Methane (CH4) emissions from landfills have source distribution in space, are sporadic, and are not known prior to measurement. Current methods of quantifying such emissions need prior knowledge about the location of the source or makeneed some geometric assumptions between the source and the sensor. This work has used TERRAFEX Lagrangian back trajectory concept to localize sources of emissions of methane at the site level, based on off-site mobile truck measurements. The process involved combining source-contribution-field reconstruction, clustering for identifying the source-area, local-maxima finder, and an IME2D-inspired approach. Controlled-release experiments conducted in 2023 were used to develop the normalization relationship, and independent experiments conducted in 2025 were used to evaluate its transferability without refitting. Normalized TERRAFEX emission estimates showed good agreement with known release rates, with (R2=0.77) and a regression slope of 0.79 for the 2023 calibration dataset, and (R2=0.66) and a slope of 0.83 for the independent 2025 dataset. Localization analysis showed that each additional downwind pass and each 10 kg hr-1 increase in release rate increased the odds of successful localization at strict spatial thresholds,; while these effects decreased as the allowable localization distance increased. The framework was subsequently applied to 41 paired landfill measurements and compared with independent Gaussian dispersion model estimates. Agreement was stronger under favourable source–sensor geometry, with (R2=0.54) and a slope of 0.96, than for measurements collected too close to the source, which produced (R2=0.19) and a slope of 0.87. Comparisons at two landfills with available Carbon Mappercarbon mapper observations showed broad spatial consistency between independently identified source areas, although the measurements were not simultaneous. These findings demonstrate that mobile methane measurements can support both candidate source-area identification and site-level emission estimation, when surveys provide sufficient downwind distance, repeated passes, and spatial coverage to constrain the source-contribution field. The framework provides a basis for measurement-informed landfill methane assessment,; while additional validation is needed across broader source types, atmospheric conditions, and survey geometries.