Abstract
Abstract. Relief inversion effect is a perceptual phenomenon that leads to an inverted perception of convex and concave shapes. This perceptual inversion occurs in scenes where the shading/shadows act as the main depth cue. In visuospatial displays, such as shaded relief maps, the positioning of the shadows in the northern slopes, thus when light source placed broadly in south, mislead the cognitive system based on the ‘light from above prior’ assumption (Mamassian and Goutcher 2001). Thus, assuming the light must come from above, our mind creates an illusion, and we perceive the landforms incorrectly. To judge the 3D spatial relationships in terrain representations correctly, the relief inversion effect must be avoided. Cartographic convention against this effect is to place the light source at northwest (NW), whereas a recent study demonstrated that north-north-west (NNW), or even north yields more precise results (Biland and Çöltekin, 2016). Since this finding goes against decades of convention, to establish its validity further, we attempted replicating the results with a different sample in South Africa. In this paper, we present our findings, which broadly confirm that the NNW (or also N) is better than NW against the relief inversion effect.
Highlights
AND BACKGROUNDThe direction at which light illuminates objects may impact the perception of the objects, especially 3D shapes (Ramachandran, 1988)
Light direction plays an important role for shaded relief maps (SRMs) where 3D landforms are represented through shading
A key difference between satellite images and the SRMs is that with the SRMs, we can precisely control the light direction. To avoid this effect, classical cartography texts recommend that the light source for SRMs should be placed at 315 degrees azimuth (NW) and 45 degrees elevation based on convention (Kraak and Ormeling 2010; Slocum et al 2008)
Summary
AND BACKGROUNDThe direction at which light illuminates objects may impact the perception of the objects, especially 3D shapes (Ramachandran, 1988). The mind still assumes that these surfaces should be below, and creates a stunningly strong illusion in which we perceive convex shapes concave and vice versa. This can be demonstrated: If one rotates Figure 1, circles on the left become craters and circles on the right become bumps for majority of the viewers. As with other types of displays, using a ‘wrong’ light direction in SRMs creates the undesired consequence of relief inversion: the 3D landforms appear inverted (Imhof, 1965). In a recent study, Biland and Çöltekin (2016) demonstrated that incident light at 337.5 degrees (NNW) results in higher accuracy than the recommended 315 degrees (NW) when participants identify 3D landforms in SRMs
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