Its my understanding that Chesterfield County was part of the Florence/Myrtle Beach DMA until the late 80s or early 90s. WBTW was the most watched tv station in that area. I was told Nielson switched Chesterfield County to the Charlotte market because WPDE didn't draw enough viewers to keep that area in the DMA. (some type of Nielson rule states that at least two tv stations in the same market need to have enough viewers to keep the county in their DMA) Back then, WBTV and WSOC were influential.
I worked at WBTW in the early 90s and late 90s during the days of Spartan Communications. What a powerful tv station. Our ratings were enormous. Even though Chesterfield County was technically in the Charlotte market, we still covered news and weather for Cheraw, Pageland, etc... A lot of people preferred to watch The Big 13.