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Sometimes it can be advantageous to measure the y-parallaxes with one of the rotations w; or w,. 
The parallaxes on the scale of the photographs (diapositives) can then be found from simple computa- 
tions in which the projector constant c is used for the flying altitude. 
2.12 The procedure. 
All y-parallax measurements should preferably be performed stereoscopically, since normally the 
stereoscopic setting of the measuring mark is more accurate than monocular setting. From special 
test series of stereoscopic and monocular settings in identical points the most accurate method for 
the y-parallax measurements can be determined by the actual operator. Before starting stereoscopic 
measurements, every operator should “warm up” his stereoscopic vision during about 10— 15 minutes. 
Before the measurements of y-parallaxes the position of the measuring device (by, or by,, ©; or w, 
or the parallax screw of the parallax bar) should be carefully read and recorded in the form; see 
Appendix 1. 
2.121 Ordinary plotting instruments. 
The y-parallax measurements can be performed with translations (by, or by,) or with rotations 
(co, Or ©). Measuring gauges can be applied in order to increase the accuracy of the readings. For 
every point the y-parallaxes should be corrected with the measuring device in at least three details 
close to the ideal point. The three corresponding readings are recorded in the form and the average 
is caleulated. The flying altitude should also be determined for each point. If the y-parallaxes were 
corrected with translations by, or by,, the y-parallaxes are found directly from the difference between 
the original setting of the measuring device (before the corrections of the y-parallaxes) and the 
averages, see the detailed directions in Appendix 1. If the y-parallaxes were corrected with rotations 
€, Or c, the corresponding rotation differences had to be transformed into linear y-parallaxes in the 
model with a simple procedure. The distance of the reading device (measuring gauge) from the 
w-axis and the enlargement from photographs to model must obviously be known for the calculations. 
2.122 Stereocomparator and mirror stereoscope 
In these instruments the y-parallaxes are measured with the parallax screws or with the parallax 
bar in the usual way. In order to correct the y-parallaxes stereoscopically, the photographs can be 
rotated mechanically or optically through 100% in their planes. The y-parallaxes are in these in- 
struments preferably corrected in four (4) details around each ideal point. The details should be 
selected symmetrically in the z- and y-directions around the ideal point. The four corresponding 
readings are recorded in the form and the average is calculated. 
2.2 Computations 
From the recorded data of the y-parallax measurements the following computations can, among 
others, be performed: 
1. The y-parallaxes. 
9. Corrections to the elements of the relative orientation and arbitrary functions of them. 
3. The mean square value of the y-parallaxes. 
4. The standard error of unit weight of the y-parallax measurements after an adjustment of the 
relative orientation according to the method of least squares. 
5. Residual individual y-parallaxes after an adjustment of the relative orientation in accordance 
with the method of least squares. 
6. The radial distortion of the reconstructed bundles of rays (in ordinary plotting instruments) or 
of the measured photographs (in the stereocomparator or in the mirror stereoscope). 
———