In a special case, a small inventory area may fall on one 
aerial photo (cluster) as a whole. In this case it is probable 
that the classes on higher elevation become overrepresented. 
The mean elevations of various classes, however, are not 
decisive for the error but the elevations on which the class 
borderlines lie. Ifthere are only two classes on a photo the 
borderlines for the classes are common and, accordingly, the 
estimates for class proportions directly on the photo scale 
can be regarded as unbias although not necessarily correct. 
Only, if the borderlines are horizontal, the proportions can 
be regarded as correct. 
Another special case, opposite to the above, would be when the 
sampling is made on a photo where the inventory district appears 
on the photo border and a part of the inventory district remains 
outside of the photo area. In this case the classes on low 
elevation tend to become overrepresented. 
For the applicability of remote imagery to inventory purposes 
it may be of great importance that the sampling directly on the 
images can be regarded as unbias. From the pure statistical 
grounds an orthoprojectional medium, like a topographic map, 
would be preferable because distortions from orthoprojection 
tend to increase the variance between clusters. Cost factors, 
however, often favor the remote imagery. 
Acknowledgements 
This paper was made in collaboration with Prof.Dr.G.Hildebrandt 
in the Abteilung für Luftbildmessung und -interpretation at the 
University of Freiburg and with the financial support of the 
Alexander von Humboldt Foundation. I want to express my sincere 
thanks for these contributions. 
References 
Loetsch, F. and Haller, K.E. 1964. Forest Inventory.Vol.1. 
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Poso, S. and Kujala, M. 1977. A method for national forest 
inventory in northern Finland. Metsántutkimuslaitoksen 
Julkaisuja.