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the fundamental purpose of the system being to store various data files 
for the same geographical area and to access these data files for 
analysis and comparison. In this system based on the National Grid, 
each primary data file contains information for one NGS, usually 500 
pixel codes representing 500 ground cells each measuring 40m x 50m. 
Secondary data files—combinations of primary data files created by some 
of the programs—represent larger geographical areas, but are still 
multi-NGS files. 
The NGS, the ground cell size, the type of categorical file 
(classification, land capability, boundary, or other categorical map), 
and other pertinent information are uniquely defined by a system of file 
names. For example, the file name E390N810.L.AP81 indicates the NGS 
coordinates are E390 and N810, the cell size is large (L = 40m x 50m), 
and the file type is a classification map interpreted from 1981 aerial 
photography (AP81). 
Pixel sizes were selected to be proportionate to the enhanced character 
size of the PET matrix printer. This permits lineprinter maps to be 
printed at approximately 1:10,000 scale when L-sized pixels are used. 
The 32-category classification scheme and other categorical coding 
schemes developed for the project include only capital letters, numbers, 
and other unshifted single keystroke characters (capitals are input 
without shift key operations on the PET). This simplified programming 
operations and decreases the time needed to key in pixel codes. 
Six programs in the system were used to process and analyze the Old 
Machar data: Storage, Boundary Determ, Combomap Anal, Map Change Detec, 
New Land Cover, and Land Capability. 
The Storage program stores pixel characters (pixel codes) input from the 
keyboard in a diskette data file. Each data file created by this 
program represents a categorical data file for one NGS. Elaborate 
prompts allow the user to label the data file with a name consistent 
with the overall system of file names. The user inputs pixel characters 
by rows and edits at two stages--during the initial typing using the 
keyboard editing controls and after examination of the row as reprinted 
by the program. By request this program prints a map of the stored file. 
For the Old Machar study, the Storage program was used to create 21 
boundary map files, 43 classification files for 1966 data, 25 
classification files for 1981 data, and 4 land capability files for a 
selected area of the parish. The remaining data files for 1981 data 
were simply duplicates of 1966 files since no change had occurred in the 
respective NGS. Existing PET disk commands were used to create and 
rename the duplicate files. 
The Boundary Determ program superimposes mapped boundaries on 
categorical data files and combines and stores as one map subdivisions 
of two or more NGS. The width of the PET lineprinter limits the width 
of this retained map to two NGS when L-sized pixels are used. 
The Boundary Determ program was used to superimpose Old Machar Parish 
boundaries on each year's classification data and to create secondary 
data files of contiguous NGS in which non-parish pixels were eliminated. 
Old Machar Parish is contained within an area which covers 8 NGS from 
east to west and 7 NGS from north to south. Therefore, the parish data 
for each year was processed as four map strips, each 2 NGS wide and 5, 
6, or 7 NGS long. The Boundary Determ program was also used to create 
a 4 km 2 land capability data file from the four primary land capability