transparently delivers data to and receives data 
from clients’ GIS environment. 
The ilustration with the vector data shows the 
general approach for handling data in Delta-X. 
This approach, where by a common defined global 
data model is materialized for actual representa- 
tion in local databases management system, is 
extended to handle the various classes of data- 
type required in GIS. Namely, raster data such 
as the arial photo and remote sensed images of 
point, line and area features, structured text that 
are related to spatial objects, free text, digital 
elevation models (DEM) of areas, etc., are all 
represented in the common global schema defi- 
nition. Since some of these data types have in- 
ternationally defined standards of representation, 
e.g., GIF, JPEG, MPEG, SGML, HTML, etc., 
these are maintained as files in their respective 
standard formats, and related to the defined spa- 
tial features through spatial indexes. A number 
of such non-vector data types are related to the 
spatial features using index techniques. 
4.2 Spatial Indexing 
Even over a small area of coverage, maintianing 
all the relevant information for all data types, 
quickly grows into a siginificant large database. 
The density of information is generally non- 
uniform. For example, sparsely populated areas 
have less accumulated data than densely popu- 
lated areas. Delta-X organizes space into hierar- 
chical tesselated regions and maintains two dis- 
tinct levels of index schemes for identifying a fea- 
ture in defined space. The first level of index 
identifies a regular polygonal cell of the region 
of coverage that has been hierarchical tessellated 
into a near uniforn grid. Tbe second level of in- 
dex associates features within each cell with other 
informtion types. The first level uses a quadtree- 
like method of spatial index [15]. The second 
level draws from a number of one-dimensional 
and multi-dimenesional index techniques depend- 
ing on the data-type to be indexed. 
The partitioning of the space into cells (referred 
to as tiles in the literature), by the first level of 
index enables the otherwise large volume of data 
to be accessed and controlled in manageble units. 
By this means, all data of a specific region or 
cell, and controlled by a particular organization 
or agency, can be grouped together. Geographic 
coordinate system (longitude and latitude) forms 
the common and basic coordinate reference sys- 
tem by which regions and features are related 
across multiple databases. The local database 
may derive the actual projection and coordinate 
reference system used in referencing spatial ob- 
jects within that particular environment. 
5 System Functionalities 
In indentifing the functions of the federated spa- 
tial information management systems it is impor- 
tant to note that Delta-X is not a Geographic In- 
formation System. It is only concerned with the 
facilities for managing a network of database that 
are to be integrated in a GIS application. 
The functions desired in such a system is better 
percieved from the stand point of users, system 
administration, and policy makers. The desired 
functions from a user’s standpoint depend on 
whether the users are data providers or data con- 
sumers. Data providers generally require to ad- 
minister a DzServer which they have control over. 
They can then make their databases available to 
others, namely researchers and application devel- 
opers. Since some cost is associated with the com- 
pilation of these databases, data providers would 
normally need some return of their investiment. 
Hence some accounting and billing functions are 
expected. The data consumers require functions 
that assist them to locate readily the databases 
relevant to their applications and to retrieve the 
data into their GIS environment in some desired 
format. 
5.1 Basic Functions 
From a local database management system point 
of view a DzServer is a layered software on top 
of the LDBMS that acts as the agent of Delta- 
X. From this standpoint, DzServer functionalities 
include: 
e registering and de-registering databases; 
e translating user queries to and from the un- 
derlying LDBMS forms; 
e  transfering data between one server and an- 
other; i.e., DzSever-to- Dzserver transfers; 
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