To have lasting quantitative value, remotely sensed data must be calibrated to physical units of re¯ectance. The empirical line method o ers a logistically simple means of generating acceptable estimates of surface re¯ectance. A review and case-study identify the ease with which this method can be applied, but also some of the pitfalls that can be encountered if it is not planned and implemented properly. A number of theoretical assumptions and practical considerations should be taken into account before applying this approach. It is suggested that the empirical line method allows the calibration of remotely sensed data to re¯ectance with errors of only a few percent.

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Ancient cultural changes have often been linked to abrupt climatic events, but the potential that climate can exert a persistent influence on human populations has been debated. Here, independent population, temperature, and moisture history reconstructions from the Bighorn Basin in Wyoming (United States) show a clear quantitative relationship spanning 13 ka, which explains five major periods of population growth/decline and ∼45% of the population variance. A persistent ∼300-y lag in the human demographic response conforms with either slow (∼0.3%) intrinsic annual population growth rates or a lag in the environmental carrying capacity, but in either case, the population continuously adjusted to changing environmental conditions. climate change | demography | hunter-gatherers | paleoecology M any cultural changes have been linked to climate change, from the collapse of civilizations (1) to the frequency of civil conflict (2). However, tests of the relationship between climate and population size have been hampered by the lack of long-term, highresolution records that are collaboratively interpreted by archaeologists and paleoclimatologists (3). One consequence is that many archaeological examples emphasize the impact of severe events and have not resolved the importance of continuous climate change in shaping cultural history. Here, we report on high-resolution approaches to paleoclimate and population reconstructions that permit us to ask how human population size responds to moisture and temperature dynamics over long timescales. We examine the hunter-gatherer population history of the Bighorn Basin ( Fig. 1) in Wyoming (United States) over the past 13,000 y and test whether that history is associated with changes in moisture and temperature.Changes in the relative size of prehistoric human populations can be reconstructed using the frequency of radiocarbon dates (4, 5) if one accounts for discovery bias and taphonomic factors. The radiocarbon dataset (Dataset S1) includes 421 dates from open-air archaeological sites and 158 dates from closed sites (caves and rock shelters); all dates have SEs ≤ 100 y and come from anthropogenic contexts. The open-and closed-site dates were separately calibrated and the summed probability values (SPVs) generated by CALPAL (6); open-site values were then corrected for taphonomic loss (7). Relative basinwide population changes derive from an average of the open-and closed-site SPVs, which was then smoothed and standardized (Fig. 2C). We note that our analysis assumes that the ratio of summed probability to actual population remains constant throughout time, and that the open and closed sites should be weighted equally. These assumptions might deserve investigation in the future.Two independent paleoclimate datasets test whether the observed demographic changes were driven by climate change. A temperature reconstruction for the region derives from representative pollen profiles from Yellowstone National Park (Buckbean Fen, 44.30 N, 110.26 W, 2,363 m elevation) (8) and the Bigho...