  The rotation curve is significant to research on dark matter and the
structure of the Milky Way. But it is a great challenge to measure rotation
curve accurately, and different ways obtain distinct results. In this work, we
use a DIY small radio telescope to carry out hydrogen 21-cm line observations
on campus, and calculate the rotation curve of inner disk by tangent method
based on the model of Przemek et al. (the rotation speed of the Sun
$v(R_0)=233.6\pm2.8$ km/s, and distance to the Galactic center
$R_0=8.122\pm0.031$ kpc). Furthermore, we obtain the distribution of spiral
arms in the Milky Way with our data by adopting different rotation curve
models. We also compare our rotation curve result with previous works, analyse
the possible measurement error in the tangent method, and discuss the
consistence of our result with others.
