WORKING GROUP 3 
RETZER 
169 
indicators previously determined by field study to be reliable for that area. 
Because of the limitations discussed above, type lines located stereoscopically 
are often more correctly placed than those plotted in the field. Areas of rock 
outcrop, rock slides, bogs, etc., can be precisely located and identified by 
stereoscope without ever visiting the areas in the field. 
The success of stereoscopic preplotting is dependent on the combination of 
four conditions, namely, 1. good, clear aerial photography 2. adequacy of the 
stereoscopic equipment 3. skill of the operator and 4. operator’s familiarity 
with the conditions of the area. 
The most commonly used photography is black and white contact prints. 
Infrared photography is available in places and has been used in one densely 
wooded area. The contrasting leaf colors are of value in helping to keep located 
in the field. In cultivated lands small differences in surface soil are readily 
visible as shades of gray on black and white photography and these are strongly 
relied on to locate type lines. In forested lands the soil surface is rarely ever 
visible on the aerial photo and the operator sees only variations in colors of 
vegetation. This is an indirect clue and may not have importance as an indi 
cator of soil differences. The advances in color photography have interesting 
possibilities but because of cost have not yet been tested in our operations. 
Most photos for field use are reproduced on paper. The use of plastic based 
photo reproduction has many superior qualities for use in the wet climates of 
Alaska and the Pacific Northwest. We anticipate that it will be used much 
more extensively. 
Improved cameras and techniques in the original photographic work are 
of tremendous values in soil surveys and we expect to take advantage of them 
whenever they apply. 
Stereoscopic equipment is our most indispensible and most used tool. In 
the hands of skilled people the stereoscope can greatly reduce the costs of 
surveys by reducing field time and by increasing the efficiency of that time 
spent in the field. In forested mountain lands many features can be seen with 
a stereoscope that are not visible in the field. 
Our equipment for office use ranges from the simple reflector types to the 
scanning types that are mobile in both X and Y directions and with individu 
ally adjustable eye pieces. Variable powered lenses are advantageous for some 
uses. Ease and comfort is important for continuous major operations. Wide 
field visibility is likewise important. The small pocket stereoscopes with and 
without magnification are used for the further study of the aerial photos in the 
field. Often they are mounted on a hardboard which is also used to carry the 
field photos. In mountainous, forested lands the stereoscope is an indispensible 
tool in the normal soil survey. Its value is related to the skill of the operator. 
Many special problems are encountered in the management of mountain 
lands for which no soil information is available at the time and interpretations 
must be largely derived from stereoscopic study of aerial photos. For example,