An Application of Eye Tracking Methodology inCartographic Research
Vassilios Krassanakis

Vassiliki Filippakopoulou

Byron Nakos

National Technical University of Athens
School of Rural and Surveying
Engineering
Laboratory of Cartography
9, Heroon Polytechnioy Str.
Zographos, GR-157 80, Greece
krasvas@mail.ntua.gr+30 210 772-1306

National Technical University of Athens
School of Rural and Surveying
Engineering
Laboratory of Cartography
9, Heroon Polytechnioy Str.
Zographos, GR-157 80, Greece
bfilippa@survey.ntua.gr+30 210 772-2732

National Technical University of Athens
School of Rural and Surveying
Engineering
Laboratory of Cartography
9, Heroon Polytechnioy Str.
Zographos, GR-157 80, Greecebnakos@central.ntua.gr+30 210 772-2733

ABSTRACT

Eye Tracking is a valuable tool for studies related to visual search and visual perception. In cartographic research, visualexperimentation is considered appropriate in order to investigate the variables of the map that can influence the map
reading tasks. In the present paper, we demonstrate how the eye tracking technology may contribute to cartographic
research by presenting an experiment on the selectivity of the visual variable of shape. Specifically, we examine how
map readers are reacting with point symbols that are characterized by the topological property of having a hole as basic
shape characteristic. The Viewpoint Eye Tracker® by Arrington Research is used for the recording of the eye
movements. The detection of the center of vision is accomplished by locating the center of the pupil. Furthermore, analgorithm is developed in order to control the accuracy and the precision of the recording data. The analysis of fixations,saccadic movements and scan paths is significant for the map reading tasks during the process of visual searching. EyeTracking seems to be very robust in cartographic research, as it represents the visualization of map reader's cognitive
process.
Keywords

eye tracking, pupil location method, map reading, visual search
INTRODUCTION

Eye Tracking methodology is providing an enriched way to examine processes related to visual search and visualperception. The fundamental metrics of the methodology provides the opportunity to represent the results of thesubject's cognitive process, which occurs during the observation of a stimulus. In cartographic research, cognitive
processes take place during the map reading tasks. Furthermore, the map elements and the elements of the cartographic
background constitute the main objects of the cartographic symbolism and can be considered as the basic stimuli of thisprocedure. In cartographic symbolization, there are some features, such as color, orientation, motion and size which aredetected immediately by the visual system prior to attention (Wolfe 1998, 2000, Wolfe and Horowitz 2004, Michaelidou
et al. 2005). Cartographers are focusing on the properties which are rapidly and accurately detected by the visual system
in order to facilitate the detection of target symbols during the map reading tasks (Michaelidou et al. 2005). In thepresent paper, we investigate the reaction of the map readers in point symbols which are located on a simple
cartographic background. Specifically, these point symbols are characterized by the topological property of having ahole as basic shape characteristic for which there is evidence that is an immediately detected feature (Chen 1982,Pomerantz 2003). An experiment, using eye movements analysis, is designed and the results are presented below.
EXPERIMENTAL DESIGN

Subjects are asked to search on a computer monitor for a target point symbol with a unique topological property feature
being that of having a hole among compact point distractors on a simple cartographic background. The cases wheretargets are located in the center, the middle and the periphery of the base map are examined. Also, we investigate thecase of target-absent trial. Totally, 16 different maps are designed. During the process of visual searching, the eye
movements are recorded, using pupil location method, in order to examine the representation of subject's cognitiveprocess. Eight subjects are used for the analysis and each subject is tested individually.
EYE TRACKING LAB SETUPThe Viewpoint Eye Tracker® by Arrington Research is installed at the Laboratory of Cartography of National Technical
University of Athens (NTUA). The operation of this system is based on the use of devices that are able to record thegaze on a visual scene by analyzing eye images. The basic components of the Eye Tracker are the recording device(hardware) and the processing tools (software). The system is embedded in a computer machine, which has anadditional card for graphics. This fact contributes to the support of two display monitors; the first one for the system'soperator (primary monitor, 17-inch diagonal, resolution 1280x1024) and the second for the stimuli projection(secondary monitor, 19-inch diagonal, resolution 1280x1024). The recording device in the central processing unit(CPU) consists of a board connected to the motherboard and externally of a system attached to the optical system of theobserver. The system of attachment supports the existence of a camera and an infrared Light-Emitting Diode (LED) for
each eye. The system's geometry is integrated by a mechanism that stabilizes the position of the observer's opticalsystem. The distance between the observer and the secondary monitor is 55.50 cm (21.85 inch). A third camera on thesystem attached to the observer's head provides the opportunity to extend the stimuli monitor to physical surface.
The function of the system is accomplished by a set of sequential operations. The infrared LED, which is located under
the observer's eye, illuminates the eyeball while its image is being captured by the camera placed next to the infrared
LED. The illumination contributes to the discrimination between the regions of the pupil and the iris. Furthermore, theapplication of predefined threshold allows the capturing of corneal reflection. The center of the pupil and the position ofcorneal reflection are detected by using segmentation algorithms. For each observer, the system creates a transformationthat associates every movement of central vision to the relative movement on the stimuli. The generation of thisfunction occurs through a process of auto-calibration.
The Eye Tracker uses three different methods in order to detect the visual center; Pupil Location Method, Glint
Location Method and Pupil-Glint Vector. The process of auto-calibration is applied by the observation of fixed point
targets that are represented as squares on the stimuli monitor. This process has several variables (e.g. total number of
fixed targets, time of target's representation) which are modified by the purpose of the experimental design.
Furthermore, the Eye Tracker supports the creation of special Regions Of Interest (ROIs) and the recording of the rangeof observer's pupil. Additionally, the system is able to compute the ratio of the tangent pupil, the eye torsion. Each
Tracker's record is accompanied by a variable of its quality. The sampling frequency can be selected by the operator
between two available choices (30Hz and 60Hz). The visual range is ±44 degrees of the visual arc horizontally and ±20
degrees in the vertical dimension. The Eye Tracker's accuracy lies between 0.25-1.00 degrees of the visual arc, when therecording resolution is 0.15 degrees of the visual arc.
Moreover, we have developed an algorithm in order to control the accuracy and the precision of the recording data. Thealgorithm is executed before and after each procedure of experimentation. Its function depends on the estimation of
statistical measures related to the spatial distribution of the records.
RESULTS

The analysis of fixations, saccadic movements and scan paths reveals the followings:• In most cases, the process of visual search starts from the place where the target symbol is located in the previous
stimulus scene. In all other cases, the place where the target is located in the previous scene is included in the scanpath.
• Scan path is more complicated and it covers a larger area when target is located in the periphery of the map, while itis simpler in the middle or in the center.
• Fixations correspond to places of the base map where point symbols are located.
• The process of visual search through target and distractors is depicted as the part of saccadic movements in the scanpath.
• Scan path is more complicated in the case where target is absent from the stimulus scene.
• In the case of target-absent trials, eye movement analysis does not reveal a special pattern of searching.
• The verification of target's detection is independent of the location that it is indicated.
CONCLUSIONSIn the present paper, we demonstrate how the eye tracking technology may contribute to cartographic research bypresenting a cartographic experiment. The results are significant for the study of the map reading process, during thevisual search of target symbols on cartographic background. Target symbols are characterized by the unique topologicalproperty feature of having a hole, and the process of searching takes place among compact distractors. Eye movements
analysis enriches the traditional methods of measuring the visual search through the accuracy and the reaction time on astimulus. Eye tracking seems to be a valuable tool for experimentation on scientific fields related to cognitive processes.
REFERENCES

Arrington Research, ViewPoint EyeTracker, PC-60 Software User Guide, Arrington Research Inc, Scottsdale (U.S.A.).
Chen, L. Topological structure in visual perception, Science, 218, 2 (1982), 699-700.
Michaelidou, E., Filippakopoulou, V., Nakos, B., and Petropoulou, A. Designing Point Map Symbols: The effect ofpreattentive attribute of shape, Proceedings of the 22th International Cartographic Conference (2005).
Pomerantz, J.R. Wholes, holes, and basic features in vision, Trends in Cognitive Science, 7, 11 (2003), 471-473.
Wolfe, J.M. Visual search. In H. Pashler (Ed.), Attention, London, UK: University College London Press (1998).
Wolfe, J.M. Visual attention. In De Valois KK (Ed.), Seeing, 2nd Edition, San Diego, CA: Academic Press (2000),335- 386.
Wolfe, J.M., and Horowitz, T.S. What attributes guide the deployment of visual attention and how do they do it?, NatureReviews, Neuroscience, 5 (2004), 1-7.