  In a first of a series of studies of the H-alpha + [NII] emission from nearby
spiral galaxies, we present measurements of H-alpha + [NII] emission from HII
regions in M81. Our method uses large-field-CCD images and long-slit spectra,
and is part of the ongoing Beijing-Arizona-Taipei-Connecticut Sky Survey. The
CCD images are taken with the NAOC 0.6/0.9m f/3 Schmidt telescope at the
Xinglong Observing Station, using a multicolor filter set. Spectra of 10 of the
brightest HII regions are obtained using the NAOC 2.16m telescope with a Tek
1024 X 1024 CCD. The continua of the spectra are calibrated by flux-calibrated
images taken from the Schmidt observations. We determine the continuum
component of our H-alpha + [NII] image via interpolation from the more
accurately-measured backgrounds (M81 starlight) obtained from the two
neighboring (in wavelength) BATC filter images. We use the calibrated fluxes of
H-alpha + [NII] emission from the spectra to normalize this interpolated,
continuum-subtracted H-alpha + [NII] image. We estimate the zero point
uncertainty of the measured H-alpha + [NII] emission flux to be $\sim$ 8%. A
catalogue of H-alpha + [NII] fluxes for 456 HII regions is provided, with those
fluxes are on a more consistent linear scale than previously available. The
logarithmically-binned H-alpha + [NII] luminosity function of HII regions is
found to have slope $\alpha$ = -0.70, consistent with previous results (which
allowed $\alpha=-0.5 \sim -0.8$). From the overall H-alpha + [NII] luminosity
of the HII regions, the star formation rate of M81 is found to be $\sim 0.68
M_{\odot} {\rm yr}^{-1}$, modulo uncertainty with extinction corrections.
