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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004
and atrocious weather. In the area where the wired is untouched,
for example, in a hard, badly, dangerous environment, the
WGIS could still work well.
(2) Wireless applicability. Being compared to the wired
network, the wireless network has a more comprehensive
employment and configure mode; otherwise, it isn't involved in
the situation of stations. Thus, the use of GIS becomes more
flexible. Wireless GIS could be realized the convenient GIS by
clients; it is applied the traditional GIS and prevalent mobile
GIS contemporarily.
(3) WGIS should provide a more extensive GIS service for
users. Location Based Service (LBS) and Mobile Based Service
(MLS) provide a realistic model for our practical world. At
different time and in different locations, the model dynamically
offers distinct information service.
(4) WGIS should improve the security for data transferring. The
wireless GIS is utilized an electromagnetic wave transferring in
the air, instead of cables, to dispatch spatial data. The
frangibility of transferring medium makes the data transfer
easily to be disturbed by the outside environment; meanwhile
the uncertain factors of a transferring process make wireless
data easily to be listened and captured to enough packets by
hackers. Hence, the increase of the wireless security for data
transfer is an important factor for our design.
(5) Transferring velocity of WGIS is restricted by the existing
standard. Although the wireless velocity has been made a great
progress in 21st century, for example the new standard for
wireless, i.e. CDMAX, IEEE802.11a and IEEES802.11g, is
protrusive recent, the velocity of wireless couldn't be exactly
and completely compared with the wired network. From
considering the wired cable transferring, the gigantic GIS data
transferring velocity is hard to satisfy the demands; therefore,
improving GIS transferring velocity is a persistent requirement
for WGIS.
(6) WGIS should solve the low efficiency of the visits of spatial
database exchange. Data organization is still the GIS bottleneck
for accesses of users. In the wireless environment, there also
exist the factors for confining exchange visits, for example
restriction of data access and uncertainty of client location.
Considering the interoperability of GIS data, it is important to
enhance the efficiency of data transfer.
As a consequence of the above strategies, in the wireless GIS,
there is generally a high degree of unavailability; thus, data
transferring is the central issue in this field. Accordingly, there
Will be a significant contribution in the establishment of WGIS
technologies from solving the problem of low data availability
and veiocity depending not only on spreading software protocol
standard, but also improving hardware technology.
4. SPATIAL DATA STORAGE FOR MOBILE GIS
As we all know, the character of GIS data including vector data,
grid data and image data is gigantic and multi-resource. In
mobile environment, for the limitation of transferring velocity,
we should consider appropriate strategy to storage and manage
GIS data in the wireless server.
307
4.1 Mobile computing
Mobile Computing and wireless networks are fast-emerging
technologies to make an environment conducive for ubiquitous
computing. In this environment, mobile users equipping with
compact battery-powered palmtops or laptops need to access
the large volume of GIS data stored in the fixed network
through Mobile Support Station (MSS) by the wireless
communication. In order to realize the GIS data storage in MSS,
adopting the mobile database technology is essential.
Mobile Host
Fig.2 The model of Mobile Environment
Fig.2 presents the model of a mobile environment. A mobile
environment consists of two distinct sets of entities: mobile
hosts and fixed hosts. Some of the fixed hosts, called MSS, are
augmented with wireless interfaces to communicate with
mobile hosts, which are located within its radio coverage area
called a cell. Mobile hosts are connected by wircless
connections to the MSS of the cell where they currently exist. A
mobile host can move within a cell or between two cells while
retaining its network connection. Further, every host and cell in
the system is assumed to be associated with a unique identifier.
The mobile computing environment is a distributed computing
platform with the following differences: the mobility and access
devices of users, frequent disconnection, limited bandwidth and
the mobile resource constrains - limited computational and
power sources.
As a part of a mobile database system, a mobile host acts as a
data client and a data server at the same time. A mobile host, as
a data server, is to support basic transaction operations such as
‘read’, ‘write’, ‘commit’ and ‘abort’. WGIS should deploy
mobile database as data management, and the use of multi-ties,
dynamic replication technology is available.
4.2 Three-ties replication model in mobile database
In mobile computing environment, users can access information
through wireless connections regardless of their "physical
location. In mobile database systems, new features, such as
mobility, disconnection, low bandwidth, high bandwidth
variability, heterogeneous networks and security risks, make
traditional database processing schemes no longer well suited.
Replication is one of the key technologies in promoting the
performance of mobile database systems. In. WGIS mobile
database system, the three-tier replication architecture, which is
shown in Fig.3, is used to solve the synchronization problem,
