Technical note: Evaluating a geographical information system (GIS)-based approach for determining topographic shielding factors in cosmic-ray exposure dating

Abstract. Cosmic-ray exposure (CRE) dating of boulders on terminal
moraines has become a well-established technique to reconstruct glacier
chronologies. If topographic obstructions are present in the surroundings of
sampling sites, CRE ages need to be corrected for topographic shielding. In
recent years, geographical information system (GIS)-based approaches have
been developed to compute shielding factors with elevation data,
particularly two toolboxes for the ESRI ArcGIS software. So far, the output
of the most recent toolbox (Li, 2018) has only been validated with a limited
number of field-data-based shielding factors. Additionally, it has not been
systematically evaluated how the spatial resolution of the input elevation
data affects the output of the toolbox and whether a correction for
vegetation leads to considerably more precise shielding factors. This paper
addresses these issues by assessing the output of the toolbox with an
extensive set of field-data-based shielding factors. Commonly used elevation
data with different spatial resolutions were tested as input. To assess the
impact of the different methods on CRE ages, ages of boulders with different
10Be concentrations at sites with varying topography and 10Be
production rates were first recalculated with GIS-based shielding factors and then with field-data-based shielding factors. For sampling sites in forested low mountainous
areas and in high Alpine settings, the shielding factors were independent of
the spatial resolution of the input elevation data. Vegetation-corrected
elevation data allowed more precise shielding factors to be computed for
sites in a forested low mountainous area. In most cases, recalculating CRE
ages of the same sampling sites with different shielding factors led to age
shifts between 0 % and 2 %. Only one age changed by 5 %. It is shown that
the use of elevation data with a very high resolution requires precise
x and y coordinates of sampling sites and that there is otherwise a risk that small-scale
objects in the vicinity of sampling sites will be misinterpreted as
topographic barriers. Overall, the toolbox provides an interesting avenue
for the determination of shielding factors. Together with the guidelines
presented here, it should be more widely used.
