lesser degree of magnification (ocular magnification 3x, linear magni- 
fication 6x). The systematic error obtained by this measurement was 
— 2.* m (ocular 8x: — 2.4) and the standard deviation + 0.80 m 
(ocular 8x: == 0.99). The standard deviation was thus somewhat re- 
duced, while the systematic error was unchanged. According to the int- 
erpreter as well ground as head appeared more distinctly at an optical 
magnification of 3x. 
Discussion of the Results Obtained in Measuring 
The results of the measurements indicate that the negative systematic 
error to be expected in measuring tree heights will be considerably 
greater on prints without shadows than on prints with shadows. In spite 
of the fact that the flying altitude is lower for the pictures with shadows 
the difference is striking. 
An explanation of the low values obtained in measuring on prints 
without shadows is perhaps to be found in the fact that the contrast 
between the light top of the tree and the ground is very low or insigni- 
ficant. The adjustment of the measuring mark will thus be directed 
towards the level of the head where the mass of branchwork and need- 
les is so dense that a contrast to the ground can be perceived. As the 
shape and density of the head may vary greatly from tree to tree the 
level of the head perceived by the interpreter will vary from case to 
case. The dispersion of the values is also much greater on the prints 
without shadows than on the prints with shadows. 
The results show, further, that the magnitude of the systematic error 
to a great extent depends on the interpreter. 
Repeated measuring on one of the models without shadows indicates 
that the systematic error of the interpreter is relatively constant in this 
type of print, whereas repeated measuring on the print with shadows 
shows that the systematic error, as it is here defined, may vary from 
one measurement to another even when performed by the same inter- 
preter. 
The results from models 410 and 15a are very good. It has thus been 
possible satisfactorily to identify the tops of the trees, which shows that 
the photographic quality of the prints was good. The errors are con- 
siderably smaller than the errors recorded in earlier investigations (1 
and 2). The diference between the systematic errors for models 410 
and 15a is about 1 metre and was not altered, when the measurement 
was repeated. The lower values in measuring on model 15a may on the 
whole be referred to differences in the rendering of details due to diff- 
erent flying altitude. 
The limitations in the accuracy of the measurements due to the 
smallest visible detail on the prints and the ground cover ought to result 
in negative systematic errors. This was also the case, except for inter- 
preter 2 on model 410 (systematic error -- 0.0). An examination of 
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