rain 
on 
are 
-he 
wi 
-8))/ 
"fects 
mi- 
yias. 
ing 
lied 
ilts 
ised 
74 
All of the photo pairs used were supplied photographically by 
an equal scheme of dots. The centrum of the scheme was placed 
according to the uniform coordinates with the help of topographic 
maps. The inventory area was first divided into squares of 8km 
x 8km on topographic maps. Half of the squares were taken into 
the sample systematically. Those photos which had the nadirs 
fall closest to the centrums of the squares of the sample, 
were those supplied by a scheme. 
It could be seen that the sample obtained was very close to that 
which would have been resulted if it had been defined on the 
basis of topographic maps, alone. This is because the differences 
in elevation are fairly small in Finland and the photos were 
rectified. On more mountaineous regions or with a photography 
with larger angle, the distortions would have been, of course, 
larger. 
Conclusions 
The application of the principles of cluster sampling is suggested 
when remote imagery is used to inventory natural resources. The 
remote images, air photos for example, are regarded as clusters. 
In the first stage, the clusters must be taken in statistically 
sound way on orthoprojection, with help of topographic maps, for 
example. In the second stage, the clusters are supplied with a 
scheme which defines the cluster elements directly on the scale 
of the image. 
It was shown that the vertical photos serve as an unbias basis 
for sampling in the conditions of cluster sampling described 
above. This conclusion seems to be valid also to tilted air 
photos and images based on scanning. The presupposition to be 
linked to the conclusion is that the terrains in the part of 
image applied should not cover each other in the imaging 
projection. 
In the remote imagery taken with large angle over a rough terrain 
it may easily appear that some low parts of the field are 
shadowed by nearby high terrains. This is true especially at 
the borders of the image. Because of this handicap it may be 
advisable to use only the central parts of images for sampling.