  Several Ising-type magnetic van der Waals (vdW) materials exhibit stable
magnetic ground states. Despite these clear experimental demonstrations, a
complete theoretical and microscopic understanding of their magnetic anisotropy
is still lacking. In particular, the validity limit of identifying their
one-dimensional (1-D) Ising nature has remained uninvestigated in a
quantitative way. Here we performed the complete mapping of magnetic anisotropy
for a prototypical Ising vdW magnet FePS$_3$ for the first time. Combining
torque magnetometry measurements with their magnetostatic model analysis and
the relativistic density functional total energy calculations, we successfully
constructed the three-dimensional (3-D) mappings of the magnetic anisotropy in
terms of magnetic torque and energy. The results not only quantitatively
confirm that the easy axis is perpendicular to the $ab$ plane, but also reveal
the anisotropies within the $ab$, $ac$, and $bc$ planes. Our approach can be
applied to the detailed quantitative study of magnetism in vdW materials.
