mmm" PEN — rm 
respect of standard deviation and systematic error as the measurements 
in mirror stereoscope and micrometer. Nor are the difficulties met with 
in measuring of such character that they can be expected to diminish in 
the bigger instruments. 
The measurements on model 15a in Balplex give an indication of the 
dificulty of obtaining sufficient sharpness from the flying altitudes 
normally used for forest purposes. 
The small extent of the experiment renders it difficult to arrive at 
definite conclusions. However, the very low values obtained in measur- 
ing on model 410 in A 7 are striking. 
Results in Measuring Stand Heights 
In forestry the interest of measuring individual trees is rather limited. 
However, there is a greater need on various occasions to measure the 
stand heights (the height of the tree representing the mean basic area). 
It may therefore be interesting to compare the accuracy obtained by 
such measurements with the accuracy obtained in measuring single 
trees. 
The investigation comprises 20 sample plots representing various 
heights and densities. Some plots were measured before the field visit. 
After control in the field of 4 representative plots the remaining 16 
plots were measured a second time. 
Table 6 shows, as previous investigations [1 and 2] have done, that 
stand heights as a rule were somewhat overestimated. The standard 
deviation shows a considerably greater dispersion in comparison with 
measurements of single trees made on the same model. The cause of 
this uncertainty in measuring seems partly to be the difficulty of ob- 
taining an accurate value for the ground level. It is often necessary to 
measure outside the area or in a gap where the ground level ist too high 
or too low. Further, shadows as a rule disturb the measurements. 
Table 6. Systematic Error and S tandard Deviation in Measuring Stand Heights 
in. Mirror tereoscope (Ocular Magnification 3x). Model 41o, A — 1890 m. 
  
  
  
  
  
  
  
  
  
  
  
Before Field Visit | After Field Visit 
Interpreter Systematic Standard Systematic Standard 
error, m deviation, m error, m deviation, m 
1 — 0.1 : 1.0 +01 + 1.5 
2 — 1.5 zo. +99 +24 
3 4-01 +20 ll + 1.7 
4 HOT ER +06 + 1.7 
5 + 0.6 +21 +06 + 1.9 
6 + L0 8.1 m — 
1 —0.2 + 1.9 — — 
8 1.8 + 8.3 — = 
9 — 00 + 2.5 E = 
10