This 23min session by Grand Master Dr. Stephen Skinner traces the connection between the trigrams and hexagrams of the I Ching/Yijing and the development of the Feng Shui luopan.
Originally, the 8 trigrams were simply markers for the 8 directions. The earliest rings were mainly astronomical such as the 28 Moon Mansions. It was not until 1019 CE that the 24 Mountains were connected to just 60 hexagrams on the San He luopan. Shao Yong, a Song dynasty Chinese philosopher, was interested in cycles of time yuan元 (Cycles) and generated circular arrangements of the hexagrams.
This was later used on the luopan as a circular arrangement of 60 hexagrams matched with the 60 jiazi. In 1827 the number of hexagrams on the luopan was increased from 60 to 64, and so San Yuan Feng Shui was born.