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Abstract
Airborne laser scanning (ALS) data can provide more than just a topographic data set for archaeological research. During data collection, laser scanning systems also record radiometric information containing object properties, and thus information about archaeological features. Being aware of the physical model of ALS scanning, the radiometric information can be used to calculate material information of the scanned object. The reflectance of an object or material states the amount of energy it reflects for a specific electromagnetic wavelength. However, the collected radiometric data are affected by several factors that cause dissimilar values to be recorded for the same object. Radiometric calibration of such data minimizes these differences in calculated reflectance values of objects, improving their usability for feature detection and visualization purposes. Previous work dealing with calibration of radiometric data in archaeological research has relied on corresponding in-field measurements to acquire calibration values or has only corrected for a limited number of variables. In this paper, we apply a desk-based approach in which radiometric calibration is conducted through the selection of homogenous areas of interest, without the use of in-field measurements. Together with flight and scan parameters, radiometric calibration allows for the estimation of reflectance values for returns of a single full-waveform ALS data collection flight. The resulting data are then processed into a raster reflectance map that approximates a monochromatic illumination-independent true orthoimage at the wavelength of the laser scanner. We apply this approach to data collected for an archaeological research project in western Sicily and discuss the relative merits of the uses of radiometric data in such locations as well as its wider applicability for present and future archaeological and environmental research. In order to make the approach more accessible, we have developed a freely available tool that allows users to apply the calibration procedure to their own data.
Highlights
Airborne laser scanning (ALS) data have become a common component of the archaeological prospection toolkit
A substantial minimization of per strip differences can be noted in the calibrated reflectance image as compared to one using the amplitude values as acquired from the scanner
The area shown was used to calculate pixel wise differences of amplitude and reflectance maps from two overlapping strips, and a spot comparison was calculated for the two points illustrated in the figure
Summary
Airborne laser scanning (ALS) data have become a common component of the archaeological prospection toolkit. The main sources of such information are visualizations derived from digital terrain (DTM) or digital surface (DSM) models, which are geometric models that are calculated from the range data produced during a laser scan. There is other information in laser pulses that, though often neglected, can be of great value to archaeological and paleoenvironmental research. In addition to range data, laser pulses reflected from a given surface carry information about its physical properties. This information is included in the data in the form of radiometric values (e.g., the amplitude of the received laser pulse signal)
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