  The advent of novel measurement instrumentation can lead to paradigm shifts
in scientific research. Optical atomic clocks, due to their unprecedented
stability and uncertainty, are already being used to test physical theories and
herald a revision of the International System of units (SI). However, to unlock
their potential for cross-disciplinary applications such as relativistic
geodesy, a major challenge remains. This is their transformation from highly
specialized instruments restricted to national metrology laboratories into
flexible devices deployable in different locations. Here we report the first
field measurement campaign performed with a ubiquitously applicable $^{87}$Sr
optical lattice clock. We use it to determine the gravity potential difference
between the middle of a mountain and a location 90 km apart, exploiting both
local and remote clock comparisons to eliminate potential clock errors. A local
comparison with a $^{171}$Yb lattice clock also serves as an important check on
the international consistency of independently developed optical clocks. This
campaign demonstrates the exciting prospects for transportable optical clocks.
