More Accurate Elevation Data for Users of Geospatial Data...
http://www.esri.com/news/arcnews/winter0405articles/more-accurate.html
Height Modernization: A Short Q and A
Stations with GPS-derived orthometric heights determined in height modernization projects will now have a statement at the top of their NGS data sheet that states:
HT_MOD - This is a Height Modernization Survey Station