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Bruce Bridgeman, Ian Cooke; Increasing eye height makes slopes appear less steep. Journal of Vision 2015;15(12):744. doi: https://doi.org/10.1167/15.12.744.
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© ARVO (1962-2015); The Authors (2016-present)
Several studies have shown that slopes of hills are greatly overestimated. We have recently demonstrated that the overestimates increase logarithmically as the end point of the domain to be estimated increases – every doubling of the distance to the end point results in a constant increment in perceived slope. A theoretical analysis showed that a critical parameter is the angle v between the observer’s line of sight and the slope of the hill, when the observer fixates the far point of the required domain. The theory predicts that increasing observers’ eye height above the surface of the hill will reduce the slope overestimates by increasing this angle. Here we test that theory by having observers stand on a 37 cm high box to increase their eye height. Observers estimated an outdoor slope in front of them in degrees, at ranges from 2 to 16 m. Estimates for various ranges defined by traffic cones again followed a logarithmic function (r2 = 0.997), with lower estimates compared to other observers standing directly on the surface of the hill. Apparent slope increased more rapidly with distance than in a group standing on the hill’s surface, however, so that at larger distances slope estimates with and without increased eye height converged. As the length of the domain to be judged increases, enhanced eye height has a smaller and smaller effect on the angle v between the line of regard and the hill’s surface. A demand characteristic might induce observers to give different estimates for the four distances tested; an analysis of just the first estimate of each observer, however, showed that the distance vs apparent slope function remained logarithmic. We conclude that anticipated effort, perceived danger and other factors play only a minor role if any in slope estimates.
Meeting abstract presented at VSS 2015
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