Prevalence of the terrain reversal effect in satellite imagery

The terrain reversal effect is a perceptual phenomenon which causes an illusion in various 3D geographic visualizations where landforms appear inverted,e.g.we perceive valleys as ridges and vice versa.Given that such displays are important for spatio-visual analysis,this illusion can lead to critical mistakes in interpreting the terrain.However,it is currently undocumented how commonly this effect is experienced.In this paper,we study the prevalence of the terrain reversal effect in satellite imagery through a two-stage online user experiment.The experiment was conducted with the participation of a diverse and relatively large population(n=535).Participants were asked to identify landforms(valley or ridge?)or judge a 3D spatial relationship(is A higher than B?).When the images were rotated by 180°,the results were reversed.In a control task with‘illusion-free’original images,people were successful in identifying landforms,yet a very strong illusion occurred when these images were rotated 180°.Our findings demonstrate that the illusion is acutely present;thus,we need a better understanding of the problem and its solutions.Additionally,the results caution us that in an interactive environment where people can rotate the display,we might be introducing a severe perceptual problem.

Fixing an illusion–an empirical assessment of correction methods for the terrain reversal effect in satellite images

Identifying land forms and land cover classes are important tasks in image interpretation.Sometimes,a phenomenon called terrain reversal effect(TRE)causes an inverted perception of 3D forms.When this inversion occurs,valleys appear as ridges and vice versa.While the TRE can severely impair the ability to identify 3D land forms,‘correcting’for the TRE in imagery can introduce new problems.Importantly,one of most commonly-proposed methods–shaded relief map(SRM)overlay–appears to impair the ability to identify land cover classes.In this paper,we report a comparative empirical evaluation of an SRM overlay solution,and its‘enhanced’versions supported by various other cues(stereopsis,motion,labels).In response to the different solutions,we measure the effectiveness,efficiency,confidence and preferences of our participants in land form and land cover identification tasks.All examined methods significantly improve the ability to detect land forms accurately,but they also impair the ability to identify the land cover classes to different degrees.Additionally,participants’visualization preferences contradict their performance with them,calling for reflection on the visual effects of the applied correction methods.Based on the study,recommendations concerning the correction of the TRE are drawn,and gaps are identified.

Comparing the terrain reversal effect in satellite images and in shaded relief maps:an examination of the effects of color and texture on 3D shape perception from shading

Terrain reversal effect(TRE)causes reversed 3D shape perception in satellite images and shaded relief maps(SRMs),and introduces difficulties in identifying landforms such as valleys and ridges.With this paper,in a controlled laboratory experiment,we compare how well 27 participants could identify valleys and ridges over 33 locations using SRMs,color satellite images and grayscale satellite images.The main depth cue is shadow both in vertical-view images and SRMs.However,the presence of texture and color in images also affect 3D shape perception.All our participants experience the illusion strongly:with the SRMs,it is very severe(2%accuracy),with grayscale images low but considerably better than SRMs(17.6%accuracy),and slightly worse with color imagery(15.3%accuracy).These differences between SRMs and imagery suggest that the participants who are able to bypass the illusion consciously or subconsciously interpret the photographic information.We support this observation further with a cue-strength analysis.Furthermore,we provide exploratory analyses of the effects of expertise,global convexity bias,and bistable perception.Our original empirical observations serve towards a better understanding of this visual illusion,and contribute towards nuanced and appropriate solutions to correcting for TRE differently for satellite images and SRMs.