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Concerning Accuracy in Measuring Tree and 
Stand Heights 
Bulletin from the Comittee of Forest Photogrammetry 
By Hugo Ericsson 
In a previous survey issued by the Committee of Forest Photogram- 
metry (Hans Axelson: “Effect of Photo Scale on the Use of Aerial 
Photographs in Swedish Forestry”) the author chiefly discusses purely 
photogrammetric errors influencing the accuracy in measuring tree and 
stand heights, i. e. tilted camera axis, height differences in the terrain, 
varying altitude, accuracy in adjusting the measuring micrometer, and 
errors due to the movement of the trees in windy weather. 
In this survey will be treated errors caused by the difficulty in identi- 
fying the ground level and the tree tops. These difficulties are above all 
due to different kinds of photographs, local height differences, and the 
shape of the head of the tree. A comparison is also made between mea- 
surements performed on different scales, on paper prints and ne- 
gatives and by means of different instruments (mirror stereoscope, 
Balplex and A 7). 
All aerial photographs and measurements in Balplex and A 7 have 
been made by the Geographical Survey Office. 
Planning of the Investigation 
Picture Material 
For the investigation have been used pictures from a test field in the 
northern part of Upland, taken in the spring of 1956 from an altitude of 
3 120 metres and in the summer of 1957 from varying altitudes and un- 
der various conditions of illumination (see table 1). The photographs 
were taken with a camera Wild Rc 5a (c 15,3 cm). The flights of 1957 
were all in the same track with the orientation N. W.—S. E. and a pho- 
tographic extension of 80 %. For the stereo-interpretation every second 
picture in each track was used. By selecting the pictures it has been 
obtained that the areas for measuring single trees were on the whole 
equally placed on all the models. 
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