We present the simultaneous optical and thermal analysis of a freestanding photonic semiconductor membrane made from wurtzite III-nitride material. By linking microphotoluminescence spectroscopy with Raman thermometry and other spectroscopic techniques, we demonstrate how a robust value for the thermal conductivity K can be obtained using only optical noninvasive means. For this, we consider the balance of different contributions to thermal transport given by, e.g., excitons, charge carriers, and heat-carrying phonons. In principle, all these contributions can be of relevance in a photonic membrane on different length scales. Further complication is given by the fact that this membrane is made from direct band gap semiconductors, designed to emit light via an InxGa1-xN (x = 0.15) quantum well embedded in GaN. Thus, III-nitride membranes similar to the one of this study have already been successfully used for laser diode structures facing thermal limitations. To meet these intricate challenges, we designed an experimental setup that enables the necessary optical and thermal characterizations in parallel. After the optical characterization by microphotoluminescence, we follow a careful step-by-step approach to quantify the thermal properties of our photonic membrane. Therefore, we perform steady-state micro-Raman thermometry, either based on a heating laser that also acts as a probe laser (one-laser Raman thermometry), or based on two lasers, providing the heating and the temperature probe separately (two-laser Raman thermometry). For the latter technique, we can obtain temperature maps over several hundreds of square micrometers with a spatial resolution less than 1 mu m. As a result, the temperature probe volume using the two-laser Raman thermometry technique can be increased by a factor exceeding 100 compared with the conventional one-laser Raman thermometry technique, which impacts the derivation of the thermal conductivity K. Only based on our largest temperature probe volume we derive K =95+11-7 W m-1 K-1 for the c plane of our approximate to 250-nm-thick photonic membrane near room temperature, which compares well to our ab initio calculations, applied to a simplified structure, yielding K = 136 W m-1 K-1. Based on these calculations, we explain the particular importance of the temperature probe volume, as quasiballistic transport of heat-carrying phonons, which is of high relevance for determining K, occurs on length scales beyond the penetration depths of the heating laser and even its focus spot radius. The one-laser Raman thermometry technique, therefore, fails to derive realistic K values, unlike the two-laser Raman thermometry that can probe temperatures over sufficiently large volumes. The present work represents a significant step towards the achievement of noninvasive, highly spatially resolved, quantitative thermometry maps on a photonic membrane made of a direct band gap semiconductor, which is of particular relevance for photonic applications.