While James does raise a number of valid points I will take issue with the listing of variables being separate from tire pressure. True, these issues will all have an effect on tire diameter; however, most of them do so by resulting in a raising or lowering of the internal pressure of the tire ... primarily those which result in temperature change will directly result in pressure change.
The concern expressed earlier about air loss when taking pressure readings was one that I shared until I witnessed Neville's system in action. The pressure guage he uses does not result in any air loss and a magnifying glass showed no difference in readings after multiple readings.