Long-distance chronometric leveling with a portable optical clock

We have measured the geopotential difference between two locations separated by $457\mathrm{k}\mathrm{m}$ by comparison of two optical lattice clocks via an interferometric fiber link, utilizing the gravitational redshift of the clock transition frequency. The ${}^{87}$Sr clocks have been compared side-by-side before and after one of the clocks was moved to the remote location. The chronometrically measured geopotential difference of $3918.1(2.6){\mathrm{m}}^2{\mathrm{s}}^{-2}$ agrees with an independent geodetic determination of $3915.88(0.30){\mathrm{m}}^2{\mathrm{s}}^{-2}$. The uncertainty of the chronometric geopotential difference is equivalent to an uncertainty of $27\mathrm{c}\mathrm{m}$ in height.