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like shape. In northern Minnesota, the apex is often to the southwest with the fan 
opening up toward the northeast conforming to the direction of the hot dry sum- 
mer winds. In the Itasca project, therefore, it was only necessary to determine 
stand ages here and there on the ground. The rest of each age class could be 
mapped out on the aerial photographs from the size of the trees and the configu- 
ration of the old burned-over areas. 
The recognition of tree species on aerial photographs is of vital importance 
in management and inventory work, and much attention has been paid to it all 
over the world (Spurr, 1948; Chapman, 1947; Stoeckeler, 1949; Raup and Denny, 
1950; Schulte, 1951, Komitteen fór skoglig fotogrammetri, 1951 among many 
others). In the sub-boreal forest of eastern United States and southern Canada, 
most of the three species can be distinguished readily of summer infrared or autumn 
panchromatic photographs. In the Itasca survey, as elsewhere, no difficulty was 
experienced in distinguishing pure stands, but the percentage of the various species 
in mixed stands could not be accurately obtained on aerial photographs. Especi- 
ally difficult were pine-fir (Pinus-Abies), spruce-fir (Picea-Abies), and birch- 
aspen (Betula-Populus) mixtures. With careful ground-checking, however, species 
distinctions could frequently be seen and applied in subsequent photo-interpretation. 
The growth estimates required in a management survey must be based upon 
site quality. In recently glaciated country such as Itasca Park, site quality is clo- 
sely related to the land forms of the glaciated landscape. The photo-interpreter 
having some training in Pleistocene geology can usually learn to distinguish the 
outwash plains, ground moraine, eskers, kames, and other features which are 
closely related to forest growth potential. Investigators at Cornell and Purdue 
Universities have done much to add to our knowledge of soll identification from 
aerial photographs. 
The Itasca stand map is being prepared by (1) mechanical radial line trian- 
gulation based upon existing triangulation and highway survey control, (2) ground 
reconnaissance before and during detail mapping, and (3) simultaneous mapping o 
planimetric detail, forest types, and age classes using the Multiscope. The estimated 
time required is one man-hour per photograph for the radial triangulation, two 
man-hours per section (1-mile square) for the field reconnaissance, and two man- 
hours per section for the transfer of detail. The cost including overhead is there- 
fore about 3.9 cents per acre (9.6 cents per hectare of $ 25 per square mile) for a 
detailed map at a scale of 1 : 12,000 showing planimetric detail, forest stands, age 
classes, and principal glacial landforms. 
A variety of methods are available for estimating volume and growth. In the 
Itasca survey, ground survey strips are carefully located on the photographs and 
are run through the principal forest types. On these strips, which are 33 feet (Ap- 
proximately 10 meters) wide, all trees are tallied by species and diameter. In a 
separate survey, growth data are collected on a series of small plots (each con- 
taining about 10 trees) which are also carefully located on the photographs. On 
these plots, tree heights, ages, and growth rates are carefully measured. The 
volume, age, mean diameter, and growth of the various stands throughout the tract 
are to be estimated by visual comparison with the sample strips and plots of known 
volume and growth. Tree diameters may be estimated from visual comparison 
with known stands or by crown diameter — stem diameter correlations (Minot, 
1951; Spurr, 1948).