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PHOTOGRAMMETRIC MEASUREMENTS FOR STAND DATA COLLECTION 
  
J. TALTS 
The Swedish University of Agrigultural Sciences, Stockholm. 
Introduction 
In Sweden stand data for forest management purposes are normally 
collected by field cruising methods. Aerial photographs in scale 
1:30 000 or 1:60 000 are used for making forestry maps in 1:10 000 
or 1:20 000, for delineations of stands and for area determinations. 
The stands are normally 5-25 hectars large. Main species are spruce, 
pine, birch and trembling aspen. 
Forest data are required for the individual stands and not only for 
larger areas. Such data include volum and basal area per hectar, mean 
height, mean diameter and species distribution. These and other data 
can be determined by measurements from aerial photograps. 
Preliminary trials 
Inspired by the trials with large-scale photos conducted by the Forest 
Management Institute in Canada /1/, /2/ and also by other researchers 
/3/ a study of the accuracy in measurements of tree heights, crown 
diameters and diameters at breast height was made. Photographs were 
taken with a Hasselblad (MK 70) measuring camera. An attachment was 
used which allowed the camera to be fixed to the wing strut of a Cessna 
150 or Cessna 172 aircraft. A number of sample plots were marked out 
on the ground and photographed in colour. The image scale varied bet- 
ween 1:2 000 and 1:4 000. Data for individual trees were collected in 
the field. Tree positions and heights were determined by means of a 
theodolite. 
Measurements of the trees in the photographs were made bv means of a 
stereo-comparator - not a very suitable instrument for this kind of 
measurements. Measurements were made of the tree top, the butt, the 
point at which the shadow of the top was cast on the ground, and two 
points for determining crown diameter. In a few cases, measurements 
of the shadow were also made to determine dbh and crown diameter. 
The subsequent calculations comprised analytical relative and absolute 
orientation and computerized determination of the measured points. 
The following results were obtained from determination of a tree height: 
Image scale Standard error (incl. bias) 
1:2 500 0.34 - 0.54 meter 
1:4 000 0.41 - 0.85 meter 
The standard error in top measurements was twice that in butt measure- 
ment. 
Determination of the tree height by means of measurements of the shadows 
were as accurate as top-butt measurements. À requirement of this method 
is that the stereo model be properly levelled. The same accuracy was