Picturing Forests from the Air 



681 



As a first step in interpreting the 

 data, foresters usually examine over- 

 lapping pairs of aerial photographs un- 

 der a stereoscope; the effect is about 

 the same as if a person were suspended 

 over a timbered area so as to be able 

 to see the three dimensions of the ob- 

 jects below. From this vantage point, 

 the forest types and stand-size classes 

 are identified and often delineated on 

 the contact print. The information can 

 be plotted on a good map by a number 

 of methods. One simple plan for flat 

 country is to transfer the forest-type 

 boundaries and other timber data by 

 using a divider and a scale. Another 

 method is to use the new radial plani- 

 metric plotter, which helps correct for 

 differences in elevation of the area 

 being mapped. 



The area of the forest land can be 

 determined directly from the aerial 

 contact prints or from a forest-type 

 map by one of several methods. One 

 way is to measure the forest area by 

 means of a planimeter, a mechanical 

 device for measuring the surface area 

 on a map by following the boundary 

 of the forest land with a pointer at- 

 tached to a tabulating indicator. To 

 get the forest-land area, the result is 

 multiplied by a conversion factor ad- 

 justed to the scale of the map. 



Another method, called "counting 

 dots," is to put a clear acetate sheet, on 

 which are regularly spaced dots, over 

 aerial photographs or a map and then 

 count the dots that fall on and off for- 

 est land. It provides the basis for com- 

 puting the percentage in forest land; 

 the figure applied to the total acreage 

 of the tract in question gives the area 

 of forest land. 



Another step is to classify and de- 

 lineate the timber according to forest 

 type, tree-size class, and density. For 

 that, a code has been developed. 

 "P5d," for example, means pine type, 

 intermediate saw timber of good den- 

 sity; "A" indicates agricultural land, 

 and "N" noncommercial forest land. 

 The classification is usually done by 

 examining the aerial photographs un- 

 der a stereoscope. The area of the dif- 



ferent classes is determined by one of 

 the methods listed previously. 



For rough exploratory work on new 

 areas or for checks on previously sur- 

 veyed tracts, volumes per acre are 

 sometimes estimated from the photo- 

 graphs for each stand-size class of 

 timber, such as saw-timber areas and 

 pole-timber areas. For a more exacting 

 timber inventory, it is considered 

 best to measure a number of sample 

 areas say one-fourth acre in size in 

 each stand-size class on the ground to 

 provide a factor for computing the 

 total volume of the area under study. 



The height of trees is sometimes used 

 to separate forest areas into height 

 classes by forest types. Several methods 

 can be used to measure the approxi- 

 mate height of trees as shown on the 

 photographs. One is to measure the 

 length of shadows and compute the 

 height of the corresponding trees by a 

 rather simple formula. A solar ephem- 

 eris, time, and latitude and longitude 

 of the tree are needed. 



Another method is to use one of sev- 

 eral stereoplotting instruments, which 

 measure the difference in parallax be- 

 tween the top and the base of a tree. 

 This factor, when it is correlated with 

 the height of the plane above ground, 

 the length of the air base, and the focal 

 length of the camera, gives the height 

 of a tree. 



Still another instrument is the paral- 

 lax wedge. It is a simple device that 

 has two converging lines etched on 

 glass or other transparent material ; one 

 of the lines has marks to indicate dis- 

 tance. When used with a stereoscope 

 and overlapping pairs of photos, the 

 lines converge into a single sloping line 

 that makes it possible to determine the 

 height of trees. All these methods are 

 considered precise enough to place 

 most timber in 10-foot height classes 

 with reasonable consistency. 



Efforts are being made to use timber 

 height and density as controlling fac- 

 tors in making volume estimates. Some 

 tests have been made with varying suc- 

 cess. The aim is to find a method of 

 inventorying timber from aerial photo- 



