28 REPORT OF COMMISSION VII 
has limitations which prevent its widespread adoption. Experienced inter- 
preters can extract virtually as much information from good quality black and 
white prints as they can from color transparencies. The former are more gener- 
ally available, photographic results are more certain, their cost is less, and the 
prints are easier to view and to annotate. At present it may be said that color 
photography may have its place in very special projects; but its potentialities 
in general inventory work cannot be exploited until it has been made the subject 
of more research, with resultant improvements in the techniques of procurement 
and use. 
Because of the advantages of having the right kind of photos for the particu- 
lar inventory job at hand, many governmental and private agencies have found 
that it now pays to fly an area to their own specifications, rather than to try 
to depend upon ground surveys, or the use of inadequate existing photography. 
In the United States there is at least one large corporation with large forest 
holdings, which has its own photographic plane, pilot, photographer, and proc- 
essing laboratory. It can select the proper time and climatic conditions for its 
work; the specifications are adjusted to the requirements of each job; the film 
can be processed and printed promptly so as to yield the maximum of usable 
information to the interpreters and photogrammetrists. Such specialization in 
the procurement and use of aerial photographs evidences the recent advances in 
photography and interpretation which make the photo inventory approach more 
economical, as well as more desirable, than the methods based strictly on ground 
surveys. 
Despite the recognized advantages of having a photo project flown to meet 
the needs of a given survey, this system is not economically suited to all projects. 
Large private concerns, such as oil companies, may be able to finance flights 
for exploratory purposes alone. A lumber company may have its cutover lands 
flown for re-inventory purposes. But governmental projects, while possibly 
designed to fill one particular original need, may have to accommodate two 
or more potential needs. If copies of photos are made available to other agencies 
and private individuals, they will be used in many and varied ways. A record 
maintained for one year by one photo distributing agency showed that sales, 
amounting to about 13,000 dollars, were broken down as follows: 
  
Federalavencles... 12. cv +... eei eee re Rn mies 23% 
State, County, and Municipai...…... Vs Le 14% 
Corporations, Utilities, and Irrigation Districts...................... 50% 
Indivieuals ose p ie ds RE IR 13% 
Ta Da 100% 
In Canada, the extensive photo coverage of that country is being used pri- 
marily both to provide maps, and to provide timber inventories. In the United 
States, several public or private agencies may put copies of the photos to use 
for as many different purposes. One may use them to assist farmers in preparing 
conservation plans; another to determine acreages for tax assessment purposes; 
another to prepare flood control plans; and still another to conduct a timber 
inventory. 
TECHNIQUES 
1. GENERAL 
Every user of aerial photographs has his own techniques. Most interpreters 
prefer to work with contact prints under a lens stereoscope of low magnification. 
In range surveys, where only broad vegetation type boundaries are needed, 
the contacts are frequently used without the aid of a stereoscope. This also 
may be used in agricultural or soil surveys. The U. S. Soil Conservation Service