34 REPORT OF COMMISSION VII 
+ 
which are based on a tree height-age relationship. Determination of site index 
is nominally a field operation, but when interpretation is carried out concurrent- 
ly with field mapping of site, an interpreter can develop a sense of ocular judg- 
ment with which site classifications can be extended by means of the photos 
into areas not seen on the ground. This method corresponds to that of the 
timber cruiser when he estimates heights and diameters of trees in a ground 
survey, checking himself with occasional measurements. 
Perhaps the most strikingly novel technique relating to U. S. forest surveys 
is the development of aerial volume tables, listed as items b(2) and b(3) in 
Wilson's quotation. Much work remains to be done in developing the best tech- 
niques for preparing volume tables and correlating them with the photo details 
discernible by the interpreter. Yet, the advancements and trials already made 
are most encouraging. The trend is certainly toward increased reliance on use 
of photos in conjunction with statistical sampling methods, with a corresponding 
reduction in the necessary cost and time of field operations. 
One novel use of photos was made in obtaining an estimate of felled timber 
in a logging operation on the Pacific Coast (Anon. 1951). Photographs at a scale 
of 1:10,000 were taken of a total of 900 acres, scattered over a 40 square mile 
area. Four classes were identified on the photos, based on species of timber 
and densities, and areas of each classification were determined. Then, from 
existing ground cruise data and other information, estimates of the volume of 
felled timber were obtained. 
3. AGRICULTURAL AND RANGE INVENTORIES 
The production of food as the most elemental of man's activities is broadly 
divided into the growing of crops and the raising of livestock. The two are often 
integrated activities, but when considered in terms of resource inventories, 
they may justify separate considerations. 
Agricultural inventories essentially summarize the kinds and acreages of 
cultivated or otherwise intensively managed crops. Such inventories may be 
for large geographical or political subdivisions, or they may be for small indi- 
vidual farms. Other objectives in agricultural surveys may include analysis of 
methods of cultivation, and the locating of wild crops of economic importance, 
and of uncropped lands potentially suited for agricultural development. 
Extensive agricultural inventories, conducted with a minimum of ground 
work, make necessary the recognition of agricultural versus nonagricultural 
areas, as well as the recognition of kinds of agricultural crops. In this respect 
there is much similarity to the basic interpretation requirements of a forest 
inventory. 
Identification of agricultural areas on photos is a relatively simple task, 
regardless of the continent or climate or type of agriculture. The common char- 
acteristics are well known: General abrupt contrast with adjacent native vegeta- 
tion; sharply delineated field boundaries (generally in straight lines or con- 
toured curves); obvious cultivation or harvesting marks; regular plantation 
patterns; presence of dikes and terraces; etc. 
As with native vegetation, the more refined determination of kinds of crops 
depends upon the interpreter's having a familiarity with a given locality, and 
his recognition of the size, shape, shadow, texture, tone, and situation char- 
acteristics of each crop (U. S. Navy 1945). J. W. B. Sisam (1947) has some 
excellent illustrations in his publication on “The Usé of Aerial Survey in 
Forestry and Agriculture" which show the differences between various crops in 
different parts of the world. 
Aerial photos of agricultural areas have been used extensively in the United