The Transpeninsular Line (at approximately 38°27′ N) is a surveyed line, the eastern half of which forms the north–south border between Delaware and Maryland. The border turns roughly north from the midpoint of the line towards the Twelve-Mile Circle, which forms much of the remainder of the Delaware border.
In 1751, a line was surveyed straight across the Delmarva Peninsula beginning at what at least some early Swedish settlers called Cape Hinlopen, which was to be the southern boundary of Delaware. This place is better known as Fenwick Island. Twenty-four miles north is another cape named Cape Henlopen near Lewes, Delaware. Various spellings of henlopen mean "entering in" or "approaching." The confusion of the names was the crux of a long standing dispute between the Penns (Delaware) and the Calverts (Maryland), the latter claiming that the Lewes' cape should have been the start of the boundary line. Ironically, it was a map commissioned by Charles Calvert in 1732, which showed Cape Hinlopen at Fenwick Island, that was used to decide the matter. Calvert had intended the Lewes' cape to be so named, but he only discovered the mistake after he had submitted it to the English court deciding the case. He failed in his later attempts to have the court reject his own map. If the actual Cape Henlopen near Lewes had been used as the start of the line, Delaware would be about one thousand square miles smaller, over a third of its current area.
The line was accepted by Charles Mason and Jeremiah Dixon (of Mason–Dixon line fame) in 1763 when they were engaged to survey the borders between Maryland and the grants to William Penn (Pennsylvania and Delaware).
A Transpeninsular Line marker can be found near the Fenwick Island Lighthouse, at the northern boundary of Ocean City, Maryland, located on 146th Street. The midpoint Transpeninsular Line marker can be found on the north side of Route 54 about halfway between Delmar and Mardela Springs, Maryland. This marker is also the southern endpoint of the Mason–Dixon Line.
- "Transpeninsular Line". Historical Marker Database. November 29, 2007. Retrieved December 28, 2007.