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This paper describes the calibration of the Volcano ray-tracing engine against channel measurements collected in an urban environment with a state-of-the-art 28-GHz directional channel sounder. A discrete set of rays, representing planar wavefronts propagating between the transmitter and receiver, were extracted from the measurements and characterized in path gain, delay, and 3D angle-of-arrival through super-resolution techniques, with average errors of only 1.2 dB, 0.55 ns, and 2.05° respectively. The extracted rays were then tracked over space as the receiver, mounted on a mobile rover equipped with military-grade GPS, traversed 66 m while amassing a total of 488 channel acquisitions. The tracked rays were then mapped to rays predicted from ray-tracing, originating through specular reflection or diffuse scattering from ambient objects. The mapping enabled object-specific calibration, namely calibrating distinct diffuse-scattering models for buildings, vehicles, and foliage. To our knowledge, this is the first effort to calibrate ray-tracing with object-specific diffuse scattering models against rays individually mapped in the path gain, delay, angle, and space domains. Results, in terms of calibrated ray-tracing parameters, fit-error statistics, and lessons learned, are included. Our chief finding was that, while most papers on millimeter-wave ray-tracing do not even consider diffuse scattering, it accounted for 20% of the total received power, whereas diffraction accounted for less than 1%. [ABSTRACT FROM AUTHOR]