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We improve the identification and isolation of individual stellar populationsin the Galactic halo based on an updated set of empirically calibrated stellarisochrones in the Sloan Digital Sky Survey (SDSS) and Pan-STARRS 1 (PS1)photometric systems. Along the Galactic prime meridian ($l=0^{\circ}$ and$180^{\circ}$), where proper motions and parallaxes from Gaia DR2 can be usedto compute rotational velocities of stars in the rest frame of the Milky Way,we use the observed double color-magnitude sequences of stars having largetransverse motions, which are attributed to groups of stars in the metal-poorhalo and the thick disk with halo-like kinematics, respectively. The Gaiasequences directly constrain color-magnitude relations of model colors, andhelp to improve our previous calibration using Galactic star clusters. Based onthese updated sets of stellar isochrones, we confirm earlier results on thepresence of distinct groups of stars in the metallicity versusrotational-velocity plane, and find that the distribution of the mostmetal-poor ([Fe/H] $<-2$) stars in our sample can be modeled using two separategroups on prograde and retrograde orbits, respectively. At $4$-$6$ kpc from theGalactic plane, we find approximately equal proportions of the Splashed Disk,and the metal-rich ($\langle {\rm [Fe/H]} \rangle\sim-1.6$) and metal-poor($\langle {\rm [Fe/H]} \rangle\sim-2.5$) halos on prograde orbits. TheGaia-Sausage-Enceladus, the metal-weak thick disk, and the retrograde halostructure(s) ($\langle {\rm [Fe/H]} \rangle\sim-2.2$) constitute approximately$10\%$ of the rest of the stellar populations at these distances.

A Blueprint for the Milky Way's Stellar Populations. II. Improved Isochrone Calibration in the SDSS and Pan-STARRS Photometric Systems