Here is another way to locate the start line. Here is the screencap from the promo......with the benchmark circled.
Here is close-up of the benchmark....
And here is a map of the location of that particular benchmark. The locations are recorded by the National Geodetic Survey. There are approximately 750,000 benchmarks across the United States. They are very precisely surveyed points. The information they contain is latitude, longitude and elevation above sea level.
Thanks to Texas Lady for Images 1 and 3, and to Chateau for Image 2.