TOOLS FOR ESTIMATING ASH FALL THICKNESS: USING REFERENCE PHOTOGRAPHS TO IMPROVE ACCURACY

Volcanic ash is the most widespread of all volcanic hazards. It causes damage and disruptions to transportation networks, infrastructure, and agriculture and poses a hazard to human health. Measuring the extent and thickness of volcanic ash deposits is an essential scientific activity to inform hazard planning, emergency response, and recovery. However, in many instances ash fall measurement occurs after eruptions, resulting in a lack of direct ash fall data. Digital media allows analysis of photographs taken by non-experts in ash fallout zones to provide almost immediate insights as to the magnitude and extent of an explosive eruption, enabling experts to remotely support the emergency response. However, thickness is notoriously difficult to measure on complex, non-flat surfaces, such as grass, cars, and roofs. This study attempts to resolve this via the creation of a suite of visual media to communicate how ash appears on common surfaces. Photographs of ash fall deposits published in the news media or posted on social media can be compared with reference photographs to estimate the ash thicknesses documented in the images.

Volcanic ash fall conditions were simulated using an ash dispersal rig. A set of reference photographs was prepared for ash thicknesses of 1 mm, 5 mm, and 10 mm on common surfaces (i.e. asphalt, grass, flax). Effective photographs show visual differences between various ash thicknesses, with a known scale. A survey (n=77) tested changes in people’s ability to accurately estimate ash thickness with the use of reference photographs of various ash thicknesses. Results confirmed that people tend to overestimate ash thicknesses. Moreover, people are more accurate at estimating ash thickness in close- to medium-shot photographs than long-shot photographs.

Images which depict ash fall deposits at close- and medium-range fields of view with a relevant scale can be readily utilized to collect data for ash fall impact assessments and isopach maps. Effective photographs informing people of volcanic ash fall risk should include universal objects for scales (i.e. a pencil or soda can) and use common perspectives (i.e. low angle shots). Such photographs of various thicknesses of ash on everyday surfaces can be used for public education internationally and can be critical for allowing remote data collection.