Visualization of Graph-Based Structures for Navigation and Tracking of Real-World Objects Vitaly Zabiniako, 2 Oleg Gorbik

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1 VOL. 3, NO. 2, February 2012 ISSN Visualization of Graph-Based Structures for Navigation and Tracking of Real-World Objects 1 Vitaly Zabiniako, 2 Oleg Gorbik 1 Mg.sc.ing., Riga Technical University, Riga, Latvia 2 Bac.sc.ing., Riga Technical University, Riga, Latvia ABSTRACT In this article authors provide description of the conception of navigation and tracking of real-world objects using graph structures. Similarities between classical mathematical model of the graph and the conception of navigation domain are being highlighted. A case-study of construction of a prototype for air traffic control (ATC) purposes is presented along with evaluation of its usability by a group of potential users and a survey of existing navigation solutions. Conclusions are being made and further researches are being defined. Keywords: visualization, graph, mobile object, traffic, comprehension. 1. INTRODUCTION Nowadays there are several different tasks that imply necessity to visualize mobile objects of all kinds. This kind of visualization can be used, for example, in air traffic control systems that allow to monitor the air traffic situation in a certain space region. Three-dimensional (3D) computer graphics might be used for these purposes. The same necessity to use visualization of mobile objects in 3D space is also important in navy navigation systems that help guiding these objects and in different simulation tasks. This work allows identifying and analyzing visualization techniques that can be used in implementation of according systems. The main problem is the following: even nowadays ATC operators mostly have only two-dimensional (2D) radar monitors in which a great amount of information is shown and they need to interpret, store and transform it into 3D space in their mind. Similar problem persists for naval navigation, especially taking into account the fact that nowadays many amateurs sail with the yacht in open sea and they may have difficulties while trying to comprehend all the necessary spatial information by looking at 2D picture. This is why development of 3D object visualization becomes important. Independently from the type of the particular domain it is possible to generalize the controllable/observable objects their mutual relations and trajectories. The particular generalized mathematical mod el can be treated as a graph structure. Looking at the visualization of real-world objects from this perspective, according graph visualization strategies and visualization techniques become dominant. upon graph structures. The tasks of this work are as follows: 1) to identify the relationship between mobile objects in the real world and the graph model; 2) to analyze visualization techniques that can improve visual management of objects; 3) to implement a prototype that demonstrates the described basic conceptions and evaluate it by using survey of potential users; 4) to make conclusions about achieved results. 2. THE SEMANTIC RELATIONS BETWEEN REAL WORLD MOBILE OBJECTS AND THE GRAPH MODEL According similarities will be analyzed based on the ATC a service provided by ground-based controllers who direct aircraft on the ground and in the air in order to prevent collisions (utilizing lateral, longitudinal and vertical separation) as well as organize and expedite the flow of traffic [1]. The primary tool for such activity is an ODS (Operational Display System) computer-driven systems with ergonomic console for showing maps, weather data, aircraft routes and digitized real-time radar tracks. Considering that most signals come in digital form, additional functionality might be optionally embedded (such as trajectory prediction, early conflict warnings, arrival optimization, etc.). Another important feature of such system is the ability for pilots to submit flight plans (information about intended flights general shape of route, waypoints, altitudes of flight control points, etc.) that could be displayed on ODS screen. Integration of all this data on ATC controller s screen usually results in complex visual image that must be monitored by a human in real time refer to Fig.1 as example. The goal of this work is to elaborate recommendations about how to efficiently visualize mobile objects in air control / simulation systems based 288

2 3. USEFUL VISUALIZATION TECHNIQUES The main problem in relation to the fact that traffic control systems must visualize information in a way that makes it easily perceivable (so the user could react quickly and adequately) is extensively researched in different works. This research describes how traffic control systems and navigation systems might be created so that people could recognize the volume of information that is shown on the screen without additional effort. In general, we assume that such systems should visualize the objects in 3D space due to the fact that the human perceives, interprets and thinks in terms of real three dimensional world. 3.1 Related works Fig. 1: Fragment of ATC display data In case of busiest airports density of such data volumes might increase even more. Considering that ATC process is a mission-critical task (same relates to any avionics software involved), it is of primary importance to relieve controller s work by providing him with all necessary information in most possible convenient way. Upon close inspection of Fig.1 it is possible to conclude that such kind of data can be interpreted as a graph structure with multiple nodes corresponding to objects such as aircrafts, waypoints, towers, etc. At the same time, such concepts as flight paths (either previously submitted via flight plans or those calculated in real-time), open communication channels between different towers, borders that separate different control space regions, etc. might be interpreted as graph edges. It is possible to derive some obvious guidelines that define possible content of such visualization systems [2]: 1) there should be only one view port on main ODS display (duplicating information in multiple view ports increases its comprehension time) and it must operate in full screen mode without any additional toolboxes (at each fraction of time controller must be concentrated only on primary ATC data without any distractions); 2) both threedimensional and planar visualization is necessary. The first one allows for natural interpretation of data, as aircrafts utilize all space dimensions equally. The second one is still necessary because of compatibility with historically dominating sweeping radar systems. 3) enhanced abilities in relation to data navigation, selection of individual elements and altering its individual appearance characteristics should be provided, as this is essential for effective data monitoring and visual clustering. More detailed analysis that is carried out based on existing researches and proposed features is presented in next chapter. Source [3] gives explanations on visualization of objects trajectories and methods how it is possible to identify potential conflicts. In case if only individual points on 2D radar monitor are represented with secondary numerical data (such as speed, altitude, course, etc) it is hard to create a 3D spatial scene in user s mind, especially, if there are many dynamic objects, which is typical situation in any airport nowadays. At each particular time moment the ATC personnel must clearly understand positions and moving vectors of all aircrafts, resolve possible conflicts and provide pilots with instructions on how to escape these. In this situation, a 3D representation would be useful not only for showing the mobile objects but also for trajectories and secondary characteristics of these. To make it possible, different detalization levels must be integrated into traffic control systems: 1) when it is possible to see more objects and according flight trajectories but less detailed information about each of these; 2) when it is possible to get detailed information about particular object without visual disturbance from other objects nearby. The information about weather conditions also might be useful [4]. While working with visualization of real world objects it is also important to allow for convenient control of the scene view. If a user cannot effectively manipulate objects or move the camera to approach and see its surrounding and detailed information, a user can find it hard to work with such system, which, in turn, potentially increases chances of an accident. Work [5] describes in detail 3D space and typical navigation tasks in it. The authors offer six-axis navigation that is implemented with a mouse and provide a description of experiments that show how users could successfully manipulate objects in 3D space by using the offered approach. For traffic control systems in general and especially for air traffic control systems, specific functionality is required so that such systems could be successfully used by traffic controllers in airports. Based upon opinions of 289

3 staff, this kind of systems must allow to identify crossing of object trajectories, show speed of an object (both vertical and horizontal) and predict the position the object will come to, while moving in the chosen trajectory. To allow for such functionality it is possible to draw 3D time sphere for the chosen object which limits the possible location in which the object can be located in the next moment [6]. Authors of [7] researched sea navigation capabilities and developed according software called Marine GIS. Based on this work we can conclude that traffic systems require visual representation of landscape features. This work also emphasizes necessity of different camera modes by providing a view from the top that allows to cover bigger space with a single view, and another mode for object close-up. The work [8] deals with visualization of a highway in South Korea province and outlines main tasks in the domain of simulation of moving objects. The mentioned visualization methods must be evaluated in terms of ability to perceive data efficiently so that all the necessary information could be received without effort. 3.2 Analysis of methods for improved comprehension For air control systems it is necessary for the background of the scene to be black or in some other of the dark colors. In most cases the landscape will not occupy all the screen and there will be some empty space. In case if it is white or of some other bright colors, the perception might be hindered and the user will soon become tired. This is not the best choice, especially taking into consideration that the operators will have to look at the monitor for several hours without break and the comfort of eyes is mandatory in this case. The necessity to depict the trajectory of an object (edges of the graph) was mentioned previously. This feature is important for the user due to the fact that there is no need for him to calculate and keep track of the trajectory in his mind. Additionally an opinion exists that there is a necessity to pay special attention to the points where the trajectory changes direction, for example, by depicting such points with a different color. In case if there are many objects in an air space, a person could not notice that the particular trajectory changes its direction because those changes can be minor, however such aspects may deny clear vision of what may happen, which can lead to an accident. As for the selection of optimal color range it is recommended to use bright colors to depict edges of the trajectory graph so that the most important information would not be lost in the general information flow (in case if the color of trajectories will be similar to the background or the landscape, its changes might be unnoticed). Trajectories of different objects must be drawn in different colors to avoid misleading. This is especially important for the camera mode with general view from above when trajectories of many objects (even if these are moving in different altitude layers) can visually overlap and cross. The trajectories of objects have to be visualized as lines this allows perceiving these from any angle of view. Width of lines is of separate concern. A line that is too thin will not be visible, while a line that is too thick is not optimal because in this case it will visually occupy too much screen space, thus, overlapping other information. This can negatively affect perception and increase time of comprehension of current situation, which leaves less time for analysis and decision-making. As it was mentioned, the traffic control and navigation systems must reflect the crossing points of trajectories so that the operator can notice a region with potential danger at once and make an appropriate decision. Such dangerous parts of the trajectory should be marked with bright red color, because in this case the operator understands danger immediately. The same should be implemented for parts of trajectory that goes through dangerous zones, such as regions with bad weather conditions or high traffic density. The system can identify such regions based on automatic analysis of airborne aircrafts and METAR weather forecasts. A similar method has to be used for objects themselves an aircraft which is marked with bright red color immediately attracts attention. The changes in trajectory must also be marked with bright colors, however there should be difference in color tone to avoid visual mixing with dangerous zones. This approach eliminates the need for an operator to follow all possible trajectories and highlights only those parts where extra attention must be paid. The representation of mobile objects in such systems does not require application of complicated textures for three-dimensional models. The texture, if it is used, has to be simple, emphasizing only the information that can help to perceive the object, for example for better visual outline of the shape of aircraft. Same can be said for complex lighting models which are not necessary for such systems due to the fact operator s attention will be subconsciously turned to objects in areas with better illumination rather than the whole scene. The transparency feature is also useful, although must be applied carefully. In case when particular object is being selected in detailed camera mode, other objects and according trajectories might become partially transparent this will allow for better concentration on characteristics of particular aircraft without unnecessary distractions. However, exception must be made in case if secondary transparent object suddenly changes its trajectory or approaches dangerous zone. In such case of danger transparency must be cancelled and corresponding 290

4 regions with potential danger must be explicitly outlined with bright red color. Another feature that must be considered is the visualization of textual information with characteristics of mobile objects. Both ATC operators and users of navigation systems must have access to detailed descriptions of according properties. The text label must be positioned close to corresponding object (the node of the graph) to avoid ambiguity. In general view camera mode only flight number and its primary characteristics such as altitude and speed must be visualized. Secondary information, such as exact type of the aircraft, etc., must be visible only in detailed view mode, otherwise it will be visually cluttered and overlapped, which will decrease interpretation capabilities. The font for text labels must be monospaced this will allow for better recognition of separate characters. While following the previously mentioned guideline that the background is dark, the font of text labels, in contrary must be of white color. This will ensure required contrast which is necessary for reading without additional efforts. While the mentioned recommendations for improved recognition were primarily suggested for ATC problem domain, these are also suitable for general information visualization, such as traffic control, simulation and graph visualization tasks [9], [10]. 4. EXPERIMENTS WITH ATC SYSTEM PROTOTYPE AND EVALUATION OF SURVEY RESULTS Fig. 2: Usage of black background Another aspect was in connection with visualization of trajectory of mobile objects two modes were presented to the users with no implementation of trajectories and with implementation in form of lines. According to evaluation of users, this experiment confirmed the importance of visualizing trajectories of the mobile objects refer to Fig. 3. Fig. 3: Visualization of trajectories The experiment with preferred color of the trajectory proved that it is useful to implement brighter colors. Additionally, it is important to use colors that ensure contrast with surrounding color, such as landscape (see Fig. 4). 4.1 Description of experiments In scope of this work a prototype of ATC system has been created for evaluation of mentioned improved aspects (the airspace and landscape of Latvia were used as an example). A set of experiments was carried out in order to clarify which color is suitable for the scene background. It was confirmed that any color which is not black or white can decrease user s concentration upon what is happening in the monitored space (in this case air space) and it is better to use one of these two colors. Fig. 4: Visualization of trajectories with contrasting color Another experiment confirmed that it is better to implement different colors for different trajectories to avoid visual mix. Fig. 5 provides more convenient approach for identifying and distinguishing the trajectories. However, it was also observed that extensive usage of white color for prolonged time periods also results in user being tired. In contrary, when using a black background, such negative effect was not observed and users reported satisfactory results. (see Fig. 2) Fig. 5: Visualization of trajectories with different colors Experiments with the width of trajectory confirmed that both too thin and too thick lines are not suitable. In the 291

5 first case it is hard to notice according line and in the second case lines occupy too much space on the screen and restrict perceiving with mutual overlapping. While experimenting, an optimal width was defined which is equal to 3-5 pixels on 17 inch diagonal monitor (see Fig. 6). reduces necessity to scan full text in search for particular text fragments (see Fig. 9). Fig. 9: Usage of fonts Fig. 6: Adjusting line width for trajectories Experiments with text labels confirmed that it is not suitable to visualize detailed information when all air space is being observed. Such data amount is suitable only for a mode with an object being selected and zoomed in. Otherwise, identifying particular textual description is hindered, which becomes even more complicated when multiple labels overlap. Visualizing only primary set of properties as a text allows obtaining balanced view (see Fig. 7). The usage of transparency in detailed view mode when particular object is selected, allowed for better focusing, as it was expected, due to visual reduction of other objects that were rendered as partially transparent (See Fig. 10). Without transparency such focusing was considerably more difficult due to equal visual representation of all objects in the scene. Fig. 10: Usage of transparency for visualization 4.2 Evaluation of prototypes in a survey Fig. 7: Visualization of text labels with primary data While experimenting with colors of text, it was concluded that black color for labels is usable at some extent, but it turns to be completely unreadable in combination with dark background. Usage of different bright colors improved perception, although reading text in such colors for prolonged time periods is not pleasant for the users. An attempt to use white color proved that in this case text is clearly noticeable both in front of landscape and black background and also it is aesthetically pleasing (see Fig. 8). A survey, which consists of two parts, was carried out in scope of this work to prove the correctness of obtained results. In the first part the developed prototype was presented to 14 respondents and they were asked about various aspects of perceiving the visualized information. In the second part respondents evaluated these aspects according to a preset scale (from 0 not important to 10 extremely important), while determining how much influence each of corresponding aspects would have in a real application. The results of the first part of survey are presented in Fig. 11. Fig. 8: Adjusting text color Another important aspect is the usage of font. Monospaced fonts Courier New and Times New Roman were used. The experiment confirmed that use of monospaced font improves perception of information and Fig. 11: Results of the survey 292

6 During evaluation 12 respondents agreed that the chosen line width for trajectories is optimal and 2 respondents pointed out that they prefer thinner lines. None of the respondents stated that thicker lines are preferable, which conforms to the theoretic assumptions described in previous chapter. Thus, 14.29% of respondents expressed different opinion and preferred thinner lines, while 85.71% of respondents agreed with authors of the work (none of the respondents preferred thicker lines). While answering the question about the textual information in general camera mode from above, 13 of the respondents agreed with authors of the work that is better to access only primary information of all objects simultaneously. One respondent pointed out that for him it would be more convenient if all the available information about all objects would be seen at once. Thus, 92.86% of the respondents agreed with the authors of the work and 7.14% had a different opinion. The aspect of contrast between the trajectory of a mobile object and the background/landscape was evaluated from quite important (6 and 7) to very important (8, 9 and 10) with the most part of respondents (71.43%) evaluating this aspect as very important. Regarding the partial transparency in case when particular object is selected, wide range of evaluations was acquired. Most of respondents evaluated this characteristic as important, many as medium and a small part as not important. This allowed concluding that such feature is required, although it is not critical. The evaluation of the importance of the background color revealed that majority of respondents thinks that the choice of appropriate color is important, however there were also some evaluations that stated that this characteristic is of medium importance. Unlike the previous question, there were no respondents who evaluated this aspect as not important. Regarding the question about partial transparency of the remaining objects when particular one is selected, 12 respondents informed that the partial transparency improves perception of information this corresponds to the opinion of authors, however 2 respondents pointed out that the version without transparency is preferable. Thus, 85.71% of respondents agreed with the authors and 14.29% disagreed. While commenting on preferable background color 13 respondents pointed out that the black background is superior to any other. At the same time 1 responded stated that he would prefer another color, however he did not specify particular one. None of the respondents mentioned white background as preferable due to the fact that it is too aggressive for human vision. Thus 92,86% of respondents agreed with the authors of the work, none stated that white is more suitable and 7,14% chose other color. Regarding the usage of monospaced versus proportional font, all 14 respondents pointed out that monospaced version is visually more appealing. By calculating the average percentage of how many respondents agreed with the authors in general, it revealed to be 77.14%, so the most part of respondents agreed that the offered solutions significantly improve perception capabilities in comparison with alternative methods. In the second part of survey respondents were offered to evaluate individual methods of visualization by using 0 10 scale for specifying the importance of particular aspect. The first question about importance of visualization of the object trajectory resulted in evaluations marks ranging from 7 to 10. All respondents evaluated this characteristic as very important only the individual marks differ, which fully conforms to conclusions made by the authors of this work. The next question there was a talk about a possibility to choose particular object for detailed examination of its characteristics. All the respondents evaluated according importance with the highest evaluation. The respondents pointed out that this is the only option which allows for convenient usage of such systems. As it was already mentioned according to opinion of authors, it is important for the user of the system to have an ability of moving the camera freely both in terms of its position and direction of view. The answers about whether such feature is necessary were divides almost equally, which allows concluding that this is recommended for implementation, although is not mandatory. However, when asked in combination with proposed ability to switch the camera mode between free flight and global view, the majority of respondents pointed out that such feature would be important, while some evaluated it as of medium necessity. In the survey part which dealt with textual information the respondents were offered to evaluate how important it would be to show full data regarding the object in a global view. Majority of respondents evaluated this characteristic as not important however there was also a number of medium evaluations. However, regarding the local view mode, many respondents evaluated the importance of this feature as top priority (57.14%) with no low evaluations. The question about the importance of color of a text label revealed generally high evaluation which was of two types 8 and 10 points. 293

7 5. CONCLUSION ACKNOWLEDGEMENT In scope of this work a research about visualization possibilities and techniques regarding mobile objects was performed. Based on the proposed assumptions, prototypes for evaluation of visualization techniques were developed and a sur vey was carried out. It is possible to conclude that for the traffic control and navigation systems the use of three-dimensional space is important because it improves the perception of information and allows even for non-professionals to monitor and interpret situation adequately. At the same time, handling simulation systems without the usage of three-dimensional space is hard and often even impossible. Based on a set of experiments and results of the sur vey, a number of desired visualization aspects was revealed: 1) the background has to be black, or in case of sea navigation /simulation system similar to the color of sky; 2) the trajectory of moving objects has to be visualized; 3) according graph edges must be visualized as solid lines; 4) the colors of the edges have to bright and have to be in contrast with both background and the surrounding landscape (a part of already travelled path might be depicted with different color); 5) changes in trajectory direction, mutual crossing of these, dangerous zones, etc., must be visually outlined with bright red color and possibly a sound signal for immediate attention; 6) monospaced font of white color must be used for textual information; 7) two separate camera modes are required the general and local views; 8) in general camera mod e only the most important textual object data must be presented, such as the number of flight and speed, while the detailed view must present full data set; 9) when in local view it is useful to decrease visual presence of other objects by using partial transparency. The results of survey confirmed theoretical assumptions of the authors. These might be applicable for development of traffic control and navigation systems, allowing for more user-friendly solutions. There is a hope that introducing fully threedimensional navigation and traffic control systems will help to decrease the number of accidents in the future and make both air and sea traffic safer and more open. The necessity to support these goals is the main reason for further research and development in scope of this work. This work has been supported by the European Social Fund within the project «Support for the implementation of doctoral studies at Riga Technical University». REFERENCES [1] Advanced Simulated Radar Client / ASRC [Online]. Available: /ASRC%20Documentation/Introduction.html [2] Zabiniako V., Rusakov P., Definition of General Requirements for Graph Visualization Software. The 49th Scientific Conference of Riga Technical University, 2009, pp [3] Azuma, R., Daily, M., and Krozel, J., Advanced Human-Computer Interfaces for Air Traffic Management and Simulation, Proceedings of 1996 AIAA Flight Simulation Technologies Conference, 1996, pp [4] M. Lange, et al., 3D Visualization, and 3D and Voice Interaction in Air Traffic management. In proceedings of SIGRAD 2003, Umea, Sweden. [5] Ousland, A., Turcato, H., Navigating 3D environments, In Proceedings of Eurescom, [6] Rozzi, S., et al., Developing Visualizations to Support Spatial-Temporal Reasoning in ATC, Proceeding of the Second International Conference on Research in Air Traffic Transportation ICRAT 2006, pp [7] Gold M., et al., The marine gis - dynamic GIS in action. In Proceedings of ISPRS th Congress, Istanbul, Turkey, [8] Choi H., Kang I., Nam K., Utilizing 3D Web-based GIS for Highway simulation, Asian Conference on Remote Sensing Proceedings, [9] Zabiniako V., Rusakov P., Supporting Visual Techniques for Graphs Data Analysis in Three- Dimensional Space. 17-th International Conference on Information and Software Technologies pp [10] Zabiniako V., Rusakov P., Analysis of Visualization Problems of Graphs and Models of Graphs. The 47th Scientific Conference of Riga Technical University, 2007, pp