The NAD 83( CORS 96) and NAD 83(NSRS2007) realizations of the North American Datum of 1983
NGS has adopted a realization of NAD 83 called NAD 83(NSRS2007) for the distribution of coordinates at ~70,000 passive geodetic control monuments. This realization approximates (but is not, and can never be, equivalent to) the more rigorously defined NAD 83( CORS 96) realization in which Continuously Operating Reference Station ( CORS ) coordinates are distributed. NAD 83(NSRS2007) was created by adjusting GPS data collected during various campaign-style geodetic surveys performed between the mid-1980's and 2005. For this adjustment, NAD 83( CORS 96) positional coordinates for ~700 CORS were held fixed (predominantly at the 2002.0 epoch for the stable north American plate, but 2007.0 in Alaska and western CONUS) to obtain consistent positional coordinates for the ~70,000 passive marks, as described by Vorhauer [2007]. Derived NAD 83(NSRS2007) positional coordinates should be consistent with corresponding NAD 83( CORS 96) positional coordinates to within the accuracy of the GPS data used in the adjustment and the accuracy of the corrections applied to these data for systematic errors, such as refraction. In particular, there were no corrections made to the observations for vertical crustal motion when converting from the epoch of the GPS survey into the epoch of the adjustment, while the NAD 83 ( CORS 96) coordinates do reflect motion in all three directions at CORS sites. For this reason alone, there can never be total equivalency between NAD 83(NSRS2007) and NAD 83( CORS 96).
Note: NGS has not computed NAD 83(NSRS2007) velocities for any of the ~70,000 passive marks involved in this adjustment. Also, the positional coordinates of a passive mark will make reference to an “epoch date”. Epoch dates are the date for which the positional coordinates were adjusted, and are therefore considered “valid” (within the tolerance of not applying vertical crustal motion). Because a mark's positional coordinates will change due to the dynamic nature of the earth's crust, the coordinate of a mark on epochs different than the listed “epoch date” can only be accurately known if a 3-dimensional velocity has been computed and applied to that mark.