The significance of system inertia, especially its nonuniform spatial distribution, is becoming paramount in the current power system scenario. This paper proposes a generalized two-level method to estimate the spatial inertia distribution, which is applicable for any given operating condition (or snapshot), as well as for ambient and transient power system conditions. The novelty of the proposed method lies in its mode-agnostic approach for estimating absolute-valued nodal inertias, which makes it more generalized than dominantmode-based methods, and more suitable for planning studies. Also, the proposed method does not require rate of change of frequency (ROCOF) measurements, or rate of change of power injections, which makes it immune to the noise associated in the estimation of these quantities. Additionally, the proposed method does not require defining near-zero ROCOF thresholds which is system-specific and a non-trivial problem. The applicability of the proposed method is tested through simulation studies performed in the IEEE-39 bus test system. The robustness of the method is also tested against modeling uncertainties and in the presence of measurement noise.