I am bit confused about datums used to define a localised coordinate system for a specific area. Definitions say that, datum defines how a certain well-established point (i.e., reference point) on earth is located on a reference ellipsoid used to model that area. So, it establishes latitude and longitude of that reference point on earth with GPS or other means. My question now is, with respect to which reference frame are these latitude and longitude measured? is it always wrt earth-centric WGS84 or ITRS? So, when a textbook says, the coordinate of a NAD27 reference point is 39°13′26.686″ north latitude, 98°32′30.506″ west longitude, is it measuring these values from Equator and Greenwich respectively on physical earth surface or on a reference ellipsoid? How coordinates of this point was established before GPS was in place?
On the other hand, when GPS is used to establish the coordinate of a reference point, isn't it always measuring the lat. and long. on the ellipsoid corresponding to WGS84?
I haven't found any textbooks that explain it in a simple way.