ts 
  
5 
all points on all the air bases below a horizontal datum instead 
of below the isobaric surface. Once these distances were known it 
was then a simple matter to add them to the depths of the model 
points below the air base to deternine the depths of the model 
points below an elevated ho orizontal. 
The Smoothing Correction 
It was now necessary to perform à smoothing operation to remove 
small de pth residuals between points common to adjacent models. 
These residuals are occasioned partly by the limiting accuracy of the 
radar altimeter measures, and partly by small residual errors which 
ind the shape of individual models. The smoothing operation 
followed the method of Blachut which will not again be described, 
d the main depth traverses 
At this stage of the proceedings true depths had been computed 
for all points lying ions the central sections of the strips, the 
depths to points off these central sections being still subject to 
errors due to the lateral tilt of the mode 18 The depths, however, 
were not all with relation to the same ho ntal datum. 'In parti- 
cular the tie strips were flown at a e ne lower height than the main 
strips so that there was a large difference equal to about 8,500 ft 
between the datums of the tie strips and those of the main strips. 
The next process was, therefore, to correct &ll depths to the 
reference of the same horizontal datum. The datum of the middle 
tie strip was accepted for this purpose. 
There were common points where the central sections of the main 
strips orossed the central sections of the tie strips. The datum of 
the main strips were thus lowered bodily by subtracting a constant 
amount from all depths on each individual main strip to make the 
depths of these common points the same on the main strips as they 
vere on the central tie strip. When this ad been done the depth 
residuals between the central sections of the main strips and the 
central sections of the North and South tie strips were as follows: 
A 
Along the North tie strip: Strip 
Strip 
Stri p 
Strip 
and along the South tie strip: Strip 
I 
four feet shorter 
thirteen feet longer 
one foot shorter 
eight feet shorter 
ei ght feet shorter 
fo ur feet longer 
five feet longer 
no residual. 
e 
M 
= 
Strip 
Strip 
Strip 
RO IQ 
> 
+ 
M 
LJ 
These residuals were removed by a sinple linear closing procedure, 
The lateral tilt Correction 
at this stage still subject to lateral tilt errors, but these errors 
] places where the central sec- 
1s of the tie strips crossed the ral edges of the main strips, 
the values from the central sections of the tie iris being clearly 
the right ones, Corrections were, therefore, made to the main 
strips at these place 3 the same sections, the 
corrections were proportionately restes back to a value of zero at 
the strip centres. At other sections across the main stri Ps where no 
tie strips had been flown the cor rections were supplied by simple 
interpolation between those sections where tie Siri had been flown, 
It was found, however, that it was not possible to extrapolate these 
corrections beyond the north of the north tie strip or the south of 
the south tie strip and it was necessary to use the coast line at 
these places for application jf the lateral tilt correction in lieu 
of tie strips. 
> 
4 
f 
- 
U 
* 
MH 
ps 
C 
He 
px 
TJ 
ct 
Depths of points off the central sections of the main strips were 
at x temm