Reprinted from 
PHOTOGRAMMETRIC ENGINEERING 
Tune 
1953 
PHOTO INTERPRETATION IN FOREST INVENTORIES* 
Karl E. Moessner, Photogrammetrist, Forest Survey, 
Central States Forest Experiment Station 
M! photogrammetrists know how important aerial photos are in prepar- 
ing maps, but few of them realize how much the popularity of this method 
is due to its early use on forest inventories. 
Foresters make forest inventories to find the location and area of timber, its 
volume, species, size, and condition. They use this information in managing, 
purchasing, or selling forest properties. Since forests are usually located in wild 
and relatively inaccessible areas where ground surveys are expensive, some use 
of aerial photos has been advocated for many years. Today many forest in- 
ventories start with flying and interpreting aerial photos. 
Inventories in which every tree on the area must be measured are rare in 
America! sampling is the rule. As in most forms of sampling, the universe must 
first be stratified—that is, broken down into groups or classes. This can be done 
most efficiently by mapping or classifying the forest on aerial photos. 
But before going into detail on classifying forest areas on aerial photos, the 
history of this phase of photo interpretation should be briefly traced. 
HISTORY OF PHOTO INTERPRETATION IN FOREST INVENTORIES 
Because mapping from aerial photos requires some photo interpretation and 
as photogrammetry is used in both, it is impossible to trace the growth of one 
without mentioning the other. Publications of the past 30 years show the grow- 
ing importance of photo interpretation in the specialized field of forest in- 
ventories. 
Although publications prior to the first World War do not mention aerial 
photos, in 1919 a Lt. Lewis suggested in the Canadian Forestry Journal that 
aerial photos be used in mapping inaccessible forest areas and ventured the idea 
that carefully analyzed stereograms could be used as standards for photo in- 
terpretation on such inventories. 
The great acreage of inaccessible forest land in Canada stimulated Canadian 
interest in aerial mapping and in the early 1920's many articles in Canadian 
forest magazines reported this novel use of aerial photos by various pulp and 
paper companies. Articles dealt primarily with preparing mosaics and stressed 
the ease with which commercial forest areas and old burns are recognized. 
Mapping by photos versus sketching from a plane was also discussed. L. A. 
Andrews writing in Canadian Forestry Magazine concluded that estimates of 
damage and distribution of species would be needed before aerial cruising be- 
came accurate. Unfortunately, after 30 years the research needed to assemble: 
the essential data is still incomplete and the information is unavailable to aerial 
estimators. 
In the late 1920's Elwood Wilson reported to the International Silvicultural 
Congress that identification of species, height measurements, forest type classi- 
fication, and even volume estimates were possible on aerial photos by means of 
stereo-interpretation. 
In the early 1930's articles became more specific as to interpretative tech- 
niques and emphasized the advantage of stereo study over the use of a single 
* Prepared for Seventh International Congress. Permission for publication granted by Inter- 
national Society of Photogrammetry. 
1 They have been used extensively in some European forests under intensive management 
and are used occasionally on small tracts in America. 
1