DIGITAL TECHNIQUES FOR MAP COMPILATION 
235 
(similarly, y and y 2 ) 
The formula for co-variance is: 
cov (x, y) = x • y — * • y 
where 
X' y = X] X iVi 
n 1 
The method of successive parallax approxi 
mations can be illustrated by a description of 
the present program which consists of four 
parts, designated (A), (B), (C), and (D). The 
first program part (A), uses the basic image 
data with a scan resolution of 16 spots/mm 
and produces new digital images of lower 
resolution. The lower resolution permits a 
given photo area to be represented by far 
fewer digits. The other three parts of the pro 
gram perform image matching at three other 
successively higher levels of resolution, ap 
proaching that of the real photo. 
The three parts of the program which per 
form matching are similar in plan. At each 
level of resolution, the shrinking sample area 
is used. Three sample sizes are used in part 
(B), a 12X12, a 6X6, and a 3X3 array of 
spots. Only the two smaller sizes are used in 
the last two parts of the program, (C) and 
(D). All correlation done with images at a 
single resolution is called a “stage” of correla 
tion or matching. Part (T) controls the first 
stage of matching. 
INPUT DATA ! PROCESSED IN: OUTPUT DATA: 
FROM SYSTEM DATA ,8M 7094 SYSTEM gy PRINTER-SCANNER 
•NOT OPERATIONAL 
(6/10/64) 
Fig. 11. Predictive programming system diagram.