An empirical assessment of the impact of the light direction on the relief inversion effect in shaded relief maps: NNW is better than NW

ABSTRACT

Relief inversion (or terrain reversal) effect is a well-known phenomenon in cartography that occurs when shadow is the main depth cue for three-dimensional shape perception. Light direction has been suggested as the main cause of this effect. However, the prevalence of relief inversion effect with regard to the changing light direction is currently not established, and there is little empirical evidence on this subject. This article systematically assesses the influence of light direction on the accuracy of landform perception in shaded relief maps (SRM). In a controlled experiment, 27 participants were asked to identify concave and convex landforms in 128 SRMs using a 5-point Likert scale where answers varied from clearly a valley to clearly a ridge. Eight different scenes were illuminated from 16 light directions to obtain the 128 SRMs. Our findings clearly demonstrate that incident light at 337.5° north-northwest (NNW) yields the highest accuracy and confidence ratings in landform identification among the investigated light directions; and leads to higher accuracy scores than at the 315° (NW) which is conventionally used in SRMs. Thus, we propose an update to this convention and recommend the light source to be placed at 337.5° when creating SRMs.

KEYWORDS:

• Hill shading
• hillshade
• shaded relief map
• relief inversion effect
• terrain reversal effect
• light-from-above-left preference
• left bias
• overhead illumination bias
• shape-from-shading
• global convexity bias

Acknowledgments

We thank Dr. Miguel-Angel Bernabé-Poveda for inspiring us to work on the relief inversion, Dr. Bernhard Jenny and Dr. Jo Wood for their help with the shaded relief map software, Dr. Alžběta Brychtová for the statistics discussions, all our participants for their patience and willingness to help, and the reviewers of this article for all their valuable inputs.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This study was partially funded by Swiss National Science Foundation (SNSF project VISDOM) [award number 200021_149670].