” Jones said.
Evolving geological history
While it may be hard to imagine, Jones said much of Pittsburgh was once about as high as the tallest spot on Montana Street, peaking around 1,300 feet, but streams have cut up the landscape, forming hills that collected into large rivers.
“The highest elevation areas are leftovers of the originally relatively flat surface,” Jones said. “They’re kind of erosional remnants.”
The timeline of Pittsburgh’s topographical changes is illustrated on a shaded relief map, Jones said, because it indicates where rivers and streams once cut through neighborhoods and where new ones are beginning to form. The deepest stretches – the Mon, Allegheny and Ohio rivers – are shown in dark green, while the smaller, feeder streams, or dendritic drainages, are lighter depending on their terrain.
Pittsburgh’s topography will continue to change, as riverbeds erode and landslides plague city hills, but for now, the highest and lowest points in the city illustrate the region’s history well. And it doesn’t take long to visit them; as the two extremes are about two miles apart.