The technology is not in the gnomon itself but in the method used to measure the length of the shadow.
The earliest measurements of the shadow's length were no doubt made with the foot-rules of the time, but as it was realised that these varied according to bureaucratic prescription and local custom, a standard jade tablet (thu kuei), which may be called the Gnomon Shadow Template, was made for this purpose only. It is mentioned in the Chou Li, and actual specimens, made of terra-cotta, one dated +164 are extant.
The moment of the solstice could thus be determined by placing the calibrated template at the base of the post due north for several days around the expected time, and taking noon of the day when the shadow most nearly coincided with it.
The purpose of the gnomon was to determine the day the solstice occurred. A template was adopted because of the inconsistency of other methods of measurement.
The template system was an attempt to overcome the chaos of primitive metrology, and did not persist...In +500 Tsu Keng-Chih made bronze instruments in which the gnomon and a horizontal measuring scale were combined. About fifty years earlier Ho Cheng-Thien had proceeded to more careful observations of the winter solstice shadow.
So there was an improvement in Chinese technology around the time of the 6th century and that is apparent in the consistency of the measurements after then. At that point the variation from the calculated obliquity varies by amounts of less than 1/10º (.083º). With an 8 foot gnomon that is a difference in shadow length of less than 1/5". A difference in timing of a few minutes and/or leveling errors can easily account for such a small difference.
Source
Yes, Lockyer got it wildly wrong. In the case of Karnak he was off by at least 1500 years and for Stonehenge about 800 years.
[edit on 7/30/2010 by Phage]
