921 
on GIS identified by Smith and 
Peuquet(1985): 
1. find what is at a specified 
location; 
2. find all occurrences of a specified 
entity or subset of entities. 
Functions for output allow users to file 
results, plot maps and generate printed 
output of statistics and related 
diagnostic information about the number of 
nodes involved in operations. 
The interface to the user is by a query 
language which allows users to issue a 
range of commands in abbreviated form. For 
example, if we wish to find the 
intersection of grit and land above 500 
feet, we enter the query: 
grit I N(height300_400 U height400_500) 
where grit, height300_400 and 
height400_500 are the names of quadtree 
encoded files representing specific 
geographic features, and the operators I, 
U and N stand for intersection, union and