This is a widget that can be invoked from IDL to display a region of SEVIRI imagery at thermal infrared wavelengths and discriminate airborne volcanic ash within it. Thresholds can be interactively set and applied across the image to create a dynamic ash mask. Thresholds are based on both a global brightness temperature difference, and on a local brightness temperature difference (the local brightness temperature difference is calculated on demand within the widget, and can be interactively adjusted by the user). A magnified display of the image allows individual pixels and areas to be selected as ash or ash-free, over-riding any threshold tests to create a final mask. The final product is a png file of the ash mask for the region and an IDL save file containing the final mask and some ancillary information. A README file is provided with full instructions on how to use the widget. This tool should be useful to those requiring a 'truth', derived from expert interpretation of the imagery, for validation of automated ash detection techniques.
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