|
Designing a Real-Time Mobile GIS The Mobile GIS applications can be divided into two main groups: system construction and system utilization. The former one concentrates on creating the GIS, such as data collection, data checking, data updating among other aspects. The system utilization aspect concentrates on the use of an existing system, for example, location searching, path finding, and information seeking. In this paper, the utilization issue is the focus. A wireless web GIS application extends the use of Internet GIS. Through the web content provider's service offering and by pointing to a web-browser on a mobile device and then to a web site, the mobile users can perform various functions. This kind of application uses all the advantages of Internet, mobile computing and GIS. This also allows users to interact with GIS data and maps on the Web without owning mobile GIS software and data. Mobile GIS applications mainly serve the public and the business to customer (B2C) services. 2.2 Limitations of Wireless Web GIS Application As wireless web GIS application combines the wireless, Internet and GIS technology, it has all the limitations of the above three technologies. In the second generation of mobile systems, the low bandwidth and low reliability are the major obstacles in application development. These obstacles constrain the applications design. Moreover, the small screen and low resolution of mobile devices degrade the visibility and create a non user-friendly interface, and certain sophisticated and enhanced functions cannot be performed through the Internet, such as three-dimensional analysis. Since the amount of data sent over web is only a quarter to one-third of the amount of corresponding raster data, the vector display has a far greater productivity and faster response. Unfortunately, the mobile web browser and plug-ins do not support vector data. Furthermore, there is no common vector-format in the Internet world. As a result, the mobile web page contents are mainly composed by text, supplemented with images, but of low resolution. Lastly, the server side performs most processing works. Hence, poor database design has downgraded the overall performance. 2.3 Location-based Service Mobile GIS is usually coupled with Global Positioning System satellites (GPS) and wireless communication to facilitate exchanges between the existing spatial server and mobile devices. Position Determining Equipment (PDE) determine the location of the mobile devices in real-time. There are generally two ways to determine the location of a mobile device :
3 Real-Time Dynamic Database A spatial temporal data set is defined as a collection of spatial objects including their whole lifetime (history). Each member in the spatial temporal data set can be treated as a snapshot of a particular spatial object in a particular moment. The notation is: ST={si(t):g(si(t)) A, I=1,2,……,n;0<t<¥} (1) where si(t) refers to a spatial object 'i' at time t. Traditionally, a change of attribute can be defined as the difference between the two elements in the same spatial temporal data set that corresponds to the same object. For example, there is only one building in zone A at time t1. In time t2, however, there are three buildings in the same zone. In a mobile environment, we should focus on the location of the user. At different locations, a mobile user may encounter different objects in different locations. The user also gets a different picture at the same location at different times. The changes of a particular object itself are meaningless unless the 'entire' scene is considered. The spatial temporal change should be described as the environmental change according to the point of view of the mobile user at his current position and time. In Real-Time mobile GIS applications, we distinguish between attribute, spatial and temporal changes. Attribute change comprises the inclusion of only the application-related attributes that are retained. Temporal change includes the alteration of the same spatial attributes at a different time, and parts of the object are discarded if they are invalid for the application. Spatial change is the environmental change while the mobile user is at a different position. After applying the new spatial temporal data set in the original database, a new subset is created and the contents will continue to be updated as the location changes. We term this as a "Real-Time dynamic database". Real-Time dynamic databases consist of the "value-based" relationships where typically, the relationship is specified at a retrieval time and the locations of related records are discovered during retrieval. Therefore, the content and number of records are altered from time and time.  1 Original DB 2 DB with Attribute Constraint 3 DB with Spatial Constraint 4 DB with Temporal Constraint Fig. 1. Spatial-Temporal set concept in spatial data Fig. 1 shows the spatial temporal data set. For experimentation, an assumption is made that a mobile service is requested to find the coffee shop at a particular location. In stage 1, it shows the structure of the original GIS database. Each row corresponds to one spatial object. The attribute constraints are added in stage 2. The relevant fields such as "ID", "Open_time", "Close_time", "District" and "Name" are preserved. "ID" acts as the unique key while "Open_time" and "Close_time" indicate the working hours. "District" is the location identification and "Name" is the label. A spatial constraint is added in stage 3, where all objects within the mobile user's scopes are extracted. For example, the spatial data are kept in the same district of where the user is located. In the final stage, a temporal constraint is implemented and the meaningless information is discarded. For example, even if the buildings are in the area of interest, but do not have the same office hour temporal properties, they can be ignored. As a result, a subset is generated. Physically, the contents in each Real-Time dynamic database are extracted from the original database and stored separately. However, the sum of records in all Real-Time dynamic databases may not be equal to the number of records in the master database. Figs. 2a and 2b illustrate that the number of records varies from time to time. For example, there are 100,000 records in the attribute table and 30,000 are categorized as fast food shops. At time 17:00, there are 30,000 records. However at 23:30, only 5,000 records are stored because they are still within working hours.  Fig. 2(a) Number of records at time 17:00, (b) number or records at time 23:30 Fig. 3 shows the attribute value changes in a Real-Time dynamic database. A particular value, i.e. cost value, is varied from time to time. While there is environmental change, some contents are also changed to reflect the truth. A typical example is the optimum path measurement. Optimum path means the best route from the source to destination path is calculated from all objective factors, such as the traffic condition, the cost to pass, the penalty for the turns, and so on.  Fig. 3. Change in attribute value |