Relation Towns2 contains information about towns 
in province B which are passed by second class roads. 
Query 2: Find towns in province B which are passed 
by second class road No. 3. 
Towns3 <- *(a:Towns2,b:Roads2) 
WHERE[(b.No = 3) AND 
(b.Li COVERS a.Pt)] 
[Pt:a. Pt, Name:a.Name, 
Population:a.Population]; 
7. Conclusions 
Spatial information is carried by spatial objects and 
their relations. Points, lines and polygons are three 
distinct types of spatial objects in the two 
dimensional space. Spatial data types are a useful 
mechanism for describing spatial objects in spatial 
databases. 
Information about instances of spatial data types are 
extracted by using functions defined on the data 
types. These functions are used either to extract 
subsets of data or to calculate new data from the 
instances. The composition and combination of 
existing functions can form new functions. 
To check the spatial relations of two objects in the 
database, a few system-defined fundamental 
topological operators are necessary. More operators 
can be built from the fundamental operators and 
other functions. By including spatial operators in 
the query language, spatial analyses can be 
performed as a number of queries to the spatial 
database. 
Implementation of operators and functions 
discussed here as well as others included in the 
spatial query language GeoSAL (Svensson et al 1991) 
is going on at the National Defence Research 
Establishment in Stockholm, Sweden. 
Acknowle dgements 
The author is grateful for valuable comments from 
Prof. Dr. Friedrich Quiel at the Royal Institute of 
Technology in Stockholm, and Dr. Per Svensson 
and Dr. Karsten Jôred at the National Defence 
Research Establishment in Stockholm, Sweden 
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