Do your homework, don't let others tell you. All Curvature formulas here, to do your own tests => Earth's Curvature (charts & formulas for your tests available HERE)

The Grand Canal of China (also known as the Beijing-Hangzhou Grand Canal) is the longest canal or artificial river in the world. It connects the Yellow River and Yangtze River. The oldest parts of the canal date back to the 5th century BC, although the various sections were finally combined during the Sui dynasty (581–618 AD). The total length of the Grand Canal is 1,776 km (1,104 mi). At a time when Chinese knew not of the curvature of the Earth, they built a 1104 Miles canal, not accounting for Earth's curvature.