C4Gnet.XYZ is constrained to the National Spatial Reference Framework (NSRS) of the National Geodetic Survey (NGS). Currently, this is NAD83-2011 Epoch 2010.00. Vertically, C4Gnet.XYZ is constrained to the NSRS, and results are expressed in ellipsoid heights. Orthometric elevations, like NGVD88 are derived in your field or office software by applying a geoid model, For instance, GEOID 2018 from the NGS. We recommend using whatever the latest geoid model is from the NGS. You can obtain the geoid files from the NGS or your GNSS rover vendor. The C4G website also has some GEOID Tools that you might find helpful.