A method of identifying, from an aerial thermal image (1) of a plurality of buildings within a predetermined area, those buildings having relatively low thermal efficiency, comprises performing ground-based measurements (2, 3) indicative of the thermal efficiency of a sample of the buildings; correlating (4) the ground-based measurements (2, 3) with the aerial thermal image (1) and estimating (6), on the basis of the correlation (5), which ones of the buildings, other than those of the sample, have relatively low thermal efficiency.