The above method of measurement provides independent values of tip and tilt for 
each vertical photograph. Consequently, summation of errors in ^ and to is 
eliminated. 
As the forward and aft horizon photographs record the same segment 
of their respective horizons when on the same flight line, it is possible to 
choose a reference picture for the determination of ^ at intervals of 20 to 25 
pictures. For the determination of W , it is advisable to choose reference 
pictures at 8- to 10-picture intervals because the viewing angle of the camera 
is only 39°. We have successfully measured differential tip and tilt and 
) using photographs of different flight lines flown on the same day. 
The determination of the differential tip and tilt for horizon 
photos of an average quality can be repeated with a precision of il c . 
2.4 Determination of Absolute and 
To determine the absolute values for tip and tilt, it is desirable 
that at least one stereo model in each flight line be leveled on ground control 
points or on water bodies. The minimum ground control requirement is that at 
least one stereo model in a group of flight lines flown on the same day be fully 
controlled in elevation. After this stereo model is carefully leveled, the tip 
and tilt of each photograph making up this stereo model is determined in the 
plotting instrument. 
The absolute values of tip and tilt for each successive photograph 
are determined by adding to the absolute 4* and Cc? Q f the controlled photo 
graph, the differences in ^ and ^ computed from horizon photos for each 
vertical photograph of a flight line. In order to check the determination of 
the absolute tip and tilt, we also provide vertical control for the last stereo 
model of each flight line. The results are considered to be correct if a 
closure is obtained with an accuracy equivalent to that of the relative orienta 
tion, usually t2 C to ^3°. 
The values of ^ and determined by the above described methods 
must be corrected for the influence of earth curvature before they can be used 
in a plotting instrument.