5 
Correction in the PG 2 
In the PG 2, the first term of (5) is taken care of by a <p x2 correction of the value: 
B.p 
(6) 
2R 
The second term is compensated by a correction to the c x2 which is the element in the PG 2 representing the 
plotting principal distance as far as the x-motions are concerned. 
The required correction is: 
b.B 
dC 
x2 
2R 
The effect of the correction (6) and (7) on the x-parallax is: 
dx + dx 
q> c 
Comparing (5) and (8), the remaining x-parallax still to be corrected is, when c = f: 
The constant part B. c has a value which is of the same order as the error made in centering the diapositive in 
2R 
the photocarrier and can therefore be considered négligeable (IV). 
The remaining correction is during operation only a function of y. In the PG 2, this correction is given automa 
tically during scanning by a special device, attached to the instrument. See fig. 2. 
A plane parallel glass plate (a), fitted in the optic carriage, between image and floating mark, is rotable around 
an axis parallel to the direction of movement of the optic carriage (y-direction) (V). 
Rotation of (a) around its axis causes an x-parallax. During scanning in y-direction the glass plate rotation is 
controlled by a curved lineal (b) which has a fixed position on the main frame of the instrument. 
A lever system (c) serves as contact between the lineal (b) and the glass plate (a). The radius of the lineal is 
variable. The required curvature can be set by rotating the lineal around its axis (d). The setting can be read 
on the scale(e).The required value for e can be determined with the help of a simple nomograph. 
The axis of the lineal (d) intersects the curve at distance + and - y Q from the center position. 
When e is set at the proper value, an x-parallax correction is obtained according to (9), apart from the constant 
value B. c which we considered négligeable. 
2R 
Advantages of the method 
The method as described above has obviously the following advantages: 
1) No effect on y-parallaxes 
2) Applicable for all scales and base-height ratios 
3) Independent of terrain elevations, as can be seen from (5) 
4) Minimum of planimetrie influence by the favourable choice of the reference plane.