84 
of a strip. In twelve cases scale «jumps » were detected (from the height- and Y- 
differences): 
In strip 2, photo 30 (between model 31/30 and 30/31) — 0.5% 0 X 
» » 3, » 52 + 0.4% o X 
» » 3, » 53 + 0.5 % 0 
» » 4, » 78 — 0.5% 
» » 4, » 65 — 0.4% 0 
» » 5, » 88 + 0.4% 0 X 
» » o, » 96 + 0.5% 0 
» » 5, » 99 + 0.5 % 0 
» » 6, » 118 + 0.5 % 0 
» » 6, » 117 + 1.2% 
» » 7, » 132 + 0.5% 
» » 8, » 157 + 0.5 % 0 X 
For the scale-transfer, three points situated close to the principal point and 
marked by small pricks, had been used. In the photographs mentioned above, the 
scale transfer points were often situated in areas covered by trees. This, of course, 
reduces the accuracy of the height-observations. Actually, it was found that in 4 cases 
(marked with a cross, above) the jump could be attributed to a gross-error in the height 
observation of one of the three scale transfer points. [In the computation (van den 
Hout programme), the average height of these three points (without setting a 
confidence limit) had been used]. It is feared that the Stereocomparator operator 
—who has a more than 20 years experience with Wild Autographs and the Zeiss 
Stereoplanigraph—has sometimes recoursed to a wrong procedure, i. e. has set the 
floating mark « upon the ground » on some neighbouring place and then has moved 
the whole carriage until one measuring mark was seen to cover the prick of the scale- 
transfer point, which was marked in (the centre of) one photograph only. It was 
only at the end of the observations, that this tendency of the operator was noticed 
(July, 1961) and checked. A later repetition of the observations of the models 118/117 
-117/116, between which the biggest scale-jump (e. g. 1.2%) had occurred, did not 
reproduce any noticeable scale difference! 
A repetition of the models 119/118 and 118/117, however, confirmed the scale- 
jump earlier found there (0.5%). Obviously that jump should be attributed to local 
errors in the aerial photography (diapositives). 
In the I.T.C.-Jerie block adjustment of the block Massif Central, each « section » 
contained two successive models. Scale jumps between successive models thus pro 
duce errors in the final result only when the two models appear in one section. Besides, 
these errors will mainly have a local character only. Nevertheless, as six of the twelve 
scale-jumps listed above, occurred within a section, scale-corrections were applied 
before starting the block-adjustment. 
As for the heights, each scale jump, of course, produces (mainly) a shift. As a 
systematic procedure, each model was, therefore, given a correctional shift, relative 
to its predecessor, equal to the weighted mean of the height-differences between 
identical points in the common overlap. 
The time needed for the block-adjustment of planimetry was exceptionally high: 
almost 3 man hours per model. This had two causes: