Superionic lithium -ion conductors of NASICON structure are promising solid electrolytes for all solid-state batteries. But still further improvement of the ionic conductivity is necessary to be competitive with today's liquid electrolytes. This requires a thorough understanding of grain and grain boundary ion transport properties. However, distinguishing between the impedance contributions of both regimes proved to be difficult before, due to their overlapping time constants, which often necessitate measurements below 0 degrees C. In contrast, we analyze a Li1.3Al0.3Ti1.7(FO4)(3) (LATP) solid electrolyte under battery operation temperatures between 10 degrees C and 50 degrees C by impedance measurements in combination with a distribution of relaxation time analysis in two dimensions (2D-DRT). By correlation with microstructural observation in the laser-scanning microscope (LSM), scanning electron microscope (SEM) and atomic force microscope (ARV!) the dominating ion transport pathway is determined within a bricklayer model on a macroscopic scale. Moreover, the ionic conductivities of grain and grain boundary are calculated. For the grain, conductivity values of 2 mS cm(-1) at room temperature are found. The ion transport activation energies of both domains are determined to be 182 meV and 430 meV, respectively. Optimization routes for further ionic conductivity improvements are derived.