Some Feng Shui schools brag about the authenticity of their teaching by examining ancient temples. According to their theory, the temples should face the so-called Xiao Kong Wang or Da Kong Wang. They use their luopans to verify their findings.
Well, if a temple was built several hundred years ago, the luopan reading today can be different from the reading when it was built by a few degrees. What is facing Kong Wang today probably did not face Kong Wang when it was built. This simply shows that their great grand masters did not practice the same kind of Feng Shui as they are now practicing.
If we assume that Yang Yun Song invented the Tian Pan according to the astronomical north, then the angle of declination at that time in Ganzhou where he researched Feng Shui theories should have been 7.5 degrees. Today, the angle of declination is about 4 degrees. Well, well!