  Using a sample of over 70, 000 red clump (RC) stars with $5$-$10$% distance
accuracy selected from the LAMOST Spectroscopic Survey of the Galactic
Anti-center (LSS-GAC), we study the radial and vertical gradients of the
Galactic disk(s) mainly in the anti-center direction, covering a significant
disk volume of projected Galactocentric radius $7 \leq R_{\rm GC} \leq 14$ kpc
and height from the Galactic midplane $0 \leq |Z| \leq 3$ kpc. Our analysis
shows that both the radial and vertical metallicity gradients are negative
across much of the disk volume probed, and exhibit significant spatial
variations. Near the solar circle ($7 \leq R_{\rm GC} \leq 11.5$ kpc), the
radial gradient has a moderately steep, negative slope of $-0.08$ dex
kpc$^{-1}$ near the midplane ($|Z| < 0.1$ kpc), and the slope flattens with
increasing $|Z|$. In the outer disk ($11.5 < R_{\rm GC} \leq 14$ kpc), the
radial gradients have an essentially constant, much less steep slope of $-0.01$
dex kpc $^{-1}$ at all heights above the plane, suggesting that the outer disk
may have experienced an evolution path different from that of the inner disk.
The vertical gradients are found to flatten largely with increasing $R_{\rm
GC}$. However, the vertical gradient of the lower disk ($0 \leq |Z| \leq 1$
kpc) is found to flatten with $R_{\rm GC}$ quicker than that of the upper disk
($1 < |Z| \leq 3$ kpc). Our results should provide strong constraints on the
theory of disk formation and evolution, as well as the underlying physical
processes that shape the disk (e.g. gas flows, radial migration, internal and
external perturbations).
