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Mobile geographic services
Network-based
- Cell Global Identity: Cellular telephone systems divide geography into base station coverage areas typically of several kilometers in size, although in urban areas they can be as small as 10 m. Only the finest resolution of data is of use to mobile geographic service users.
- Uplink Time of Arrival.
U-TOA measures the time it takes for signals to
travel from a handset. This system supports
existing as well as new equipment, but is
expensive to implement because of the high cost
of upgrades to all base stations. Accuracies are
around 100m.
Handset-based
- GPS. The Global Positioning System offers the highest locational data quality. Since the removal of selective availability accuracies of better than 10 m are possible. However, there are some limitations in the use of GPS, especially the requirement for line of sight (especially a problem in urban ‘canyons’), added cost, and the time it takes to obtain a signal. Some systems may be complemented by additional GPS receivers located a fixed positions. This improves location calculation from 20-45 seconds to 1-8 seconds.
- Enhanced Observed Time
Difference. E-OTD triangulates data received
from base stations. This requires that the
location of base stations is accuracy known and
that data signals are synchronised. The accuracy
of this method is estimated to be around 125m.
Fig. 6: GOMCT Tracking.
Geographic application server
A critical part of a mobile GIS is an advanced geographic application server able to provide a range of geographic services. Such a server must offer the following capabilities:
- Rich functionality: The range of applications listed earlier must be supported. At a minimum the following services must be available: high quality mapping, geographic and attribute queries, data download, gazetteer, proximity analysis (i.e. find closest object of a given type), geocoding, and routing.
- Good performance: Performance is critical for applications servers of this type because they must be able to process many requests simultaneously and potentially millions of requests per day.
- Scalability: This includes the ability to deal simultaneously with both very large data sets and very large numbers of application requests (e.g. thousands of users requesting routing across the whole US street database of 37 million streets). It must also be possible to add processing capability without interrupting operations. As a rough guideline a medium sized database is approximately 200 Mb – 1 Gb, a large data base is 1 – 10 Gb and a very large database is greater than 10 Gb.
- Extensibility: For all organizations the transition to mobile applications is very new and no one can foresee long-term future requirements. It is essential, therefore, that application servers can be extended to support new services and increased numbers of users.
- Reliability: Because geographic services need to be available 24 by 7 they must be robust and reliable. The use of commercial off the shelf technology such as standard hardware, GIS and DBMS software configurations provides this reliability.
- Standards-based: Although
wireless systems are still immature and are
developing rapidly, standards such as XML are
beginning to emerge. Building systems based on
these standards will help ensure compatibility
with future systems and applications.
Geographic database
The final component is a geographic database containing the content that will be made available via the geographic services in the application server. Data management and high throughput for large databases is enabled by the use of commercial off the shelf relational database management systems such as DB2, Informix, Oracle or SQL Server. The same software can be used to store and manage both the geographic and associated attribute data.
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