Aerial photographs have many uses in forestry.

In the management of forest and range lands, aerial photographs supplement and sometimes supplant planimetric maps and ground examinations in locating roads, trails, telephone lines, firebreaks, recreation areas, and other improvements. They are used in mapping and administering timber sales and range allotments and appraising timber for sale. They provide basic reference material for forest-management plans. They are an indispensable aid in certain types of forest research, such as country-wide forest surveys. They record forest conditions at a given time and place, and supply the basis for essential measurements for classifying timber. If they are supplemented by additional study and measurements of the timber on the ground, the results rate high as a statistic in computing total forest area, volume, and growth; the kind, age, condition, and size of trees; general accessibility; areas of forest depletion by cutting, fire, and disease; and location of the timber in relation to transportation.

They were first used in a practical way during the First World War. Methods of making and applying them expanded greatly during the Second World War. Between the wars, progress was moderate, and possibly the widest application was in planimetric and topographic mapping, with forestry a secondary objective.

Aerial photography is employed in Australia, Canada, the Soviet Union, Europe, Africa, Central America, South America, and the United States, where photographs are used in appraising forests.

The techniques in the United States and probably in other countries are not yet perfected to a point where they fully meet the needs of foresters, but because increasing use is made of the photographs in forestry, study and effort to improve the technique of taking the pictures and interpreting them are going on all the time.

The first use of air photographs in the United States probably was in 1917 in mapping part of the Columbia National Forest in Washington. During the past two decades about two-thirds of the United States has been photographed from the air—often called "flown." But on at least half of the area, the pictures are more than 8 years old.

That is unsatisfactory, because forests are changing all the time. Timber stands decline in area and volume because of cutting, fire, wind, decay, insects, and other losses. On the other hand, they expand in area, size, and volume through growth. Because of the changing factors, the old photographs may not correctly show the current condition of the forests. They may, however, correctly represent old growth timber areas and topographic features, such as streams and roads that have not changed materially. It is important to have up-to-date photographs in timber surveys and periodically—say every 5 or 10 years—to refly areas where the forest cover has changed significantly. Although old aerial photographs still have high value for engineering purposes, new pictures are preferred.

Another problem is the scale. Foresters require a larger scale for resource studies than engineers need for their work. So far, it has not been possible to agree on one scale that would be best for both kinds of work. Experienced engineers and foresters feel that (considering costs and usability) the best arrangement would be to have a special kind and scale of photographs for forestry and another for engineering purposes. Actually, that is only a part of the problem; the other part is the need for improvement in the technical aspects of taking photographs (such as the best kind of film, the scale, season of the year, focal length of camera), and concerted efforts to adapt the pictures to the major use for which they are being taken.

The scale of usable photographs in forestry work is somewhat restricted in range—from 1:12,000 to 1:22,000. A scale of 1:15,840 is commonly preferred; the figure means that 4 inches on the photograph covers 1 mile of forest on the ground. Because the scale is the factor that primarily controls the relative size of the objects that appear on the photographs, its selection is important and must be adjusted as far as possible to the purpose for which the photographs are taken and the allowable cost. Some caution in using scales to determine distances on photographs is necessary, because changes in altitude of the plane, its tip or tilt, and variations in elevation of the country being photographed may introduce errors of, say, 10 percent in area determinations on individual photographs.

The two general types of aerial photographs are verticals and obliques. Vertical photographs—taken with the camera in as nearly a vertical position as one can keep it in a fast-flying airplane—are preferred for forest surveys, topographic and planimetric mapping, and on-the-ground forestry practices. Oblique photographs are taken with the camera intentionally inclined to the vertical at a given angle. They cover large areas at a low cost and are sometimes used for rough mapping.

The best type of film for forestry photographs from the air is still to be determined and perfected. Three types now used are panchromatic, infrared with various filters, and color films.

Panchromatic is most common, but fails to meet fully the foresters' needs in differentiating between forest types and species of trees. The infrared film, with a minus blue filter, has produced photographs showing an improved contrast between species and forest types in summer pictures, but needs further trial tests and experimentation. Color film has not been tried over a large area. In theory, it looks good for identifying species of trees, particularly for hardwoods in the fall when seasonal coloring of the leaves is at its height.

Besides type of film, the season best suited to bringing out forest characteristics is important in interpreting the aerial photographs in surveys or other economic and management investigations. For forest-survey purposes, spring and fall are believed to be the best seasons for photographing forests.

As a first step in interpreting the data, foresters usually examine overlapping pairs of aerial photographs under 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 objects 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 planimetric 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 attached to a tabulating indicator. To get the forest-land area, the result is multiplied by a conversion factor adjusted 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 forest land. It provides the basis for computing 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 delineate 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 density; "A" indicates agricultural land, and "N" noncommercial forest land. The classification is usually done by examining the aerial photographs under a stereoscope. The area of the different classes is determined by one of the methods listed previously.

For rough exploratory work on new areas or for checks on previously surveyed tracts, volumes per acre are sometimes estimated from the photographs 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 approximate 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 ephemeris, time, and latitude and longitude of the tree are needed.

Another method is to use one of several stereoplotting instruments, which measure the difference in parallax between 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 parallax 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 distance. 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 factors in making volume estimates. Some tests have been made with varying success. The aim is to find a method of inventorying timber from aerial photographs which requires only a minimum of costly supplemental ground work. In somewhat oversimplified terms, that means the ability to identify tree species, to measure diameter, height, and width of crown, and to determine factors of tree condition, such as soundness, quality, and thrift, on aerial photos with accuracy and adequacy. The results could them be applied to special tables to get volume, quality, defect, and possibly growth, without any on-the-ground measurements.

To summarize: We need to know much more about taking and reading aerial photographs, but present techniques are good enough to aid greatly in the Forest Survey and to meet emergency needs for a quick inventory.

An example is the inventory of the forest fire in Maine in 1947, when 220,000 acres burned over in a few days and a critical situation developed because it was felt that the fire-killed timber had to be utilized within a year before insects and storms could destroy it. A map and timber inventory to show the location, kind, and volume of the timber was immediately needed to aid in the necessary salvage plans. The area was flown, maps were prepared from the photographs, ground plots were measured, and reports made ready in only 8 weeks.

Charles F. Brannan

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