What is the difference between NAD 83 and the ITRF?
Much of the information below was based on a draft version of a paper relating NAD 83 to the ITRF written by Stephen Frakes of the National Geodetic Survey. NAD 83 was implemented in 1986 with an adjustment combining terrestrial data with space-based data such as 3-D positions from satellite Doppler and 3D vectors from VLBI. The global parameters used in NAD83, X,Y,Z shifts and X, Y rotations plus scale were fixed to a priori values based on Doppler. The Z rotation was defined by astronomical azimuths. VLBI rotations and scale were solved as a means to compare systematic differences between the VLBI and NAD 83 coordinate systems. The NAD 83 adjustment did not include GPS. Positions determined in this adjustment are labeled NAD 83 (1986). The adoption of GPS revealed inadequacies in the system which prompted upgrade surveys (described below). If your question is how the US National Geodetic Survey Continuously Operating Reference Stations (CORS) station positions indicated as “NAD 83 Pr
