  Measurements of the resistivity anisotropy can provide crucial information
about the electronic structure and scattering processes in anisotropic and
low-dimensional materials, but quantitative measurements by conventional means
often suffer very significant systematic errors. Here we describe a novel
approach to measuring the resistivity anisotropy of orthorhombic materials,
using a single crystal and a single measurement, that is derived from a
$\frac{\pi}{4}$ rotation of the measurement frame relative to the
crystallographic axes. In this new basis the transverse resistivity gives a
direct measurement of the resistivity anisotropy, which combined with the
longitudinal resistivity also gives the in-plane elements of the conventional
resistivity tensor via a 5-point contact geometry. This is demonstrated through
application to the charge-density wave compound ErTe$_3$, and it is concluded
that this method presents a significant improvement on existing techniques in
many scenarios.
