Understanding NAD83 datum information on QGIS - State Plane System

Question

I'm trying to make sense of the CRS definition in QGIS 3.4 for State Plane projections.  I believe the datum should be NAD83, but I'm not seeing this in the definition that shows up in QGIS, and I want to find out 1)is this correct, and if so 2) why is the GIS able to use this definition?
For example, for NAD 83 / Massachusetts State Plane Maineland, here is what shows up:
Extent: -73.50, 41.46, -69.86, 42.89
Proj4: +proj=lcc +lat_1=42.68333333333333 +lat_2=41.71666666666667 +lat_0=41 +lon_0=-71.5 +x_0=200000 +y_0=750000 +ellps=GRS80 +towgs84=0,0,0,0,0,0,0 +units=m +no_defs

So GRS80 is specified as the ellipsoid, but I don't see anything about the datum.  Is the ellipsoid enough by itself?  What's going on here?

FSimardGIS · Accepted Answer

In order to be well-defined, a datum needs two things:

A reference surface (commonly a spheroid, also called an ellipsoid
of revolution) with specific parameters for its size and flattening
An attachment to the physical Earth (usually though reference points on the ground and azimuths, or with a link to a geocentric reference frame)

In the case of NAD83, its reference spheroid is GRS80, and since it is sometimes considered equivalent to WGS84 (depending on the application, a 1-2 meter difference could be considered acceptable), many applications will, by default, set the transformation to zero (in other words, no datum transformation is performed when converting between NAD83 and WGS84).
If we look at the proj string, you can see those two elements, hence NAD83 is well-defined (at least within the accuracy mentioned earlier) The ellipsoid is mentioned: +ellps=GRS80 and a link to WGS84 is also present: +towgs84=0,0,0,0,0,0,0. In this case, as you can see, the transformation is set to zero.
Another way to write this with a proj string, is to use the parameter +datum=NAD83, which explicitly states the datum in use, and in most cases has the same effect as using +ellps=GRS80 +towgs84=0,0,0,0,0,0,0
If your data requires higher accuracy, then it becomes important to know which version of NAD83 is used in order to choose the best transformation to other datums. However, if you are only dealing with data in NAD83, everything should overlay just fine.

Understanding NAD83 datum information on QGIS - State Plane System

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