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Designing a Real-Time Mobile GIS

Chen Shanchun and Shi Liangping
Beijing University of Post and Communication, China
csc80@tom.com

Wang Liang
Chinese Academy of Surveying and Mapping


1. Introduction
There is a famous aphorism “ time is money”, this shows the value of time in our daily life. Furthermore, about 80% of all data are somehow spatial related and most users have experiences with maps, being a simple and intuitive kind of visualization for complex spatial themes. This leads to the idea of using map-based visualizations and GIS in time critical applications (e.g. emergency management).

And currently, workforce trends appear to be increasing service mobility and seizing opportunities presented by the Internet. As the Internet usage widely increases, the Internet system becomes more complicated and more difficult to deal with. This creates many network problems. In the past, most solutions were to increase resources on the web server such as hardware improvements, for example through expansion of communication bandwidth. However, these are not low-cost methods or ideal solutions.

In general, the mayor concern of most web sites is the overload on servers. The system bottleneck occurs in four main areas: database, network, application server and web server. The major setback for the system bottleneck originates from the database. So optimizing the existing system such as improving the design of existing databases is an important step to enhance the overall performance.

The database is an essential component in Geographic Information Systems (GIS) and without doubt, a poor design is a burden on performance. In this paper, we concentrate on the optimizing of an existing database. We propose the idea of a spatial temporal data set in wireless GIS applications that can create the Real-Time dynamic database to support the mobile application. A location-based web service is used to illustrate how it works. The objectives are to minimize the process time and to improve the data accuracy. Thus, these can provide an alternative way to enrich the work in mobile GIS applications. In addition, the assumption is made that only a simplified spatial structure will be used for the temporal dimension. The remainder of this paper is organized as follows: Section 2 gives a brief introduction to mobile GIS. Section 3 describes the spatial and related attribute data sets and the real-time dynamic database for mobile GIS. Section 4 analyses the performance of the proposed real-time dynamic database. Section 5 concludes the work.

2 Mobile GIS
Mobile computing is creating fundamental changes by adding the ability to take GIS with you into the field and interact directly with the world around you. Mobile GIS is composed of a number of technologies.
  • GIS
  • Mobile hardware in the form of lightweight devices and ruggedized field PCs
  • Global positioning systems (GPS)
  • Wireless communications for Internet GIS access
Traditionally, the process of field data collection and editing has been time consuming and error prone. Geographic data has traveled into the field in the form of paper maps. Field edits were performed using sketches and notes on paper maps and clipboards. Once back in the office, these field edits were deciphered and manually entered into the GIS database. The result has been that GIS data has often not been as up-to-date or accurate as it should have been. Consequently, GIS analysis and decisions have been delayed.

Recent developments in mobile technologies have enabled GIS information to be taken into the field as digital maps on compact, powerful mobile computers, providing field access to enterprise geographic information. This enables organizations to add real-time (and near real-time) information to their enterprise database and applications, speeding up analysis, display, and decision making by using up-to-date, more accurate spatial data.

2.1 Mobile GIS Application Compare with Wireless GIS Application
Mobile GIS and Wireless GIS is very similar to wire-line Web-GIS, There are three main components common to both:
  • The client
  • The server (combination of web, map and data servers)
  • The network
The main differences are the client devices and the network type (mobile uses wireless communication service providers). The server functions and structures are similar to Internet GIS. Mobile applications are a bit like 'thin client' applications of Internet GIS.

There is an additional element in mobile, which is the location.


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 :
  • A handset-based system that relies on GPS, which is placed at the mobile handset. GPS provides unequalled accuracy and flexibility of positioning for navigation, surveying and GIS data capture, and
  • A network-based system that relies on triangulation of the cellular information, where PDE is at the switch centres
Position-Processing Technology (PPT) can then be used to process, track, manage and help other applications to query and retrieve the location information sent from a PDE.Thus, mobile GIS becomes an essential component to wireless mobility.

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

4 Performance Evaluation
The model discussed in this paper has been simplified with the following assumptions:
  1. Time is one-dimensional and linearly ordered;
  2. Relational databases are used to develop the GIS model;
  3. Connection to the server and data retrieval via an active TCP/IP connection using the Hyper Test Transfer Protocol (HTTP) utilized as data transfer protocol; the Mobile user provides an initial position.
A simplified location-based service has demonstrated how the Real-Time dynamic database works. A mobile user chooses a district, a current location, pass and destination points, and maximum walking distance. Based on the above criteria, the optimum path can be found and displayed on the mobile device. We characterize the performance of this function in the following way. The average response time from the mobile client is measured as the time spent (in seconds) from the moment the query is issued to the moment the results of the query are generated.

4.1 Analysis of the Results
The result is depicted in Fig. 4, which is plotted as a two-dimensional graph to illustrate the results of average response time. The line with square nodes corresponds to the time measurement from the dynamic database, while the line with diamond nodes represents the time measurement from the original database. We observe that the time saved increases as the database size expands. For the number of records, over 60,000, the time required in the original database is three times more than that in the Real-Time dynamic database. It is noted, however, that this is related to the actual time of the query, since the database volume is varied as time changes. The response time is slightly different to the result shown in Fig. 4. Nevertheless, we can easily discover that the time response on the Real-Time dynamic database is faster than the original database.


Fig. 4. Average response time.

5 Conclusions
This paper described a new conceptual model – a spatial temporal data set for mobile GIS and correspondingly a Real-Time dynamic database. In traditional GIS, spatial objects are the primary focus whereas in mobile GIS, the focus has been changed to a particular scene of a particular location at a particular time. The relevant objects are only a subset of the overall objects in the GIS and meaningful to the mobile user for a particular situation – involving location and time. The Real-Time dynamic database is automatically updated to mirror the current world. The application and test were developed using a wireless web GIS environment. We compared the difference between a traditional database with the proposed Real-Time dynamic database in terms of response. From the results we can see that the processing time of the conventional database is proportionally increased with the data size in the database while the response time can be dramatically reduced in the proposed Real-Time dynamic database. This is even more effective for a database with a large data volume. In the Real-Time dynamic database, the response time is minimized and query accuracy is improved. These two advantages will greatly benefit mobile users in their applications.

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