International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, V ol XXXV, Part B-YF. Istanbul 2004
first query parameter in the URL is used to determine which
application the client wants to communicate. The application
name follows the “?” sign. Each argument and value pair is
seperated with “&” sign. Arguments and values are used to call
the functions that perform specific mapping operations. In this
URL address, “Name” is the argument and “MapServiceName”
is the value. Arguments transmit the information to the
application that which function will be executed by the GIS
application. Values are the parameters for these functions.
The GIS application transmits an HTML page to the user for the
request. HTML code is generated in the GIS application by
using WriteString method of WebLink activex control.
Requested map is also sent in HTML code. Before transmitting
the map, it is exported to a bitmap image file by Map Objects.
Since the bitmap image file format is not a compressed one, it is
converted to JPG image file using BMP2JPG method of
WebLink activex control (Selcuk, 2001).
Microsoft Internet Transfer Control and a Timer Object were
also used as components to update the current earthquake
information. The timer control that runs at intervals is used to
update the current earthquake information. In every ten
minutes, the application connects the URL address using the
Microsoft Internet Transfer Control. After the connection, the
text file that contains current earthquake information is
downloaded from the web page and converted to shape file.
The application is easy to use and it provides a valuable
resource for accessing spatial datasets. On the client side of the
application, a web browser and a network access only is
needed. Client side of the application is shown in Figure 7.
e
d
T^ Went) Eohas ar
Voie
E repeiates Planen
“current
Earthquakes
waa |j. Peu DONT j
ross) A
|: zb 2]
D o6 s
Cunanerhuske Lit J
rar]
Figure 7: Client side of the application (Web page)
The map is at the center of the page. Map Layers are located on
the left and the legand is on the right side of the page. A
thumbnail map marks the displayed zone with a red rectangle
Combo boxes, check boxes, and radio buttons are used for user
interaction. Geographic features can be identified by clicking
on the map and a query can be made by defining criterias.
3. CONCLUSIONS
This study is a web-based geographic information system that
provides access to spatial datasets pertaining to earthquakes in
Turkey. It is designed to allow viewing, querying, and analysis
148
of geographic information. There are two aspects of the Internet
GIS. One is the web-based application and the other is
Server/Client balance. This application is a software-
independent system that users do not have to buy a GIS
software and do not have to read manuals to use it but they can
access GIS data and analysis functions over the Internet. It
offers maximum functionality with the minimum effort.
The drawback in such a distributed system is that data come
from different sources based on different forms and datums
which cause havoc in a GIS. It causes that the web-based
application does not represent an online service entirely. Since
all the spatial information stored and displayed in a GIS depend
on a geodetic datum for their meaning, it is necessary to
compile data before the development of the application. For this
reason, it is online partially. Only current earthquakes layer is
created automatically during the application, the other layers
were prepared before. It means the study has both offline and
online data sources.
The response time for generating maps is related to the map
server configuration (memory, disk, and processor). The map is
also depending on the network connection. It can take a while
to download the maps. In addition, system downtime for
maintenance and updates for a web site is unavoidable. But this
sytem is designed for easy maintenance and repair without
interruption.
An entrance page will be created to make a simple presantation
of the service. This provides users an option for choosing a
specific area of interest to start with. Furthermore, new datasets
and functions will be added to the service.
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