  We study the correlation heights, which indicate the formation height of
Extreme Ultraviolet (EUV) lines in an active region using observations from the
EUV Imaging Spectrometer (EIS) and Solar Optical Telescope (SOT) on board
\emph{Hinode}. The nonlinear force-free field (NLFFF) optimization method is
adopted to extrapolate the 3D magnetic fields to higher layers. Three
subregions with different characteristics are selected in the active region for
this study. The results show that the formation heights in different subregions
vary with their different magnetic fields or velocity patterns. After solving
the line blending problem between the He {\sc \romannumeral 2} 256.32 \AA and
Si {\sc \romannumeral 10} 256.37 \AA lines by the double Gaussian curve
fitting, we find that the transition region lies higher in a strong magnetic
area. In a pre-flare heating area there possibly exist multithermal loops as
implied by comparing the Doppler velocity and the magnetic field on the solar
disk.
