Optimizations of raster map visualization in mobile GIS

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1 Optimizations of raster map visualization in mobile GIS DEJAN RANCIC, BRATISLAV PREDIC, ALEKSANDAR DIMITRIJEVIC Computer Science Department, Faculty of Electronic Engineering University of Nis A. Medvedeva 14 SERBIA AND MONTENEGRO Abstract: - Development of geographic information systems for mobile and hand held computer devices requires significantly modified approach in comparison to development of more traditional GISs. Main issues to be addressed are efficient handling of limited memory and processing power, appropriate management and presentation of information on highly restricted devices and design of appropriate user interfaces. This paper addresses issues concerning efficient raster map visualization in the wirelessly networked client / server environment. Analyzed scenario is mobile GIS client retrieving raster maps from the map server using wireless network connection. Raster map segments are downloaded upon being needed and cached locally on the mobile device. Main focus of this paper are optimizations of this downloads and local caching process. Key-Words: - mobile GIS, raster map visualization, wireless clients, quad tree, resolution pyramid 1 Introduction Geographic information systems (GIS) have been around for a number of years and all benefits of their implementation and everyday usage are well known and have been target of thorough research. Spatial data visualization and analysis concepts have been thoroughly tested in applications in various fields of application like climate and geology changes analysis, maintaining cadastre information, social phenomena tracking or managing fleet of mobile objects [1]. The development and implementation of Internet technologies in organizations and companies that utilize in their work process some aspects of geographic space and spatially referenced objects inevitably led to development of Web based and mobile GIS solutions. Concepts of distributed computing and s implemented in GIS allowed spatial data to be physically located at the site where the data is collected or generated and updated yet to be available for use to distant, road-warrior type, users wirelessly connecting to data storage [2]. These technologies present support for field work at locations remote to company or organization headquarters simultaneously increasing productivity and efficiency of business processes and procedures performed on site where objects spatial data is modeling are located. Second section of this paper presents characteristics and functionalities of mobile GIS imposed by introducing mobility into classical GIS. Third section gives top level and detailed component architecture view of GinisMobile, proposed component framework for development of Mobile GIS. Fourth section presents in more detail components of the framework tasked with acquiring needed raster map segments from map server. This section also explains in more detail caching techniques on the mobile client side and dynamic creation of resolution pyramid. We will conclude this section with example application demonstrating proposed concepts in dynamic environment. Fifth and final section concludes this paper and gives directions for further development. 2 Mobile GIS Characteristics and Functionalities Considering the fact that mobile GIS is targeting services that have field work as part of their work process we need mobile hardware platform with significant processing power and memory capacity that simultaneously retains mobility characteristics. In this usage scenario, when mobile GIS is used by professionals as a tool for their on site activities personal digital assistant (PDA) is a perfect match for everyday usage requirements. Hardware and software limitations of this class of mobile devices as well as limitations imposed by fieldwork usage style considerably influences architecture and other usage characteristics of future mobile GIS solutions. Hardware limitations are probably the first obstacle development team will face. The most obvious attribute of this class of devices is low resolution display. PDA devices usually have vertically oriented displays with resolution up to 640x320 pixels. These displays are usually TFT type so their readability in bright daylight conditions should be considered. These, display, limitations primarily have impact on mobile GIS user interface design. Since visual representation of spatial data in the form of some sort of map is primary source of information to mobile GIS user it should be assigned

2 with maximum available space on the screen. Page based user interface organization fulfills this requirement most successfully. Processing power available in typical PDA is not very restrictive factor as it may seem at first glance. Processors used are usually from Intel XScale family [3] and are operating at MHz frequency range. This enables comfortable work with processing intensive applications. Attributes of wireless network interfaces built into PDAs available today vary significantly and require somewhat more thorough analysis. Implemented network adapter can be compliant to b/g standard. From mobile GIs development viewpoint this type of network interface requires no special attention in regard of latency, transfer speed or reliability. It s shortcoming is that it requires WLAN access points network implemented by the company utilizing mobile GIS or some third party. This infrastructure must cover the whole area field work may occur in. This area is in most cases vast and sites requiring field work are not densely packed. This is both technologically difficult and economically inefficient. Second alternative is to use packet data services provided by cellular phone operators. This approach solves the problems like the need for independent access points network. On the other hand it introduces problems of high connection latency, transfer speed and selective service availability and coverage. Cellular phone operators implementing 2.5G and 3G technology offer GPRS speeds of Kb/s and EDGE, up to 384Kb/s. Considering all these characteristics of cellular packet data transfer services network subsystem of mobile GIS should be designed to minimize the amount of data being transferred as well as to be able to adapt to temporary unavailability of network service. 3 GinisMobile Component Framework for Development of Mobile GIS Researchers in Laboratory for Computer Graphics and Geographic Information Systems (CG&GIS Lab), Faculty of Electronic Engineering, University of Nis, developed component framework GinisMobile as support for development of mobile GIS solutions. This framework represents the result of continuous development and advancement of GIS frameworks intended for rapid development of desktop and Web GIS applications (Ginis and GinisWeb). Well tested GIS concepts of Ginis and GinisWeb have been supplemented with application components from mobility domain [4]. Mobile GIS architecture is somewhat similar to architectures used for Web or WAP GIS solutions [5]. Extended client/server approach is used as basis. The most frequent modification of this architecture which is used in commercial solutions is three-tier model. It consists of presentation layer, GIS logic layer including mobile components and external GIS services layer. Third layer encompasses GIS services like Web Map Server (WMS), Web Feature Server (WFS) etc. The main advantage of this approach is clear separation of functionalities into independent modules, easily upgradeable and substitutable. Grouping of layers in the case of mobile GIS is shown in Fig.1. Fig.1 Layered architecture of mobile GIS Presentation layer is tasked with information visualization (spatial data in the form of maps and attributes in alphanumerical tabular form). This layer also receives user s requests and queries, interprets them and activates corresponding functions in application logic layer. Using adapted HTTP protocol client is supplied with static sections of maps in the form of raster images and vector type spatial data which is XML encoded. XML encoding also contains attribute data. All data the user interacts with and can change is classified and transferred in the vector form. All other data, regardless of its form when stored on the server side, is rasterized and transferred to the client in the form of raster images. Since its role is only auxiliary this is the most appropriate form of visualization. In our usage scenario (mobile GIS) client possesses significant processing power and memory capacity which can therefore be used for performance improvement of this typical layered architecture. That is the reason the client side component in the case of mobile GIS is named usually called rich [6]. Application logic which is physically located at the client device contains portion of GIS functionalities, basic functionalities which can be performed on the client side solely without data transfer to/from server side. Third party GIS services layer (e.g. Web Map Server) is physically located on the server side. Other GIS services belonging to this layer (WMS, Web Feature Server - WFS) are also controlled by GIS functionalities of this layer. WMS is tasked with

3 Visualization User interface Sensor interfaces Segmented raster map Vector layers Commands buffer Context interpretator and integrator XML profile Usage hostory of raster map segments Mobile objects history HTTP POST request Internet HTTP POST response Display style Rule based expert system Creation of raster maps (layers) Profiles <XML> Context information Creation of GIS commands GIS functionalities Coordinates conversion RDBMS Geospatioal Sensors Server side Client side Fig.2 Detailed component view of mobile GIS architecture supplying raster map segments which are used at the client to form continuous georeferenced map. More detailed overview of the mobile GIS architecture with separate components depicted is given in Fig.2. Component Integrator and interpretator represent software interfaces with external devices, mainly sensors. In our scenario these are usually positioning devices (GPS). This component enables field personnel to use GPS to either directly input coordinates of spatial objects or to orient themselves on displayed digital map. Specific class of applications is Location Based Services (LBS) which automatically use information about user s geographic location to adjust its functionality [6, 7]. For the optimization purposes spatial data that is supplied to mobile clients are classified in two groups: spatial data that is static in nature in regard to individual user in question and spatial data that user interacts with, modifies, adds and deletes. Data from the first group are auxiliary in nature and their primary role is user s orientation in space. That is the main reason this class of data is rasterized on the server and separately transferred to the client. Whole continuous raster map consists of regular rectangular matrix of raster map segments. Regarding static nature of these map segments it is optimal to maintain local cache of raster map segments already used at the client. This caching mechanism significantly increases performance of map visualization. The caching mechanism is realized as two level cache. First level is using secondary memory medium to store raster map segments. This is usually some sort of memory card. Since secondary memory has relatively slow transfer time second level of caching is introduced. If we rely on locality characteristic of spatial data it is expectable that the user will request map segments adjacent to map segment that was the last requested. Second level cache keeps last N map segments in client device RAM. The number N is dynamically calculated and depends on currently available amount of memory. Secondary caching algorithm adapts to currently available RAM on the client device. When the amount of available memory becomes critically small it removes from secondary cache raster map segments that haven t been used recently. Statistics of map segment usage is maintained and decision which segment is to be removed from the cache is made according to Least Recently Used (LRU) algorithm. Another layer of visualization on the client device screen is vector type. Vector type spatial entities are stored into mobile local storage on the mobile device. Visualization and manipulation of this type of spatial data is handled separately from raster layer. 4 Dynamic creation of continuous georeferenced raster map When being used in mobile, highly dynamic environment, most important feature of mobile GIS is fast and seamless creation and visualization of georeferenced raster map. Use case we are analyzing implies user who is being located by connected GPS device. The goal is to have the system that does not need map precreation and preloading phase as it is usually case with today s mobile GIS solutions. Usage of proprietary raster image formats should also be avoided. Proposed framework is using WMS to supply mobile client with needed raster map segments. There is no need for the end user to technically proficient in the procedures of raster map creation and conversion. Raster maps are created and maintained by specialists and available from central WMS. Web and desktop GIS applications acquiring raster maps from WMS usually repeatedly request and transfer

each raster map segment whenever it is needed. Since it is presumed that network connections are fast and cheap web clients usually have no local caching of raster segments.

4 each raster map segment whenever it is needed. Since it is presumed that network connections are fast and cheap web clients usually have no local caching of raster segments. Priority is to create the thinnest client possible. In scenario we are referring to we have mobile device with significant processing and memory capacity which is using wireless network to access WMS (WLAN or packet data transfer over cellular network GPRS, EDGE ). Apart from relatively speed and high price of such network bandwidth another limiting factor is selective availability, meaning that connectivity can not be guarantied. Therefore, proposed framework contain two stage caching mechanism. Functioning of this caching mechanism was explained in more detail in the previous chapter. Since we have GIS application running on mobile device whose processing power is significantly lower that its desktop counterparts it is imperative to include some raster map segments drawing optimizations. One of the most commonly used methods in GIS is quad-tree raster image resolution pyramid. Since this is well known optimization algorithm we will not go into detailed description of its functioning. The basic idea of the algorithm is to remove repetitive raster image scaling from raster map drawing procedure. Since raster map drawing is the most common operation in GIS and therefore in our proposed mobile GIS and image scaling is one of the most time costly operations this would result in significant increase in application responsiveness and speed. More details about quad tree resolution pyramid creation and implementation can be found in [8]. The basic quad tree resolution pyramid had to be adopted to already implemented two stage caching to achieve full effect. Generally, quad tree resolution pyramid maintains full images at each level of resolution. In our case, it is inefficient to either download images over wireless network for each resolution level or keep these images in primary cache. The role of main caching controller is given to quad tree resolution pyramid component. Upon user changes zoom level (or translates the map) command is processed through command queue and sent to quad tree component. This component, based on current zoom level, selects the most appropriate pyramid level images will be taken from. It is important to note here that primary cache stage (memory card) stores only raster segments in basic resolution (leaf nodes of quad tree). All upper levels are dynamically generated from leaf nodes and temporarily cached in secondary stage cache (RAM). This change reduces the network traffic and usage costs without significant speed degradation. Consequently, quad tree resolution pyramid first checks whether secondary stage cache contains raster images at the needed resolution level. If it does not needed images are created from lower level images if these are available in the primary stage cache. If neither primary nor secondary stage cache contains all the needed images missing ones are downloaded from WMS and included in the caching scheme. Newly created higher level resolution pyramid images are cached in the secondary level cache for as long as the application is active and there is enough RAM available. Merging of quad tree resolution pyramid and two stage cache is shown in Fig.3. Fig.3 Merging of quad tree resolution pyramid and two stage caching Since creation of images belonging to higher level of resolution pyramid is done dynamically costly operation of image scaling is done only first time user views a region. This produces observable delay when the map is drawn for the first time but removes the need to transfer data over network that is already available on the client therefore minimizing network costs. All subsequent drawings of the same map region is significantly faster since needle resolution level images are available in secondary cache (RAM). This mechanism is illustrated in Fig.4.

Fig.4 Quad tree resolution pyramid Fig.5 shows user interface of realized GinisMobile demo application demonstrating concepts presented in this paper.

5 client application is displaying various raster maps with some of the segments being in the download queue and downloaded by the downloading thread. Fig.

5 Fig.4 Quad tree resolution pyramid Fig.5 shows user interface of realized GinisMobile demo application demonstrating concepts presented in this paper. User interface is page oriented giving map display most available area. Other important pages include GPS sensor configuration controls. Concrete example shown in Fig.5 client application is displaying various raster maps with some of the segments being in the download queue and downloaded by the downloading thread. Fig.5 PocketPC application demonstrating proposed concepts 4 Conclusion Considering the general trend in development of information technologies and computer industry in general we can notice constant migration into mobile and ubiquitous computing [9]. With further development of wireless communication technologies data stored in heterogeneous distributed s will be available at any specific time instant and at any location. The most important issue in grasping concepts of mobile GIS is the understanding that mobile GIS client is mainly extension of centralized GIS. By no means will the professional geographic information system as we know today be substituted, mobile GIS will merely redefine the manner of using GIS. Success of mobile GIS very much depends on support it will be getting from traditional GIS servers. References: [1] Worboys, M. and Duckham, M., GIS: a Computing Perspective, Second edition, CRC press, Boca Raton, Florida, [2] Integrated Mobile GIS and Wireless Internet Image Servers for Environmental Monitoring and Management, [3] Intel XScale technology, /design/intelxscale/. [4] Predić, B. i Stojanovic, D., Integration of Location Based Services with With Web Geographic Information System, YU INFO 2004, Kopaonik, , Proceedings on CD-ROM. [5] Fangxiong, W. and Zhiyong, J., Research on a Distributed Architecture of Mobile GIS Based on WAP, in Proceedings of XXth ISPRS Congress, July 2004., Istanbul, Turkey [6] Predić, B., Milosavljević, A., i Rančić, D., Rich J2ME for mobile object tracking, ETRAN 2005, June 2005., Budva [7] Stojanović, D., Đrođević-Kajan, S. i Predić, B., "Incremental Evaluation of Continuous Range Queries over Objects Moving on Known Network Paths, 5th International Workshop on Web and Wireless GIS, , Lausanne, Switzerland Lecture Notes in CS 3833, Springer-Verlag, pp [8] Scott Mitchell, Ferko Csillag, Christina Tague, Advantages of open-source GIS to improve spatial environmental modeling, Proceedings of the Open source GIS - GRASS users conference Trento, Italy, September [9] Annika Hinze, Agnes Voisard, Location- and Timebased Information Delivery, In Proceedings of 8th International Symposium on Spatial and Temporal Databases, July 24-27, Santorini Island, Greece.

CONTENT MODEL FOR MOBILE ADAPTATION OF MULTIMEDIA INFORMATION

CONTENT MODEL FOR MOBILE ADAPTATION OF MULTIMEDIA INFORMATION Maija Metso, Antti Koivisto and Jaakko Sauvola MediaTeam, MVMP Unit Infotech Oulu, University of Oulu e-mail: {maija.metso, antti.koivisto,