4 
FILE ORGANIZATION — GIS operates on variable length logi- 
ical records composed of fixed length segments of information. 
Up to 255 different segment types may be defined for a file 
and these are organized hierarchically with multiple segment 
types permitted at the lowest level. Up to fifteen levels of 
hierarchy are permitted. Each segment type is composed of a 
predefined collection of fixed length fields. 
Files may be stored and searched sequentially on tape or 
disk or indexed sequentially using disk. Records stored using 
the sequential access method may be variable length. Those 
stored using indexed sequential access are stored physically 
as fixed length records by padding out each record to a maximum 
size. 
Each file is described to the system through a Data Defi¬ 
nition Table (DDT) which defines the physical characteristics 
of each field and the logical arrangement of fields into segments 
and segments into records. It also defines error conditions 
for the content of each field so that automatic error analysis 
may be performed by GIS in the course of file generation and 
maintenance. 
FILE CREATION — GIS files are created by the system from 
fixed fielded input on cards, tape or disk based upon Data Defi¬ 
nition Tables previously stored in the system. It is necessary 
to provide a DDT for both the master file being created and the 
transaction file being used as input. Complex error detection 
procedures may be invoked automatically based upon specifications 
in the DDT. 
FILE MAINTENANCE — A GIS file may 
or deleting complete records , adding or 
an existing record or changing individua 
detection procedures are similar to thos 
creation. 
Two types of file maintenance are a 
largely data directed and requires only 
specification to control it. Here, oper 
forward and are based upon the content o 
The second is language directed and 
logical decisions and arithmetic calcul 
the operation to be performed or to deri 
This mode may also utilize a transaction 
be used to change field values. 
During file maintenance multiple tr 
multiple master files may be involved in 
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RETRIEVAL -- The user communicates with GIS through a 
high level English-like language in which he defines his 
selection, processing and output requirements. Depending on 
the complexity of the operations to be performed, the lan¬ 
guage may be very simple or quite complex and sophisticated. 
In all cases, the language is data independent in that the 
physical characteristics of the' data are deduced by the 
