Reprojecting points in NAD 1983 to LLA (WGS84)?

Question

I've read documentation about using NAD_1983_To_WGS_1984_1 for the entire North American continent or NAD_1983_To_WGS_1984_5 for the 48 contiguous United States but what I'm confused is how can I use these when my data is in IL plane because 0,0,0 is the corner of IL whereas conventional NAD83/WGS84's 0,0,0 is the equator?

For example, if I have a point 1012111, 1276111 in the IL plane how do I turn it to LLA?

Data source:

PROJCS["NAD_1983_HARN_StatePlane_Illinois_East_FIPS_1201",
    GEOGCS["GCS_North_American_1983_HARN",
        DATUM["NAD83_High_Accuracy_Regional_Network",
            SPHEROID["GRS_1980",6378137,298.257222101]],
        PRIMEM["Greenwich",0],
        UNIT["Degree",0.017453292519943295]],
    PROJECTION["Transverse_Mercator"],
    PARAMETER["False_Easting",300000],
    PARAMETER["False_Northing",0],
    PARAMETER["Central_Meridian",-88.33333333333333],
    PARAMETER["Scale_Factor",0.999975],
    PARAMETER["Latitude_Of_Origin",36.66666666666666],
    UNIT["Meter",1],
    AUTHORITY["EPSG","102271"]]

Answer 1

You might look into http://desktop.arcgis.com/en/arcmap/latest/map/projections/choosing-an-appropriate-transformation.htm

Conversions between NAD83 and WGS84 are always done on lat-long coordinates.

So you have to convert your Illinois coordinates from transverse mercator to NAD83 lat-long with the parameters you have got, then use the transformation from NAD83 to WGS84.

NAD_1983_To_WGS_1984_1 is very simple, because all shift values are set to zero.