The 175 meter error is of the right magnitude to be caused
by improper matching of UTM "spheriods". The DLG data was
likely produced referenced to the "clark66" (NAD27) spheroid,
and your GPS may be using a more modern spheroid.
You could use m.ll2u and m.u2ll to convert your GPS
control points to the different datum.
Jim Hinthorne
GIS Lab
GIS Lab