e in procuring data from other agencies, user must 
purchase all the information compiled for that map 
sheet and extract the feature relevant to the 
applications, and 
eo large volumes of datasets are currently not 
maintained with any DBMS. They are maintained 
simply as large sequential files on reels of tapes. 
These and other related problems are what the federated 
multi-database spatial information management system 
(code named the Delta-X project) is concerned with. Such a 
model of operation is not exclusive to GIS. Similar 
problems have been addressed in purely corporate database 
environments. The Delta-X is a multi-database system with 
a common integrated global conceptual schema definition. 
The global database is intended to achieve interoperability 
between DBMS that have been specialized for three distinct 
data types: spatial data (vector and raster), structured text 
and free-text. Our design provides an integrated access to 
data stored in relational databases, object oriented 
databases, simple file systems and information retrieval and 
document management systems. The current implementation 
does this through Remote Procedure Calls by formulating 
queries in the format of the target systems. Where the target 
systems are relational DBMS, SQL is used as the language 
for remote data access. Simple file systems are mapped into 
relational tables. Thus, Delta-X establishes interoperability 
between relational DBMSs and GISs. 
To provide a user accessible data warehousing user 
assistance for inventory and metadata review, a 
complementary system: MetaView/GIS Spatial Browser 
(MV/GIS) was developed as a front end to the Delta-X. The 
MV/GIS and Delta-X systems are the basic component of 
our Division's spatial data infrastrucure and provide a 
solution to Geomatics Canada data warehousing. 
2.. THE DELTA-X SYSTEM 
2.1 The Delta-X Common Spatial Data Model 
This is the conceptual data model into which other GIS 
internal representations can be mapped. For vector data, the 
Delta-X spatial data model maintains up to degree four 
topology. ^ This global conceptual scheme forms the 
intermediate transition schemes for data exchange between 
different GISs. The global conceptual data model has a 
mapping onto either a relational database, as a collection of 
relational tables, or into an object-base as a colony of 
categories. A Delta-X server maintains the vector data in a 
relational database if the underlying database management 
system is relational, e.g., Ingres, Oracle, Informix, etc. 
Similarly, the Delta-X server retains the data in a set of 
equivalent category classes in an object oriented database, if 
the underlying DBMS is object-oriented, e.g., ODE, 
ObjectStore, Objectivity. The significant idea is that Delta- 
X server transparently delivers data to and receives data 
from clients' GIS environment. 
The illustration 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 representation in local databases 
2 
management system, is extended to handle the various 
classes of data-type required in GIS. For example, raster 
data such as remote sensed images, structured text that are 
related to spatial objects, and free text are all represented in 
the common global schema definition. Since some of these 
data types have internationally 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 spatial features 
through spatial indexes. A number of such non-vector data 
types are related to the spatial features using index 
techniques. 
2.2 Spatial Indexing 
Even over a small area of coverage, maintaining all the 
relevant information for all data types, quickly grows into a 
significant large database. Delta-X organizes space into 
hierarchical tessellated regions and maintains two distinct 
levels of index schemes for identifying a feature 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 unicorn grid. The second level of 
index associates features within each cell with other 
information types. The first level uses a quadtree-like 
method of spatial index. The second level draws from a 
number of one-dimensional and multi-dimenesional index 
techniques depending on the data-type to be indexed. 
The partitioning of the space into cells (tiles), by the first 
level of index enables the large volume of data to be 
accessed and controlled in manageable 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 system 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 
objects within that particular environment. 
2.3 The Delta-X Features and Services 
Delta-X performs multiple client-server roles. First, Delta- 
X servers control the data storage in commercial DBMS 
systems. Second, it is a client-server transaction processing 
system, and handles message and data exchange between 
Delta-X database and/or other specialized servers and Delta- 
X clients. Third, Delta-X performs specialized server 
functions, such as data conversions to and from the internal 
data interchange format. Finally, it acts as a proxy client on 
behalf of GIS systems that produce or consume data - it is 
the source or destination of data in Delta-X transactions 
A single Delta-X server can be accessed by many Delta-X 
clients at the same time. Even a single Delta-X client can 
start many transaction on the same Delta-X server. In our 
current implementation, all conversion processes share the 
same CPU. The Delta-X server architecture, however, can 
also be implemented on a parallel machine or on a cluster of 
workstations on a dedicated LAN, where each transaction 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B2. Vienna 1996 
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