Geo-Joint: Delaware’s Twelve-Mile Circle

The Geo-Joint brings up the subject of borders every once in a while because they’re so basic to how humans define their place on earth. We solidify our styles, foods, cultures, and even our gene pools according to borders that define “us” and “them.” So as artificial as they often are, their importance can’t be denied. We’ve also talked about how borders get to be the shapes they are, with wiggly, bendy ones usually following natural features like rivers or mountain ridges, and straight ones resulting from the feats of kings and Congress, and the neat pens of cartographers. One shape you don’t often see, however, is a continuously curved line. Specifically, the curved line of a circle, or part thereof.
Americans, though, especially if they are from the East Coast or have an interest in state geography, may have a familiarity with a borderline just as round and even as a full moon. It’s not a full circle, and in reality it’s not part of a perfect circle, but on a map it sure looks like it. That border sits atop the state of Delaware, where it butts its northern end against Pennsylvania, and sits aside Maryland. Was this just a capricious doodle by some mapmaker weary of the straight lines that define more than half of those three states’ borders? No, it was the decision of the Duke of York in 1682, when he awarded Delaware to William Penn (who already owned Pennsylvania). The Duke chose to define the northern border as the arc formed by a circle with a twelve-mile radius, the center of which was the courthouse in the town of New Castle, on the Delaware River. The situation is referred to as the Twelve-Mile Circle, but of course it’s the radius that’s twelve miles long. It seemed a neat definition, no doubt, but complications of the design led to decades of legal conflict between landowners at first, and then between the states themselves.

The circle adds a subtle curve to Delaware’s western border, exaggerated here.

The disputes arose from all the other lines that were being surveyed to define other parts of these young states. Remember, this was far less settled territory in those days, and survey methods and equipment had their limitations. People who lived in these parts had a working but non-technically-defined knowledge of the lay of the land, and the wealthy owners had far less of an idea what was really out there. The southern border of Pennsylvania, for instance, was originally defined as the 40th parallel, and was thought to intersect this Twelve-Mile Circle, but didn’t. Another gaffe was that Penn had chosen Philadelphia as the intended capital of Pennsylvania, but it lies south of the 40th parallel. Much needed to be redefined as these inconsistencies came to light. After what must have been contentious negotiations, Pennsylvania’s southern border was lowered to 39 degrees, 49 minutes North, and all lands within the Twelve-Mile Circle were to go to Delaware. There are a lot of geometric details involved, but suffice it to say that this proviso meant that the north-south border between Delaware and Maryland isn’t as straight as it seems on a map of the whole US. A bit of the circle makes a swipe at that border, and so it actually has about a one-mile section defined by that arc.

New Castle, Delaware, the city that anchored the arc.

The Duke of York further muddied the waters, so to speak, with another unusual decision. Where the Twelve-Mile Circle crossed the Delaware River, as it did in two places, Delaware would own all the water and islands to the far shore, not merely to the middle of the river, as was standard practice. Disputes with New Jersey about that proviso were in court, the Supreme Court in fact, just ten years ago. The border definition stands. The arc-shaped border caused more consternation on the opposite side of the state, where Delaware shares its western edge with Maryland. Straight and curved borders can meet at low angles, which ends up creating some very thin, pointed slivers of land. Through the complicated surveying of Delaware, Pennsylvania, and Maryland, a seemingly unclaimed shard of territory materialized, and though it amounted to just over a square mile, everybody wanted it. Known as The Wedge, lawyers argued over each state’s claim to it, and for many years it was something of an ungoverned margin, a haven for freewheeling vice. It took til 1889 for Delaware to win its legal case and emerge as the rightful owner, but it wasn’t until 1921 that Congress and the State of Delaware tied up all the official loose ends.

New Castle County Courthouse, whose cupola was the center for some of the surveys.

So that’s the simplified story of Delaware’s compass-curve border, but here’s one more crazy complication: the border has been surveyed multiple times, and for some strange reason, different circle centers have been used. Some spot in New Castle other than the courthouse cupola got used as the starting point for various survey executions. That, of course, delivered curves of different paths in different surveys. Apparently over time those various curves were all averaged together to settle on the border as it exists today, although with modern equipment the line could probably be drawn with millimeter accuracy. But introducing that new data would likely start a whole raft of new border wars, so it’s best left alone at this point. It’s good to remember, after hearing of all the complications that arose from this circular border, that even the most laser-straight border of Wyoming or New Mexico or Tennessee is in reality a curved line too, because they’re all drawn on the great big spherical shape of the earth’s surface. And if you look at large-scale US maps like USGS topo quads, you’ll see that even along those seemingly straight borders are tiny zigs and zags caused by surveyors’ errors or obstacles that got in the way of a transit line. Borders—so crucial, and yet so inexact.


Feel like a trip to Delaware to put your feet on the Twelve-Mile Circle? Get’s wall map of Delaware to start your planning! Click here:



caption: The circle adds a subtle curve to Delaware’s western border, exaggerated here.
source: Wikimedia Commons: Lasunncty at English Wikipedia (CC by SA 3.0)


caption: New Castle, Delaware, the city that anchored the arc.
source: Flickr: Ken Lund (CC by SA 2.0)
caption: New Castle County Courthouse, whose cupola was the center for some of the surveys.
source: Wikimedia Commons: Pknelson (CC by SA 3.0)

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