  With the increasing numbers of large stellar survey projects, the quality and
quantity of excellent tracers to study the Milky Way is rapidly growing, one of
which is the classical Cepheids. Classical Cepheids are high precision standard
candles with very low typical uncertainties ($<$ 3%) available via the
mid-infrared period-luminosity relation. About 3500 classical Cepheids
identified from OGLE, ASAS-SN, Gaia, WISE and ZTF survey data have been
analyzed in this work, and their spatial distributions show a clear signature
of Galactic warp. Two kinematical methods are adopted to measure the Galactic
rotation curve in the Galactocentric distance range of $4\lesssim R_{\rm GC}
\lesssim 19$ kpc. Gently declining rotation curves are derived by both the
proper motion (PM) method and 3-dimensional velocity vector (3DV) method. The
largest sample of classical Cepheids with most accurate 6D phase-space
coordinates available to date are modeled in the 3DV method, and the resulting
rotation curve is found to decline at the relatively smaller gradient of
($-1.33\pm0.1$) ${\rm km\,s^{-1}\,kpc^{-1}}$. Comparing to results from the PM
method, a higher rotation velocity (($232.5\pm0.83$) ${\rm km\,s^{-1}}$) is
derived at the position of Sun in the 3DV method. The virial mass and local
dark matter density are estimated from the 3DV method which is the more
reliable method, $M_{\rm vir} = (0.822\pm0.052)\times 10^{12}\,M_\odot$ and
$\rho_{\rm DM,\odot} = 0.33\pm0.03$ GeV ${\rm cm^{-3}}$, respectively.
