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FORESTER'S GUIDE

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AERIAL PHOTO INTERPRETATION

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AGRICULTURE HANDBOOK Siôr^(^"i ñíSURí

U.S. Department of Agriculture

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FORESTER'S GUIDE TO AERIAL

PHOTO INTERPRETATION

T. Eugene Avery

U,S, Department of Agriculture Agriculture Handbook No. 308

Forest Service October 1966

Slightly revised December 1969

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402 - Price 75 cents
This handbook is a revised and enlarged version

of Occasional Paper 156, issued by the Southern

Forest Experiment Station, New Orleans, La., in

January 1957. The text and illustrations have

been broadened to include most of the United

States and parts of Canada.

Dr. T. Eugene Avery, formerly with the Division of Forest Economics Research at the Southern Forest Experiment Station, is Professor of

Forestry and Head, Department of Forestry,

University of Illinois, Urbana, 111.
CONTENTS

Page

Types of aerial photographs 1

Obtaining aerial photography 2

Preparing photographs for stereo-viewing- _ 5

Equipment needed 5

Preparing the photographs 6

Photo scale, bearings, and distances 7

Identifying forest types and tree species. _ 9

Mapping from aerial photographs 11

Measuring areas 24

Tree height, crown diameter, and crown

closure 26

Tree height 26

Tree crown diameter 29

Tree crown closure 30

Aerial cruising 32

Individual tree volumes 32

Stand volume per acre 32

Photo stratification for ground cruising. _ 38

Literature cited 40

lU

FORESTER'S GUIDE TO AERIAL PHOTO INTERPRETATION

This handbook was written as a practical reference on techniques of aerial photo interpretation in

forest inventory. It is for the forester with a

casual knowledge of the subject. While oblique

photographs are occasionally useful for interpretation, this manual emphasizes stereoscopic interpretation of vertical aerial photographs available

from various agencies of the U.S. Department of

Agriculture (TJSDA). Figure 1 (foldout, inside

back cover) is an example of a vertical aerial

photograph.

With training, the forester can use aerial photographs to locate property boundaries and trails,

determine bearings and distances, identify classes

of vegetation, and measure areas. Experience will

enable him to improve the efficiency of forest inventories by distributing field samples on the basis

of photo stratifications. In some instances, he may

even be able to estimate timber volumes directly

from the photographs.

While photo interpretation may make the forester's job easier, there are limitations. Accurate

measurements of such items as tree diameter, form

class, and stem defect are possible only on the

ground. Aerial photographs are best used to complement, assist, or reduce field work rather than

take its place.

TYPES OF AERIAL PHOTOGRAPHS '

Oblique photographs are taken with the axis of

the aerial camera at various angles between the

horizon and the ground. High obliques show the

horizon on the photograph; low obliques do not.

An oblique photograph covers a larger area compared with a vertical photograph from the same

altitude, but its usefulness is limited to fairly level

terrain where the view is not obstructed by ridges.

For this reason, and because obliques do not readily

lend themselves to stereoscopic viewing, they are

seldom used by foresters in the United States.

Vertical photographs are taken with the aerial

camera pointed straight down at the ground, or as

nearly so as feasible. Overlapping exposures in

each fight line permit an interpreter to study vertical photographs three-dimensionally with a stereoscope. Because such prints have become so useful

for mapping and interpretation, the term "aerial

photo" is normally assumed to denote a vertical

photograph and will be thus applied in this handbook.

Composite photography is accomplished either

by several cameras taking simultaneous pictures,

or by one camera having several lenses to provide

multiple exposures.

* Adapted from "Forestry uses of aerial photographs,"

an unpubUshed training manual of the U.S. Forest Service, Pacific Northwest Forest and Range Experiment Station, 1961.

The first approach is exemplified by trimetrogon photography, which uses one vertical

and two oblique cameras. Combining the three

pictures gives photo coverage from horizon to horizon. The system provides relatively inexpensive,

small-scale coverage for reconnaissance mapping

of large or remote areas.

A second technique, used by the U.S. Coast and

Geodetic Survey, employs a special camera having

nine lenses. This multiple lens system has proved

successful for topographic mapping,, but requires

expensive photographic and plotting equipment.

Its specialized nature makes it generally unsuitable for foresters.

Mosaics are composite pictures assembled from

vertical aerial photographs. Individual photographs are cut, matched, and pasted together to

give the appearance of a single large print. A

mosaic assembled by reference to known points on

the ground is called a controlled mosaic ; if ground

control of linear distances is lacking, the mosaic is

called uncontrolled. When carefully assembled,

both types provide good map approximations.

Mosaics are often used as pictorial displays in

offices and public buildings. Professionally prepared mosaics are expensive and cannot be studied

three-dimensionally ; hence their use by foresters is

somewhat restricted.
OBTAINING AERIAL PHOTOGRAPHY

Users of this guide will ordinarily rely on prints

of existent photography flown for public agencies.

Thus there is no need here to discuss specifications

for new aerial photography. Details of flight

planning and photographic contracts are covered

in the Manual of Photogrammetry and the Manual

of Photographic Interpretation (i, 2).^

Sources of aerial photography,—^Most of the

United States has been photographed in recent

years for various Federal agencies. The key to

this photography is available free in map foim

as the "Status of Aerial Photography in the

U.S." Copies may be obtained by writing to :

Map Information Office

U.S. Department of the Interior

Geological Survey

Washington, D.C. 20240

This map shows all areas of the United States,

by counties, that have been photographed by or

for: Agricultural Stabilization and Conservation

Service, Soil Conservation Service, Forest Service,

Geological Survey, Corps of Engineers, Air Force,

Coast and Geodetic Survey, and commercial firms.

Names and addresses of agencies holding the negatives for the photographs are printed on the back

of the map, and inquiries should be sent directly to

the appropriate organization. There is no central laboratory that can furnish prints of all Government photography.

The Agricultural Stabilization and Conservation Service (ASCS) of the Department of Agriculture holds the largest share of the recent aerial

photography. ASCS was formerly known as the

Agricultural Adjustment Administration (AAA),

the Production and Marketing Administration

(PMA), and the Commodity Stabilization Service

(CSS).

Forest industries often have aerial surveys made

of their lands, and ownerships adjacent to or

within such areas may be included. Such surveys

often are an additional source of photography.

The aerial mapping company usually retains the

negatives and must have the written permission

of the original purchaser before it can sell prints.

Films^ season,, and date of photography.—Two

types of black-and-white aerial film are in common use: panchromatic and infrared. Infrared

is usually modified by a minus-blue filter to reduce

extreme contrast and improve image resolution.

Panchromatic photography offers better resolution and lighter shadows but exhibits little tonal

contrast among forest types. Modified infrared

photography presents a maximum of contrast be-

^ Italic numbers in parentheses refer to Literature Oited,

page 40.

tween conifers and deciduous hardwoods, but wet

sites and shadows register in black, thus restricting interpretation. In Western United States,

where coniferous trees predominate, panchromatic

film is more likely to be used. Conversely, foresters managing eastern hardwoods often prefer

infrared photography because of easier forest type

separations. In figure 2, dark-toned pines (A)

are readily differentiated from the light-colored

upland hardwoods (B) on the infrared stereogram, but there is little tonal contrast on the panchromatic exposures. Also, the farm pond at (C)

is immediately seen on the infrared prints, but

may be completely overlooked on the panchromatic photographs.

If measurements are needed for deciduous

species, both kinds of photography should be taken

during the growing season. Infrared is likely to

be flown specifically for forestry purposes and

usually meets this requirement. On panchromatic

photographs taken in winter, conifers can be distinguished from hardwoods, but interpretation of

hardwood stand-sizes is difficult. Modified infrared photography is occasionally available for

tracts near large forest ownerships or National

Forests, but most interpreters will be limited to

use of USDA panchromatic photography.

If there is a choice between photography of different dates, one should consider the most recent.

In regioiis characterized by rapid tree growth and

frequent cutting, the value of photos more than 5

years old is questionable.

Photo scales and enlargements.—Available

aerial photographs range in nominal scale from

about 1:12,000 to 1:20,000. Contact prints are

standardized at the 9- by 9-inch size. Forest industries often contract for photography at 1:15,840

(4 inches per mile), and some areas in Western

United States have been photographed at 1:12,000.

USDA photography is normally at a scale of

1: 20,000 ( 1,667 feet per inch). This is the smallest

scale that nonprofessional interpreters with a

minimum of equipment can use efficiently. Although enlargements of USDA photography can

be obtained, they cost more, yield no additional

detail, and cannot be viewed with simple stereoscopes. Contact prints are recommended for general use, unless enlargements present definite advantages in transfer of photo detail to base maps.

Paper surface^ weighty and contrast.—^USDA

prints are available with either glossy or semimatte

finishes. Glossy prints exhibit a high degree of

resolution and tonal contrast, but their shiny surfaces are difficult to write upon. Semimatte prints

are often preferred because they have a minimum

of glare, an easily marked on "toothed" surface,

and fair resolution qualities.
r "-.

1 . •. ^-'-''--

Figure 2.—Modified infrared (aboye) and panchromatic photography made in

the North Carolina Piedmont during June. Exposures were made simultaneously with a dual aerial camera system. Scale is 1,320 feet per inch.

Prints may be viewed three-dimensionally with a lens stereoscope.

Photographs are available on either single- or

double-weight jiaper. Single-weight paper is suitable for office use, conserves filing space, and is

easy to handle under the lens stereoscope. Doubleweight paper is preferred for field use, however,

because it is more durable and less subject to dimensional changes. Most USDA photography

is furnished on double-weight, semimatte paper

unless otherwise specified. Contact prints may

be obtained on special low-shrink, waterproof

paper at a slight increase in price.

Some USDA laboratories offer three degrees of

print contrast : "soft" for mapping, "normal" for

general use, and "contrast" for timbersurvey work.

The contrast of aerial prints sliould be specified if

a choice is available. The photograpli should present maximum tonal differences between timber

types without loss of image detail in light and

dark areas. Although contrast is limited primarily by the quality of the original exposure, new

electronic printing devices get excellent contrast

from all but the poorest negatives.

Ordering Agricultural Stabilization and Conservation Service photography.—Photo index

sheets or index mosaics, showing the relative

positions of all individual photographs within a

given county (fig. 3), can be examined at local

offices of the Agricultural Stabilization and Conservation Service or the Soil Conservation Service. The number of index sheets per county varies

from one to six or more. Foresters working regularly with aerial photographs may want to purchase a set of index sheets for areas where their

work is concentrated.

To decide wliich photographs to order, outline

tlie desired area on the photo index. Include all

photographs which overlap this area, partially or

completely. Prints are ordered by county symbol,

roll number, and exposure number (fig. 1). Other

items to specify are:

Date of photography—appears in upper left

corner of each print.

Scale—such as 1:20,000 or 1,667 feet per

inch.

Reproduction—such as 9-

prints, double-weight,

with "contrast" finish.

by 9-inch contact

semimatte paper,

3
j s OEt'ARTMENT OF flGmCULTuRE

COMMOOiTv ST»BiLir*TiON «»ViCE

MORGAN - GEORGIA

Figure 3.—Portion of an aerial photo index sheet issued by the U.S. Department of Agriculture. Prints are

lapped in mosaic fashion to form a composite picture of the county. The serpentine white line indicates

that the county boundary is a river.
Photographs of the following States should be

ordered from :

Western Laboratory

Aerial Photography Division

Agricultral Stabilization and Conservation

Service

U.S. Department of Agriculture

2505 Parley's Way

Salt Lake City, Utah 84109

Arizona

Arkansas

California

Colorado

Hawaii

Idaho

Kansas

Photography

available throu^

the remaining I

Louisiana

Minnesota

Montana

Nebraska

Nevada

New Mexico

North Dakota

Oklahoma

Oregon

South Dakota

Texas

Utah

Washington

Wyoming

of Alaska and Vermont is not

rh the ASCS. Photographs of

states should be ordered from:

Eastern Laboratory

Aerial Photography Division

Agricultural Stabilization and Conservation

Service

U.S. Department of Agriculture

45 South French Broad Avenue

Asheville, N.C. 28802

Alabama Maine North Carolina

Connecticut Maryland Ohio

Delaware Massachusetts Pennsylvania

Florida Michigan Rhode Island

Georgia Mississippi South Carolina

Illinois Missouri Tennessee

Indiana New Hampshire Virginia

Iowa New Jersey West Virginia

Kentucky New York Wisconsin

Photo index sheets, usually at a scale of 1 inch

per mile, cost $2.50 for a sheet 20 by 24 inches.

Prices for 9- by 9-inch contact prints, untrimmed

and on single- or double-weight paper, are as

follows :

Quantity

First 25-

Over 25_-

Prioe per print

$1. 25

. 90

Contact prints on waterproof (low-shrink)

paper are available at an additional 50 cents per

print. Quantity prices apply only when an order

is shipped to one address. Enlargements are available at four scales ranging from 1:15,840 on prints

14 by 14 inches to 1: 4,800 on prints 40 by 40 inches.

Prices range from about $2.50 to $8.00 per print.

These prices are as of Oct. 1968, but ¡are subject

to revision periodically. Payment must be made

in advance, preferably by money order, and 6 to 8

weeks should be allowed for delivery.

Ordering Soil Conservation Service photography,—Some areas of the United States, notably

counties bordering the Atlantic Ocean, are not

covered by ASCS photography. In some instances, photo coverage of these areas may be purchased from :

Soil Conservation Service

Cartographic Division

U.S. Department of Agriculture

Hyattsville, Md. 20251

Ordering Forest Service photography,—Aerial

photographs of the National Forests are available

at scales of 1:15,840 (1,320 feet per inch) or

1:12,000 (1,000.feet per inch). Coverage includes

the Chugach and part of the Tongass National

Forests in Alaska, but does not include Hawaii.

Some photographs are on infrared film. The

prices quoted for ASCS coverage will generally

apply to the Forest Service.

Forest Service photography of Eastern United

States may be ordered from :

Chief, Forest Service

U.S. Department of Agriculture

Washington, D.C. 20250

Photographic coverage of Western United

States (Forest Service, Kegions 1-6 and 10) is obtained through the appropriate Regional Forester

as follows :

Region Address

1 Federal Building, Missoula, Mont., 59801

2 Federal Center, Building 85, Denver, Colo., 80225

3 Federal Building, 517 Gold Ave. SW., Albuquerque,

N.Mex., 87101

4 Forest Service Building, Ogden, Utah, 84403

5 630 Sansome St., San Francisco, Calif., 94111

6 P.O. Box 3623, Portland, Oreg., 97208

10 Regional Forester, U.S. Forest Service, P.O. Box 1628,

Juneau, Alaska, 99801

PREPARING PHOTOGRAPHS FOR STEREaVIEWING

Equipment Needed

Equipment considered essential by one interpreter may be of limited use to another; nevertheless, the forester who anticipates a continued use

of aerial photographs will probably find the following list closely approximates his minimum

needs.

Lens stereoscope, folding pocket type.

Parallax bar or parallsix wedge for measuring object

heights.

Engineer's scale, graduated to 0.02 inch.

Tree crown-density scale.

Tree crown-diameter scale.

Dot grids for acreage determination.

China-marking pencils for writing on photographs.

Carbon tetrachloride and cotton for cleaning photographs.

Drafting instruments, triangles, and drafting tape.

Needles for point-pricking.

Tracing table.

Magnetic or spring-clipboard for holding stereo-pairs.

Proportional dividers.

367-547 0—69-
If purchased with discretion, the items can be obtained for less than $250. For a price list of photo

interpretation aids available from the Forest Service, write to the Division of Engineering, Forest

Service, U.S. Department of Agriculture, Washington, D.C. 20250.

Many foresters will already own drafting instruments, dot grids, and pocket stereoscopes. A satisfactory tracing table can be improvised by installing a uniform light source under a glass surface.

Fluorescent tubes are bettpr than incandescent

bulbs because they produce less heat. A sheet of

frosted cellulose acetate between two pieces of ordinary single-weight glass serves well when special

frosted glass is not available.

If the forester uses aerial photographs regularly

and becomes adept in interpretation and mapping,

he may wish to acquire more expensive equipment,

such as a mirror stereoscope, vertical sketchmaster,

reflecting projector, or radial plotting devices.

The Manual of Photogrammetry (1) contains information on such equipment.

Preparing the Photographs

Photographic flights are planned so that prints

will overlap about 60 percent in the line of flight

and about 30 percent between flight strips. For

effective stereo-viewing, prints must be trimmed to

the nominal 9- by 9-inch size, preserving the four

fiducial marks at the midpoint of each of the edges

(fig. 1). Then:

1. Locate the principal point (PP) or center of

each photograph. Aline opposite sets of fiducial

marks with a straightedge or triangle. Draw a

cross at the center with a wedge-pointed pencil,

and make a fine needle hole at the intersection.

2. Locate the conjugate principal points {GPP'^s)

on each photograph—the points that correspond

to PP's of adjacent photographs. Adjust the

stereoscope until distance between centers of

lenses corresponds with interpupillary distance

(usually about 2.5 inches). Arrange the first

two photographs of a given flight line so that

corresponding gross features overlap. Shadows

should be toward the observer ; if they fall away

from the viewer there is a tendency to see relief

in reverse. Clip down one photograph. Move

adjacent photograph in direction of line of flight

until corresponding images on each print are

about 2.2 inches apart. Place lens stereoscope

over prints parallel to line of flight so that the

left-hand lens is over the left photograph and

the right-hand lens is over the same image on the

right photograph. The area around the PP will

be seen as a three-dimensional image. The movable photograph should then be fastened down.

While viewing this area through the stereoscope,

place a needle in the same area on the adjacent

photograph until it appears to fall precisely in

the hole pricked for the PP. This locates the

CPP (fig. 1), though a monocular check should

be made before the point is permanently marked.

Eepeat for all photographs; each will then have

one PP and two CPP's except that prints falling at the ends of the flight lines will have only

one CPP. Ink a 0.2-inch diameter circle around

each PP and CPP (fig. 1).

3. Locate the flight lines on each print hy alining

the PP^s and CPP''s. Connect the edges of the

alined circles with a fine ink line (fig. 1). Because of lateral shifting of the photographic

plane in flight, a straight line will rarely pass

through the PP and hoth CPP's on a given print.

4. Determine the photo hase length for each stereooverlap by averaging the distance between the

PP and CPP on one print and the corresponding distance on the overlapping print. Measure

to the nearest 0.02 inch and record on the back of

each overlap. There will be two average base

lengths for each print, i.e., one for each set of

overlapping flight lines.

Alining prints for stereoscopic study,—A print

is selected and clipped down with shadows toward

the viewer. The adjacent print is placed with its

CPP 2.2 inches from the corresponding PP on the

first print. With flight lines superimposed, the

second print is positioned and clipped down. The

stereoscope is placed with its long axis parallel to

the flight line and with the lenses over corresponding images. In this way an overlapping strip 2.2

inches wide and 9 inches long can be viewed by

moving the stereoscope up and down the overlap

area (fig. 4).

CD

O)

ro FLIGHT

Q: LINE

->

CPP ^ P P

A. PRELIMINARY PHOTO ORIENTATION

STEREO

VIEWING

AREA

PP

O—

V

CPP

a

2.2"->|

PP

—O

B. FINAL PHOTO ALIGNMENT

Figure 4,—Method of alining 9- by 9-inch contact prints

for viewing with a lens stereoscope. Compare this

diagram with figure 1.

6
While tlie photos are still clipped down, the

prints can be nipped into reverse position with the

opposite photo on top. This presents another area

of the overlap for stereo-viewing. To study the

narrow strip between, the edge of one print must

be curled >ipward or downward and the stereoscope

moved parallel to the fliglit line until the "hidden

area" comes into view ( tig. 5).

Figure 5.—Slotted clip board for viewing the "hidden area" of overlapping contact prints with a lens stereoscope.

The area indicated by arrows cannot be stereoscopically studied when prints are lapped as in figures 1 and 4.

PHOTO SCALE, BEARINGS, AND DISTANCES

The vertical aerial photograph presents a true

record of angles, but measures of horizontal distances vary widely witli changes in ground elevations and flight altitudes. The nominal scale (as

1: 20,000) is representative only of the datum, an

imaginary plane passing through a specified

ground elevation above sea level. Calculation of

the average photo scale will increase the accuracy

of subsequent photo measurements.

Aerial cameras in common use have focal lengths

of 6, 8.25, or 12 indies (0.5,0.(5875, or 1.0 ft.). This

information, with the altitude of the aircraft above

ground datum, makes it possible to determine the

representative fraction (RF) or natural scale:

j¡p^ Focal length (ft.)

Flying height above ground (ft.)

The height of the aircraft is rarely known to the

interpreter, however, and photo scale is more often

calculated by this proportion :

A'/''=H

Photographic distance

between two points (ft.)

Ground or map distance

between same points ( ft.) _

Determining scale frotn ground measurements.—

Select two points on opposite sides of the print so

that a line connecting them passes near the PP.

If the points are approximately equidistant from

the PP, the effect of photographic tilt will be minimized. (Tilt results when the camera axis

deviates from the vertical at the instant of exposure. It is often present, but generally not

enough to be readily noticed.) Points must be

easily identifiable on the ground so that the distance between them can be precisely measured,

(las and power line rights-of-way, highways, and

railroads offer clearings where ground distances

can be quickly chained. It is not necessary to calculate the scale of every photograph in a flight
Strip. In hilly terrain, every third or fifth print

may be used; in flat topography, every tenth or

twentieth. Scales of intervening photographs can

be obtained by interpolation.

0-ßce checks of photo scale.—Scale determination from ground measurements is laborious and

expensive; hence other methods should be used

wherever possible. If a U.S. Geological Survey

quadrangle sheet is available, the map distance

can be measured and used in the formula, provided the same distance can be identified for photo

measurement (fig. 6).

Another alternative is presented in areas of flat

terrain where General Land Office subdivisions

of sections, quarter-sections, and forties are visible

on the photographs. Since the lengths of these

subdivisions will be known, they can also be used

as ground distances. A given section may rarely

be exactly 5,280 feet on a side, but determining

scale by this method is more accurate than accepting the nominal scale.

Compass hearings and distances.—Although

flight lines usually run nort.h-south or east-west,

few photographs are oriented exactly with the

cardinal dii-ections. For this reason, a reference

line must be located before bearings can be determined. The method described is used by field

teams on the U.S. Forest Survey :

1. Select a straight-line feature such as a highway,

section line, or field edge and determine its bearing. Draw this reference line on the photograph, extend it as necessary, and record the

bearing (fig. 7).

Figure S.—Determination of photo scale from a U.S. Geological Survey map. Map scale is 1: 24,000 or

per inch. Map distance between X and Y is 2.00 inches, or 4,000 feet on the ground. Photo distance is 4

Thus the photo scale is 4.80^^4,000x12, or 1: 10,000. (Courtesy of Abrams Aerial Survey Corporation.

2,000 feet

.80 inches.

)

8
. Pick a point of beginning (PB) from which

the line of approach to a field location will be

run. This should be some feature visible on

both photograph and ground, such as a fence

corner, bam, road intersection, or stream fork.

Draw a line on the photograph from the PB to

the location, and extend until it intersects the

reference line. Measure the angle between the

two lines with a protractor and determine the

bearing of the line of approach.

3. Measure the distance between the PB and the

field location to the nearest 0.01 inch and convert to feet or chains at the calculated photo

scale. At a scale of 1: 20,000, a measure of 0.01

inch equals 16.67 feet or about 25 links. In figure 7, the distance from PB to the center of the

circular field plot is 1.15 inches, or about 29

chains. Common scale conversions are given in

table 1.

TABLE 1.—Scale conversions jor vertical aerial photographs

Representative fraction (scale)

(1)

7,920__

8,000__

8,400__

9,000. _

9,600--

10,000-

10,800-

12,000-

13,200-

14,400-

15,000-

15,600-

15,840-

16,000-

16,800-

18,000-

19,200-

20,000-

20,400-

21,120-

21,600-

22,800-

24,000-

25,000-

31,680-

Method of calculationFeet per

inch

(2)

660. 00

666. 67

700. 00

750. 00

800. 00

833. 33

900. 00

000. 00

100. 00

200. 00

250. 00

300. 00

320. 00

333. 33

400. 00

500. 00

600. 00

666. 67

700. 00

760. 00

800. 00

900. 00

000. 00

083. 33

640. 00

RFD

12

Chains per

inch

(3)

10.00

10. 10

10.61

11.36

12.12

12.63

13.64

15.15

16.67

18.18

18.94

19.70

20.00

20.20

21.21

22.73

24.24

25.25

25.76

26.67

27.27

28.79

30.30

31.57

40.00

RFD

792

Inches per

mile

(4)

8.00

7.92

7.54

7.04

6.60

6.34

5.87

5.28

4.80

4.40

4.22

4.06

4.00

3.96

3.77

3.52

3.30

3.17

3.11

3.00

2.93

2.78

2.64

2.53

2.00

63,360

' RFD

Acres per

square inch

(5)

10.00

10.20

11.25

12.91

14.69

15.94

18.60

22.96

27.78

33.06

35.87

38.80

40.00

40.81

45.00

51.65

58.77

63.77

66.34

71.11

74.38

82.87

91.83

99.64

160. 00

(RFD)2

6,272,640

Square

miles per

square inch

(6)

0. 0156

.0159

.0176

.0202

.0230

.0249

.0291

.0359

.0434

.0517

.0560

.0606

.0625

.0638

.0703

.0807

.0918

.0996

. 1037

.1111

.1162

.1295

.1435

.1557

.2500

Acres/sq. in.

640

1 Conversions for scales not shown can be made from the relationships listed at the bottom of each column. With

the scale of 1:7,920 as an example (column 1, line 1), the number of feet per inch is computed by dividing the representative

fraction denominator (RFD) by 12 (number of inches per foot). Thus, 7,920-^-12 = 660 feet per inch (column 2). By

dividing the RFD by 792 (inches per chain), the number of chains per inch is derived (column 3). Other calculations can

be made similarly. Under column 4, the figure 63,360 represents the number of inches in one mile; in column 5, the figure

6,272,640 is the number of square inches in one acre; and in column 6, the number 640 is acres per square mile.

IDENTIFYING FOREST TYPES AND TREE SPECIES

Forest types and tree species can often be delineated with greater accuracy and lower cost on aerial

photographs than by ground methods. The degree to which species groups can be recognized

depends on the quality, scale, and season of photography ; the type of film used ; and the interpreter's training. A general knowledge of forest

associations and plant ecology is helpful, but field

experience in the specific area to be mapped is far

more valuable.

A generalized map of forest regions in the

United States is shown in figure 8. The tree

species listed provide the first step in identification, i.e., the elimination of those cover types not

likely to occur in a given locality. The second

step, heavily dependent on a knowledge of local
'â^-- ,;.

Figure 7.—Determination of compass bearing and distance to a field plot on a USDA contact print

1,667 feet per inch.

Scale is

vegetation, is to establish which forest types may

be encountered in the area to be mapped.

Recognition of an individual species, often feasible only on large-scale photography, is normally

the culmination of intensive study. Obviously

the forest interpreter must be familiar with

branching patterns and crown shapes of all important species in his region. Mature conifers in

sparsely stocked stands can often be recognized

on 1: 20,000 USDA photography by the configuration of their crown shadows falling on level

ground (fig. 9). Studying oblique photographs

of trees has been helpful to many foresters.

Recognition of species on vertical photographs

also requires a familiarity with tree crowns as seen

in overhead views (fig. 10). While overhead

crown characteristics are not always apparent on

1:12,000 to 1: 20,000 photographs, they can be

highly useful on large-scale stereograms. Photo

scales as large as 50 feet per inch have been utilized

for tree gjecies identifications in Canada {16, 20).

Aside from shadows, crown shapes, and branching patterns, the features to consider in identifying tree species are photographic texture (smoothness or coarseness of images), tonal contrast, relative sizes of tree images at a given photo scale, and

topographic location or site. Most of these are

rather weak clues when observed singly, yet

together they may be the final link in identification

by elimination.

For some parts of the United States and Canada,

vegetation keys are available aids in tree species

recognition. Keys are useful for training new

interpreters and for reference by more exjierienced

personnel. Depending on the method of presenting diagnostic features, photo interpretation keys

may be grouped into two general classes: selective

keys and elimination keys. Selective keys are

usually illustrations and descriptions of trees in a

specific region, e.g., pines of Florida. They are

organized for comparative use; the interpreter

merely selects the key example that most nearly

coincides with the forest stand he must identify.

Hy contrast, elimination keys require the user to

follow a step-by-step procedure, working from the

general to the specific. One of the more common

forms of elimination keys is tlie dichotomous type.

Here the interpreter must continually select one

of two contrasting alternatives until he progressively eliminates all but the item being sought.

A sample elimination key for identifying northern conifers is reproduced here. It may be used

to identify tree species pictured in tiie large-scale

stereograms in figure 11. In original form, this

Canadian key had additional descriptive materials

and illustrations such as those shown in figures 9

andlO(7<>).

Vegetation keys are most easily constructed for

northern and western forests where conifers predominate, because there are relatively few species

to be considered and crown patterns are rather

distinctive for each important group. By con10
trast, few if any, reliable keys are available for the

highly variable hardwood forests of Southern and

Eastern United States.

While the scope of this handbook does not permit a detailed treatment of all important forest

types throughout the United States and Canada,

examples are given in figures 12 through 18.

Key to the Northern Conifers^

1. Crowns smaU, or if large then

definitely cone-shaped.

2. Crown broadly conical, usually rounded tip, branches

not prominent cedar

2. Crowns have a pointed top,

or coarse branching, or

both.

—crowns narrow, often

cylindrical, trees frequently grow in swamps swamp type

—crowns conical, decidu- black spruce

ous, very light-toned in

fall, usually associated

with black spruce tamarack

—crowns narrowly conical, very symmetrical, top

pointed, branches less

prominent than in white

spruce balsam fir

—crowns narrowly conical, top often appears obtuse on photograph (except northern white

spruce ), branches more

prominent than in balsam

fir white spruce,

—crowns irregular, with black spruce (except

pointed top, has thinner swamp type)

foliage and smoother texture than spruce and balsam fir jack pine

1. Crowns large and spreading,

not narrowly conical, top

often not well defined.

3. Crowns very dense, irregular or broadly conical.

4. Individual branches

very prominent,

crown usually irregular white pine

4. Individual branches

rarely very prominent, crown usually

conical eastern hemlock

3. Crowns oi)en, oval (circular

in plain view) red pine

MAPPING FROM AERIAL PHOTOGRAPHS

Type maps are no longer considered essential by

all foresters, but at times their cost may be justified. A general ownership map showing principal

roads, streams, forest types, and condition classes

may be desired for management planning and

illustrative purposes. Also, in making a photocontrolled ground cruise where precise forest-area

estimates are required, it may be necessary to measure stand areas on controlled maps of known scale

rather than directly on contact prints. This is

particularly important where topography causes

wide variation in photo scales.

The wise interpreter will delineate only those

types that he can consistently recognize. For

maximum accuracy, type lines should be drawn

under the stereoscope. As lines made with a chinamarking pencil are easily removed with carbon

tetrachloride or benzene, this method is recommended for preliminary work. Water-soluble ink

is also suitable. Permanent markings can be made

with drawing ink after types have been verified

by ground reconnaissance.

Uncontrolled maps,—^Where area measurement

is not critical, simple uncontrolled type maps can

be prepared at photo scale by direct tracing. Tract

boundaries are drawn within the "effective area'' ^

^Reproduced by permission of L. Sayn-Wittgenstein,

Forest Research Branch, Canada Department of Forestry {16).

* If 9- by 9-inch photographs with 60-percent endlap and

30-percent sidelap are assumed, alternate photographs wiU

have effective areas of about 7.2 by 6.3 inches.

of alternate prints in each flight strip. Photographs are interpreted under the stereoscope, and

all types and planimetric data are outlined. The

detail is then traced onto frosted acetate or vellum,

from one annotated print at a time. If boundaries

are accurately drawn on interpreted prints to include all of the tract without duplication, the

traced data from adjoining prints should match up

without difficulty. To assure standardization in

type recognition, it is desirable to have one man

perform all interpretation work.

Uncontrolled maps are no more accurate for

area measurement than the photographs from

which they were prepared, but they may suffice

for management plans and illustrations. Where

flat terrain predominates, average photo scales

vary only slightly and tracings may approach the

accuracy of controlled maps.

Controlled maps.—Base maps of uniform scale

may be required for photo-controlled ground

cruises, as precise measures of forest area by type

and stand size are essential for accurate volume

estimates. In flat terrain, average photo scale can

be determined and acreages measured directly on

contact prints, but in steep topography variations

in photo scale require the transfer of forest types

to base maps for accurate area measurement. The

most common way of adjusting variable photo

scales to controlled maps is by constructing "radial

line plots" or by preparing plat sheets from field

notes of the General Land Office. As construction

of a radial line plot is complex and has been ade11
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TA¿I

367-547 O- 13
white pine red pine white spruce

'S*.

jack pine black spruce eastern

hemlock

tamarack balsam fir eastern white

cedar

white birch

(old)

white birch

(young)

aspen

basswood sugar maple red maple silver maple yellow birch

14

beech oak

I

balsam poplar

^n

ash elm

Figure P.—Silhauettes of forest trees. When tree shadows fall on level ground, they often permit

identification of individual species. (Reprinted by permission of L. Sayn-Wittgenstein, Forest

Research Branch, Canada Department of Forestry.)
Black

Spruce

Aspen

^^

f4

Red Pme

White

Birch

Eastern

White

Cedar

Bolsom

Fir

Figure 10.—Vertical views of tree crowns. Compare with

stereograms in figure 11. (Reprinted by permission of

L. Sayn-Wittgenstein, Forest Research Branch, Canada

Department of Forestry.)

Figure 11.—Large-scale stereograms

showing star-shaped white pine

crowns (top), cone-shaped balsam

fir crowns (center), and a red pine

plantation surrounded by a stand

of basswood (bottom). The two

upper stereograms are on panchromatic film at a scale of 50 feet

per inch. The bottom pair is on

infrared film at a scale of 330 feet

per inch. (Reprinted by permission of L. Sayn-Wittgenstein,

Forest Research Branch, Canada

Department of Forestry.)

u
Figure 12.—Summer panchromatic photography in Ontario. In the upper stereogram

(No. 35) stand 1 is balsam fir and stand 2 is black spruce. In the lower stereogram

(No. 66) stand 1 is aspen-white birch and stand 2 is beech. Scale is 1,320 feet per inch.

(Reprinted by permission of V. Zsilinszky, Ontario Department of Lands and Forests,

Canada.)

16
Figure 13.—Infrared stereogram illustrating forest associations in central Alaska.

Types are (1) paper birch, (2) aspen, and (3) white spruce. A portion of the

Alaska Railroad is shown at (4). Scale is about 420 feet per inch.

17
Figure //.—USDA panchromatic stereogram from Del Norte County, Calif. Enclosed trees are primarily redwood, with lesser numbers of Douglas-flr, spruce,

and hemlock. Clear stereo-viewing is hindered by the heavy displacement of

trees, some of which are 250 to 300 feet tall. Scale is about 1,667 feet per inch.

ií^í

¿"sai .X*S tSlR^P^

m

Figure 75.—USDA panchromatic stereograms from Jackson (above) and Clarke

Counties, Miss. At (1) is a natural stand of longleaf and slash pines; loblollyshortleaf pines are pictured at (2) and leafless upland hardwoods at (3). Scale "is

about 1,667 feet per inch.

18
Figure 16.—USDA panchromatic

stereograms from Columbia

(above) and Sevier Counties,

Ark. Two loblolly pine plantations are pictured at (1),

while the mountainous terrain

at (2) supports a mixture of

upland hardwoods and shortleaf pine. Scale is about 1,667

feet per inch.

Figure 17.—USDA panchromatic

stereograms from Bolivar

County, Miss. The fine-textured stand at (1) is willow

and cottonwood growing on a

recent deposition of sand and

silt. At (2) is a stand of bottom-land hardwoods, i.e., oaks,

gums, maples, and other wetsite species. Scale is about

1,667 feet per inch.

19
Figure 18.—VSDA panchromatic stereograms from Lafayette County, Ark. Pure

stands of southern cypress are shown at (1); the irregular and somewhat starshaped crowns of this species can be clearly discerned against the water. At (2)

is a mixture of cypress and túpelo gum, while a pure stand of túpelo gum is

pictured at (3). Scale is about 1,667 feet per inch.

quately treated in several readily available publications (7, J7,18), it will not be reviewed here.

General Land Office {GLO) flats.—M.ost of the

United States west of the Mississippi River and

north of the Ohio River, plus Alabama, Mississippi, and portions of Florida, was originally subdivided under the U.S. Public Land Survey.

Township, range, and section lines are oft«n visible

on aerial photographs. If enough such lines and

corners can be identified, GLO plats can be constructed for use as base maps from field notes

available at State capitals or county offices. To do

this, a GLO plat showing sections, quartersections,

and forties is drawn to average photo scale from

field notes. As many as possible of the same lines

and corners are pinpointed on the aerial photographs, preferably arranged in a systematic fra,mework throughout the forest property. Ownership

maps, county highway maps, and topographic

quadrangle sheets may help to identify such corners, but additional points must usually be found

by taking the photographs into the field. The

accuracy of this method depends upon the number

of grid lines and corners which can be pinpointed

on the aerial photographs. The technique described is adapted from Johnson (7).

When photo interpretation of forest types has

been completed, the detail is transferred to the

plat, one square of the grid being completed at a

time. If the plat and photographs are of the same

scale, transfer can be made by direct tracing on a

light table; where scales differ, a vertical sketchmaster is recommended (fig. 19). By movmg this

instrument up or down on its adjustable legs, maps

can be drawn at scales of seven-tenths to one and

one-half times the scale of the contact print. Tlie

sketchmaster accommodates a single, annotated

print, which is placed face up on the platform

under a large mirror. Photo images are reñected

from the large mirror to a semisilvered mirror in

the eyepiece housing. The semitransparent eyepiece mirror thus provides a monocular view of the

reflected photo image superimposed on the base

map. When the grid points on photograph and

map have been matched, the transfer of detail

becomes a simple procedure (fig. 20).

Topographic and county mapn.—Topographic

quadrangle sheets and county highway maps are

often useful in preparing base maps and identifying features on aerial photographs. When available, 7-V2-ininute quadrangle maps at a scale of

1:24,000 provide excellent base maps, and can be

20
Figure 19.—Vertical sketchmaster for transferring detail from single contact prints to a base map. A semisilvered

eyepiece mirror enables the operator to view photograph and map simultaneously. (Courtesy of Aero Service

Corporation.)

purchased for about 30 cents per copy. Maps for

areas west of the Mississippi River, including all

of Louisiana and Minnesota, can be purchased

from

U.S. Geological Survey

Distribution Section

Federal Center

Denver, Colo. 80225

For areas east of the Mississippi River, including

Puerto Rico and the Virgin Islands, maps may be

purchased from

U.S. Geological Survey

Distribution Section

Washington, D.C. 20240

Maps of Hawaii may be ordered at either address. All orders must be paid in advance. Other

sources of topographic quadrangle maps are the

Maps and Surveys Branch of the Tennessee Valley

Authority, Chattanooga, Tenn.; the Mississippi

River Commission, U.S. Army Corps of Engineers,

Vicksburg, Miss. ; and the U.S. Coast and Geodetic

Survey, Department of Commerce, Washington,

D.C.

County maps, usually obtained from State highway departments, may serve as base maps when

a high level of accuracy is not required. They show

township, range, and section lines, in addition to

geographic coordinates (longitude and latitude) to

the nearest 5 minutes. Tliey are usually printed at

a scale of about one-half inch per mile. Although

bearings of section lines are not shown, such maps

are more reliable than oversimplified plats showing

idealized sections oriented exactly with the cardinal directions. Portions of county maps can be

enlarged to photo scale and sections subdivided

into quarters or forties by proportionate measurement. These maps may fe especially helpful in

preparing GLO plats as previously outlined.

21
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23
MEASURING AREAS

Both aerial and ground cruising require accurate measurement of forest areas. Photographs

offer an inexpensive means of making such determinations. In locations where photo scales are

not appreciably altered by topographic changes,

areas can be measured directly on contact prints.

Greater accuracy is possible if forest types are

transferred to controlled base maps before area is

determined. This procedure is essential for mountainous terrain. On the other hand, if only relative proportions of the forest types are needed

and measurements are limited to the effective

areas of contact prints, no bias results from using

photographs, even when local relief ranges from

500 to 1,000 feet {12,19). (This statement assumes

that, for large tracts, errors in measurement of

areas below the datum plane are compensated by

errors of a similar magnitude above the datum

plane.) Small tracts in rough topography should

be measured on controlled maps. Measurement

techniques are similar in both cases, so the procedures outlined for measuring area on photographs

generally apply to map determinations.

Devices for area measurement,—^^The principal

devices for area measurement are polar planimeters, transects, and dot grids.

The planimeter is relatively expensive and its

use somewhat tedious. The pointer is carefully

run clockwise around the boundaries of an area

two or more times ( for an average reading). From

the vernier scale, the area is read in square inches

and then converted to the desired units, usually

acres, on the basis of photo or map scale.

The transect method is analogous to determining

forest area by a strip cruise in which the chaînage

encountered in each forest type is recorded. An

engineer's scale is alined on the photographs to

cross topography and drainage at approximately

right angles. The length of each type along the

scale is recorded to the nearest tenth of an inch.

Proportions for each type are developed by relating the total measure of that type to the total linear

measure. For example, if six equally spaced

parallel lines 6 inches long are tallied on a given

photograph, the total transect length is 36 inches.

If hardwoods are intercepted for a total measure

of 7.2 inches, this particular type would be assigned an acreage equivalent to 7.2-^36, or 20 percent of the total area. The transect method is

simple and requires a minimum of equipment. For

making rough area estimates on photo index sheets,

special transparent overlays for location of transect lines can be improvised {9).

Use of a dot grid is the preferred method for

determining area on aerial photographs. A dot

grid is a transparent overlay with dots systematically arranged on a grid pattern. In use, the grid

is alined with a straight-line feature to avoid positioning bias, and then dots are tallied for each

24

forest type (fig. 21). Type areas are calculated by

proportions : the number of dots on a given type

divided by the total number of dots counted yields

a percentage value that is multiplied by the total

area to obtain the acreage of the type. If total

acreage is not known, the number of acres per

square inch is determined. This figure is then

divided by the number of dots per square inch to

find the acreage represented by each dot.

Intensity of dot sampling.—^The number of dots

that must be counted for a given accuracy depends

upon the estimated proportion of the total area

occupied by the most important classification to be

recognized. For example, suppose area breakdowns are needed for 10 square miles photographed at a scale of 1:15,840. Estimated proportions of each area classification are as follows :

forest, 25 percent; agricultural lands, 60 percent;

urban areas, 10 percent; and rivers and lakes, 5

percent. If the most important category is agricultural land, the dot grid intensity would be calculated on this basis. If the population being

sampled is large, the number of samples may be

computed by a formula based on the binomial

distribution :

n=p{l-4IJ

Where: n=total number of samples (dots) to be

counted.

^=estimated proportion of the total area

in the most important type classification (0.60 in this case).

t=Si constant denoting the reliability of

the estimate or level of statistical significance (the value 1.96 used here

denotes a statistical probability of

being correct 95 times in 100).

^=maximum permissible error, expressed as a percent of total area.

(In this case, ±2 percent or 0.02.)

Substituting the above values, we get :

^=0.60(1-0.60) r¿^T=2,305 dots

Thus if we are correct in the assumption that

the true percentage of agricultural land is 60, then

2,305 dots should be spaced over the total area to

insure a 95-percent probability of obtaining an

estimated proportion lying between 58 and 62 percent. At the specified photo scale of 1:15,840,

the total print area represented by 10 square miles

would be 160 square inches. Therefore, the number of dots needed per square inch would be 2,305

-~160, or about 14. Here, it would be most expedient to select a square-spaced grid having 16 dots

per square inch.
Figure 21.—Dot grid positioned over part of an enlarged USDA

print for acreage determination. The delineated stand contains

about 25 acres. Photo scale is 660 feet per inch.

The principal drawback to the formula method

is that a prior knowledge of ;; is required, whicli

is actually the value being sought. By definition,

however, p always lies between 0 and 1, and it can

be shown that p(l — p) reaches a maxinuun value

of 0.25 when p is set at 0.50 or 50 percent. Therefore, the equation may be revised to read :

.=0.25 [IJ

And, substituting

solution :

values from the previous

ri 96T 71=0.25 ^r^ =2,401 dots, an increase of 96 dots

over the first estimate.

A grid with 16 dots \>ev square inch will still

be a(fequate for the desired degree of accuracy

(16X160 = 2,560 dots).

For areas of 1,000 acres or less, grids witli

closely spaced dots are neeaed. (jri-lv.is »itii 64

dots per square inch are connnonly used with

1:20,000 photographs. As 1 square incli equals

63.77 acres at photo scale, each dot represents 0.996

acres. For 1:15,840 photography (40 acres per

square inch), grids having 40 dots per square inch

are available. It is not essential, of course, that

a sei)arate grid be used for each photo scale or that

the number of dots per square inch be equal to the

acreage per square inch. ^Vherever feasible, the

preceding equation should be used to determine

the grid intensity.

25
TREE HEIGHT, CROWN DIAMETER, AND CROWN CLOSURE

Tree Height

Tree heights are commonly determined on aerial

photographs by measuring either stereoscopic

parallax or shadow lengths. Though more difficult for the beginner, the parallax method is

faster, requires fewer calculations, and is more

adaptable to a variety of stand conditions. Anyone who can use a stereoscope can train himself

to measure stereoscopic parallax, and even the

occasional interpreter will find this method of

measuring heights advantageous.

Shadow-length measurements are reliable only

in open-grown stands where individual shadows

fall on level groimd. Accurate measurement is

almost impossible in dense, irregular stands or on

slopes. Furthermore, the conversion of shadow

length to tree height is complex in comparison to

the conversion of parallax measurements. For

these reasons only the parallax method will be

discussed. Foresters interested in details of

shadow-height calculations should refer to Johnson's article (5), which includes a special form

for making tree-height conversions.

The concept of parallax.—The Manual of

Photogrammetry (7) defines parallax as "the apparent displacement of the position of a body with

respect to a reference point or system, caused by

a shift in tlie point of observation." In measuring

object heights on stereoscopic pairs of aerial

photographs, two types of parallax must be measured or approximated—the absolute and the differential. The absolute stereoscopic parallax of a

point is "the algebraic difference, parallel to the air

base, of the distances of the two images from their

respective principal points"' {18), The parallax

difference, or differential parallax, of an object

being measured for height determination is the

difference in the absolute stereoscopic parallax at

the top and the base of the object, measured parallel

to the air base or night line.

The formula for converting parallax measurements to tree height on aerial photographs is :

h- HXdP

'P+dP

wliere A = height of object

i7= altitude of aircraft above ground datum

/*=absolute stereoscopic parallax at base of

object being measured

<^P=differential parallax

If object heights are to be determined in feet,

the height of the aircraft (H) must also be in feet.

Absolute stereoscopic parallax (P) and differential parallax (dP) must be expressed in the same

units; ordinarily, these units will be thousandths

of inches or hundredths of millimeters.

The height of the aircraft above ground datum

26

is calculated from the basic scale formula, transposed to the more convenient form : Flight altitude

(Zr)= focal length X scale denominator. With

photographs of 1:20,000 scale and a camera of

8.25-inch focal length, the flight altitude would be

0.6875X20,000=13,750 feet.

In flat terrain, the average photo base length

is ordinarily substituted as the absolute stereoscopic parallax (P). If the ground elevation at

the base of the tree being measured differs from the

elevation of the principal points by more than 200

or 300 feet, however, the following method (17)

should be used to calculate a new value for P:

1. Orient the stereo-pair with flight lines superimposed and images separated about 2.2 inches.

Clip down photographs.

2. Measure the distance between the two principal

points to the nearest 0.01 inch with an engineers

scale.

3. Measure the distance between corresponding

images on the two photographs at or near the

base of the tree to the nearest 0.01 inch.

1. Subtract (3) from (2) to obtain the absolute

stereoscopic parallax at the base of the tree.

Differential parallax (dP) is usually measured

stereoscopically with either a parallax wedge or

parallax bar, employing the "floating mark" principle. Such instruments are discussed in the section that follows. The concept of differential

parallax can be illustrated by direct measurement

of severely displaced images such as in the largescale stereogram of a water tank in figure 22.

With a photo scale of 1:7,800 and a camera focal

length of 6 inches, flight altitude above ground

(H) is computed as 0.5 X 7,800 or 3,900 feet. Average photo base length (P) for the stereo-pair is

3.70 inches.

Absolute stereoscopic parallax at the base and

top of the water tank is measured parallel to the

line of flight with an engineer's scale; the difference (2.27-2.13 inches) is dP, the differential

parallax of the displaced images. As the tank is

somewhat tapered in form, measurements were

made at the midpoint of the base and vertically

above this position at the center of the top. Substituting in the parallax formula :

Parallax wedges.—Parallax wedges are usually

printed on transparent film or glass. The basic

clesign consists of two rows of dots or graduated

lines beginning about 2.5 inches apart and converging to about 1.8 inches apart. The graduations on each line are calibrated for making parallax readings to the nearest 0.002 inch. The "training wedge" in figure 23 is graduated for reading

to the nearest 0.01 inch only.
Figure 22.—Large-scale stereogram showing an industrial plant and heavily displaced water tank. Measurements of

stereoscopic parallax shown can be used to determine height of the tank above ground. (Courtesy of Abrams

Aerial Survey Corporation.)

Figure 23.—Parallax wedge correctly oriented over a large-scale stereogram of

a railroad trestle. Graduations on right-hand row of dots refer to separation

of the converging lines in inches. (Courtesy of Abrams Aerial Survey

Corporation.)

27
Figure 24.—Modern mirror stereoscope with inclined magnifying binoculars. Positioned across the two photographs

is a parallax bar (stereometer) for measuring heights of objects. (Courtesy of Wild-Heerbrugg Instruments, Inc.)

The parallax wedfje is placed over the stereoscopic image with tlie converging lines perpendicular to the line of flight and adjusted until a

single fused line of dots or graduations is seen

sloping downward through the stereoscopic image.

If the photo images are separated by exactly 2

inches, a portion of the parallax wedge centering

around the 2-inch separation of converging lines

will fuse and appear as a single line. Tlie line

will appear to split above and below this section.

Using the fused line of graduations, the diiferential parallax is obtained by counting the number

of dots or intervals between the point where a

graduation appears to rest on the ground and the

point where a graduation appears to "float" in the

air at the same height as the top of the object. In

figure 23, for example, the dot resting on top of

the railroad trestle yields a reading of 2.22 inches.

"When this value is subtracted from a ground reading of 2.26 inches, dP is determined as 0.04 inch&s.

If flight altitude {H) of 4,000 feet and a

photo base length of 3.56 inches, the parallax

formula yields:

xj-w r. .7 /i\ 4,000X0.04 160.00 ,, , ,

Height of trestle W= ..'.gQ-f 0.04 =i:6r=^' ^'''

The interpreter should remember that the degree

of stereoscopic parallax normally encountered on

1:12,000 to 1:20,000 tlSDA photograplis is much

less than that illustrated here. Nevertheless, the

principle and the metliods of measiirement are

exactly the same. Where P is large in relation to

dP, tables 2 and 3 can be used as sJiortcut approximations in converting parallax measurements to

object heights on I'SDA pliotographs.

The paralJax bar.—This instrument is more expensive than the parallax wedge and yields results

of comparable accuracy. But many interpreters

prefer the parallax bar because the floating dot is

movable and tlnis easier to place on the ground

and at crown levels. A parallax bar designed to

measure heights with a mirror stereoscope is

shown in figure 24.

Tlie bar has two lenses attached to a metal frame

tliat liouses a vernier and a graduated metric scale.

Tlie left lens contains the fixed reference dot; the

dot on the right lens can be moved laterally by

means of the vernier. The bar is placed over the

stereoscopic image parallel to the line of flight.

Tiie right-hand dot is moved until it fuses with

tiie reference dot and appears to rest on tlie ground

at the base of tlie tree. The vernier reading is

recorded to the nearest 0.01 millimeter. The

vernier is then turned until the fused dot appeal's

to "float" at treetop level, and a second reading

recorded. The difference between the readings

is tlie parallax dift'ei-ence {dP) in millimetere.

Tiiis value can be substituted in the parallax formula without conversion if the absolute parallax

(/') is also expressed in millimeters.

Arcxtracy of height memurements.—Accuracy in

28
TABLE 2.—Parallax-wedge conversion factors for

wedges reading to 0.002-inch parallax (dPy

Photo

base

Object height in feet per 0.002 inch of dPy

at flight altitudes of —

iP)

(inches) 6,000

feet

8,000

feet

10,000

feet

12,000

feet

14,000

feet

2.6

2.7

2.8

2.9

3.0

3.1

3.2

3.3

3.4

3.5

3.6

3.7

3.8

3.9

4.0

4.6

4.4

4.3

4. 1

4.0

3.9

3.7

3.6

3.5

3.4

3.3

3.2

3. 1

3. 1

3.0

6. 1

5.9

5.7

5.5

5.3

5.2

5.0

4.8

4.7

4.6

4.4

4.3

4.2

4. 1

4.0

7.7

7.4

7. 1

6.9

6.7

6.4

6.2

6. 1

5. 9

5.7

5.6

5.4

5.3

5. 1

5. 0

9.2

8.9

8.6

8.3

8.0

7.7

7.5

7.3

7.0

6.8

6.7

6.5

6.3

6. 1

6.0

10.8

10.4

10.0

9.6

9.3

9.0

8.7

8.5

8.2

8.0

7.8

7.6

7.4

7.2

7.0

1 To use table, measure parallax difference (dP) of

object to nearest 0.002-inch (as 0.016 or 8 dot intervals

on wedge). If average photo base (P) is 3.6 inches and

night altitude is 14,000 feet, the table value of 7.8 is

multiplied by the 8 dot intervals for an object height of

62 feet.

TABLE 3.—Parallax-bar conversion factors for use

with devices reading to 0.01-mm, parallax (dPy

Object height in feet per millimeter of dP

Photo at flight altitudes of—

base

(P)

(inches) 6,000 8,000 10,000 12,000 14,000

feet feet feet feet feet

2.6 90 119 149 179 209

2.7 86 115 144 172 201

2.8 83 111 139 166 194

2.9 80 107 134 161 187

3.0 78 104 130 155 181

3.1 -- - 75 100 125 151 176

3.2 73 97 122 146 170

3.3 71 94 118 142 165

3.4 69 92 114 137 160

3.5 67 89 111 133 156

3.6 65 87 108 130 152

3.7 63 84 105 126 147

3.8 62 82 103 123 144

3.9 60 80 100 120 140

4.0 58 78 97 117 136

1 To use table, measure parallax difference (dP) of

object to nearest hundredth pf a millimeter (as 0.57 mm.,

for example). If average photo base (P) is 3.1 inches and

flight altitude is 12,000 feet, the table value of 151 is

multiplied by 0.57 for an object height of 86 feet.

measuring total heights of trees and stands depends upon a number of factors, not the least of

which is the interpreter's ability to determine

stereoscopic parallax, usually interpreters can

detect differences in parallax of about 0.002 inch,

or 0.05 mm., and this graduation interval is used

on most parallax wedges.

The interpreter who can detect a diflerence oí

0.002 inch of stereoscopic parallax will be able to

stratify forest stands into 10-foot total height

classes on contact prints of 1:15,840 to 1: 20,000

scales {18). Greater accuracy may be possible m

flat terrain where photo scale changes are not

pronounced and less skill is required in selecting

the point for the base parallax reading. The

ground parallax must be read on the same contour

as the base of the tree.

The interpreter should consider the following

points to improve accuracy in height measurement :

1. In rough terrain, a new photo scale and flight

altitude should be calculated for each overlap.

For stands on high ridges or in deep ravines, it

is better to calculate new values for absolute

stereoscopic parallax than to use the average

photo base length.

2. Once a pair of photographs have been aimed

for stereo-viewing, they should be fastened

down. A slip of either photograph between the

parallax reading at the base and the top of a

tree may cause highly inaccurate height readings. . .

3. To avoid single measurements of high variability, several measurements should be made of the

same tree or stand and the results averaged.

Tree Crown Diameter

For most conifers and many hardwoods, tree

crown diameter is related to stem diameter. It is

thus a useful photographic measurement when

estimating individual tree volumes or stand-size

classes. Actual determination of crown diameter

is a distance measurement, somewhat complicated

by the small sizes of tree images and the effects of

crown shadows.

Crown diameters are measured with either

wedges or dot-type scales reading in thousandths

of an inch. With a crown wedge, the diverging

lines are placed tangent to both sides of the crown

for making the reading. Dot-type scales have

circles of graduated sizes for direct comparison

with tree crowns (fig. 25). For converting measurements, the scale of photography is ci^lculated

in feet per thousandth of an inch. At 1: 20,000,

each 0.001 inch of crown measure equals 1.667 feet.

A reading of 0.010 inch would imply a crown diameter of 17 feet (table 4).

Tree crowns are rarely circular, but, because

individual limbs are often invisible on aerial photographs, they usually appear roughl}^ circular or

elliptical. Since only the parts visible from

above can be evaluated, photo measures of crown

diameter are often lower than ground checks of

the same trees. Nevertheless, most interpreters

can determine average crown diameter with reasonable precision if they take several readings.

29
CROWN DENSITY SCALE

CROWN DIAMETER SCALE

CENTRAL STATES FOREST EXPERIMENT STATION

I ' I '5 I 25 I 35 I 45 I 55 I ¿5 I TÍS I ¿5 I 95 I |¿5 I »s

0 10 20 30 40 50 60 70 80 90 100 110

NUMBERS INDICATE DOT SIZE IN THOUSANDTHS OF AN INCH

Figure 25,—Dot-type scale for measuring crown diameters. Such scales are usually printed on transparent

film.

Obviously, crown diameter measurements of individual trees are most accurate in open-grown

stands. In dense stands, measurements are generally confined to determination of an average for

the dominant trees. Crowns of mature conifers

can usually be classified into 5-foot classes without

difficulty {18).

Tree Crown Closure

Crown closure percent, also referred to as crown

density, is the proportion of the forest canopy

occupied by trees. Crown density may refer to

all crowns in the stand regardless of canopy level

or only to the dominants. Estimates are purely

ocular, and stands are commonly grouped into

10-percent density classes. Printed density scales

(fig. 26) may aid the interpreter, though the patterns of black dots on a white background bear

little resemblance to photographic images of trees.

Comparative stereograms illustrating various

stand densities have also proved useful (fig. 27).

Evaluation of crown closure is much more subjective than the determination of tree height or

crown diameter. Actual measurement is virtually

V"

?

Q

•••"i'

L2J M m

R

§

^ m ü m m

.*• m m m m m m Ü m m

ho m ^

^ m m m m m

PERCENT CROWN COVER

I : 15840

FOREST SURVEY-CENTRAL STATES FOREST EXPERIMENT STATION

Figure 26,—Density scale for comparison with photo

images in estimating crown closure percent.

impossible on most USDA photographs, and

accuracy is thus dependent on the interpreter's

judgment. Inexperienced interpreters tend to

overestimate closure by ignoring small stand openings or including portions of crown shadows.

Devices for checking closure on the ground fail

to provide estimates that are truly comparable to

those made on vertical photographs. Thus the

neophyte must rely on practice, checked by skilled

interpreters, to develop proficiency.

Crown closure is useful because of its relation to

stand volume per acre. It is applied in lieu of

basal area or number of trees per acre, as these

cannot be accurately determined on available photography. Measurements of crown diameter and

estimates of closure should always be made under

the,stereoscope.

Tree counts.—Complete tallies of individual

trees can seldom be made accurately on available

USDA photographs. In dense stands, suppressed

trees and many intermediates cannot be seen.

Clumps of two or three trees often appear as single

TABLE 4.—Actual crown widths for various photo-crown widths and photo scales

Photo crown width

(thousandths of an inch)

1:10,000

or 833

ft./in.

1:12,000

or 1,000

ft./in.

1:15,840

or 1,320

ft./in.

1:18,000

or 1,500

• ft./in.

1:20,000

or 1,667

ft./in.

1:24,000

or 2,000

ft./in.

2.5

Feet

2

4

6

8

10

12

15

17

19

21

23

25

27

29

31

33

35

37

40

42

Feet

3

5

8

10

13

15

18

20

23

25

28

30

33

35

38

40

43

45

48

50

Feet

3

7

10

13

17

20

23

26

30

33

36

40

43

46

50

53

56

59

63

66

Feet

4

8

11

15

19

23

26

30

34

38

41

45

49

53

56

60

Feet

4

8

13

17

21

25

29

33

38

42

46

50

54

58

63

67

Feet

5

10

15

20

25

30

35

40

45

50

55

60

5.0

7.5

10.0

12.5

15.0

17.5

20.0

22.5

25.0

27.5

30.0

32.5

35.0

37.5

40.0

42.5

45.0

47.5

50.0

30

crowns, and ragged individual crowns may look

like two trees. Only in even-aged, open-grown

forests can all trees in a stand be separated.

Counting all trees on a plot is tedious, and this

measure of density is seldom used. Where largescale photographs are available (1:1,000 to

1:5,000), individual-tree counts may be much

more reliable.

AERIAL CRUISING

Individual Tree Volumes

Ordinary tree volume tables can be easily converted to aerial volume tables when correlations

can be established between tree crown diameters

and stem diameters {10). The photographic determinations of crown diameter and total tree

height are merely substituted for the usual field

measures of stem diameter (d.b.h.) and merchantable height, respectively (fig. 28). Photographic

measurements are usually limited to well-defined

open-grown trees, and crown counts are required

to obtain total volume for a given stand of timber.

The construction and application of aerial tree

volume tables depends on a well-established relationship between photographic measures of crown

diameter and ground determinations of tree d.b.h.

Such relationships can often be established for

individual species or species groups, notably evenaged conifers in the middle diameter classes {18),

By contrast, stem-crown diameter correlations

rarely have been used for estimating volumes of

mixed hardwoods.

The aerial tree table (table 5) provides volumes

in terms of gross cubic feet. In making an aerial

GROUND

MEASUREMENT PHOTO

MEASUREMENT

STEM

DIAMETER

at 4 1/2 FT.

Figure 28,—Comparison of ground and photographic

measurements in the determination of individual tree

volumes.

cruise, photographic measurements may include

all trees on 0.2- to 1-acre circular plots, or stands

may be delineated according to height classes for

determination of the average tree per unit area.

In the latter instance, a tree count must be made to

obtain the total stand volume.

In general, the individual-tree approach to

aerial erasing is of limited value when the interpreter is restricted to use of 1:12,000 to 1: 20,000

USDA photographs. At this scale, images are

usually too small for accurate assessment of individual trees.

Stand Volume Per Acre

If recent photographs and reliable aerial standvolume tables can be obtained, average stand

vojume per acre can be estimated with a minimum

of field work. Estimates are made in terms of

gross volume, as amount of cull or defect cannot

be adequately evaluated. Even-aged stands of

simple species structure are best suited for this

type of estimating, especially if gross and net volumes are essentially identical. All-aged stands

of mixed hardwoods are more difficult to assess,

but satisfactory results can be obtained if field

checks are made to adjust the photographic estimate of stand volume per acre and to determine

allowance for defect. Though volumes from photographs cannot be expressed by species and diameter classes, total gross volumes for areas as small

as 40 acres can be estimated within 10 to 15 percent

of volumes derived from conventional ground

cruises {13).

Most aerial stand-volume tables for mixed

species are constructed in terms of cubic feet per

acre. Tables for species in pure stands, such as

Douglas-fir, may be expressed either in board feet

or cubic feet per acre. Three photographic measurements of the dominant stand are generally required for entering an aerial stand volume table :

average total height, crown diameter, and crown

closure percent.

Aerial volume tables have been constructed for

many of the important timber associations in the

United States. Included here are tables for

Douglas-fir (table 6), Eocky Mountain conifers

(table 7), and Kentucky hardwoods (table 8).

Composite tables, applicable in mixed stands, are

presented for northeast Mississippi (table 9) and

for northern Minnesota (table 10). Crown diameter was eliminated as a variable in tables 6 and

10.

32
TABLE 5.—Volumes oj individual second-growth

southern pines ^

Crown Total tree height, in feet

diameter

class 50 60 70 80 90 100 110

(feet)

Cu. Cu. Cu. Cu. Cu. Cu. Cu.

ft- ft. ft- ft- ft- ft- ft.

10 9.5 11.5 12.5 15.0 17.5 19. 5

12 12. 5 14. 5 16.,5 18. 0 20.5 22. 5

14 15. 0 17.0 19. 0 23.5 25.0 27.5 3Ö. 5

16 17.5 20.5 24.0 27.5 30.5 33.0 36.0

18 23.5

28.0

27.0

33.5

30. 5

36.0

34. 5

40.0

38.0

45. 5

42.5

20 49. 0

22 32.5 37.0 42.5 46.5 52.0 57.5

24 37.0 42.5 48.5 54. 5 60.0 66.0

26 42.5 47. 5

53. 0

60.5

54. 0

60.5

68.0

61.0

70.5

78. 0

67.5

76.0

85.5

75. 5

28 83.0

30_ __ 94.5

Ï Based on 342 trees in Arkansas, Lousiana, and Mississippi. Gross volumes are inside bark and include the

merchantable stem to a variable top averaging 6 inches

i.b. Reprinted from (4).

One of the several procedures for making aerial

volume estimates is as follows :

1. Outline tract boundaries on the photographs,

utilizing the effective area of every other print

in each flight line. This assures stereoscopic

coverage of the area on a minimum number of

photographs and avoids duplication of measurements.

2. Delineate all forest types. Except where type

lines deñne stands of relatively uniform stocking and total height, they should be further

broken down into homogeneous units so that

measures of height, density, and crown dianieter will apply to the entire unit. Generally it

is unnecessary to recognize stands smaller than

5 to 10 acres.

3. Determine the acreage of each condition class

with dot grids. This determination can often

be made on contact prints.

4. By stereoscopic examination, measure the variables for entering the aerial stand volume table.

From the table, obtain the average volume per

acre for each condition class.

5. Multiply gross volumes per acre from the table

by condition class areas to determine gross

volume.

6. Add class volumes for the total gross volume

on the tract.

A practical application is illustrated by the

stereogram in figure 29 and the corresponding

cordwood volume summary. In this particular

inventory, hardwood components in each stand

were ignored, and estimates were derived only for

pine timber. The aerial cruise, fortified by freTABLE 6.—Aerial stand volume per acre values ^ for even-aged Douglas-fir in the Pacific Northwest

Stand height 2 (feet)

Crown closure percent ^

15 25 35 45 55 65 75 85 95

40

Cu. ft.

500

700

900

1,200

1,400

1, 700

2, 100

2,400

2, 800

3, 100

3,500

4,000

4,400

4,900

5,400

5,900

6,400

7, 000

7,500

8, 100

8,700

9,400

10, 000

Cu. ft.

800

1, 100

1,500

1,900

2,400

2,800

3,300

3,900

4,500

5, 100

5, 700

6,400

7, 100

7,900

8,700

9,500

10, 400

11,300

12, 200

13, 200

14, 200

15, 200

16, 300

Cu. ft.

1, 100

1,500

2,000

2,500

3, 100

3,800

4,500

5,200

6,000

6,800

7,700

8,600

9,600

10, 600

11, 700

12, 800

14, 000

15, 200

16, 500

17, 800

19, 200

20, 600

22, 100

Cu. ft.

1,300

1,800

2,400

3, 100

3,800

4, 500

5,400

6,300

7,300

8,300

9,400

10, 500

11, 800

13, 000

14, 400

15, 800

17, 300

18, 800

20, 400

22, 000

23, 800

25, 600

27, 400

Cu. ft.

1,400

2,000

2,700

3,400

4,300

5,200

6, 100

7,200

8,300

9, 500

10, 800

12, 200

13, 600

15, 200

16, 800

18, 400

20, 200

22, 000

23, 900

25, 900

28, 000

30, 100

32, 300

Cu. ft.

1, 500

2, 100

2,900

3,700

4,600

5,600

6,700

7,900

9,200

10, 600

12, 100

13, 600

15, 300

17, 000

18, 800

20, 800

22, 800

24, 900

27, 100

29, 300

31, 700

34, 200

36, 700

Cu. ft.

1,500

2,200

3, 000

3,900

4,800

5,900

7, 200

8,500

9,900

11,400

13, 000

14, 800

16, 600

18, 500

20, 600

22, 700

25, 000

27, 400

29, 800

32, 400

35, 100

37, 900

40, 800

Cu. ft.

1,400

2, 100

2,900

3,900

4,900

6, 100

7,400

8,800

10, 300

12, 000

13, 800

15, 700

17, 700

19, 800

22, 000

24, 400

26, 900

29, 500

32, 200

35, 100

38, 000

41, 100

44, 300

Cu.ft.

1,300

50 2, 000

60 2,800

70 3,800

80 4,900

90 6, 100

100 7, 500

110 9,000

120 - -

10, 600

130 12, 400

140 -

14, 300

150 16, 300

160 18, 500

170 - --

20, 800

180 23, 200

190 25, 700

200 --

28, 400

210 31, 300

220 34, 200

230 37, 300

240 _

40, 600

250 43, 900

260 47, 400

1 Gross volume, in trees 5.0 inches and larger, from stump to top limit of 4.0 inches diameter inside bark. Reprinted from (13).

2 Average height of dominants and codominants as measured in the field.

3 Average estimates of several experienced interpreters includes all trees in the major canopy (occasionally excludes

small trees definitely below the general canopy).

33
TABLE 7.—Aerial volume per acrejor Rocky Mountain conifers

4- TO 7-FOOT CROWN DIAMETER

Average stand height

Crown closure percent

(feet)

5 15 25 35 45 55 65 75 85 95

30

Cu. ft. Cu. ft.

100

200

300

400

500

600

800

1,050

Cu. ft.

300

400

500

600

700

850

1,100

1,400

Cu. ft,

450

550

650

750

850

1, 050

1,350

1, 700

Cu. ft.

550

650

750

850

1,000

1,200

1,600

1,950

Cu. ft.

650

750

850

950

1, 150

1,400

1, 800

2, 150

Cu. ft.

750

850

1,000

1, 100

1,350

1, 600

2,000

2,350

Cu. ft.

850

1, 000

1, 150

1,300

1,600

1, 850

2, 250

2,550

Cu. ft.

1, 000

1,200

1,400

1,550

1,850

2, 100

2,500

2, 800

Cu. ft.

35 1, 150

40 1, 400

45 50

100

200

350

500

1, 650

50 1, 800

55„__ 2, 100

60 2, 350

65 2, 750

3, 050

30_

35-

40 _

45 _

50 _

55_

60 _

65-

70-

75-

80-

85-

30-

35-

40-

45-

50_

55-

60-

65-

70-

75-

80-

85-

90-

95-

100-

105-

110-

8- TO 12-FOOT CROWN DIAMETER

100

300

450

700

900

1,200

1, 700

2, 200

150

250

350

400

500

700

900

1,200

1, 600

2, 000

2, 500

3, 000

350

450

550

650

700

900

1, 200

1, 600

2, 000

2,400

2,800

3, 300

500

600

700

800

950

1, 100

4, 500

1, 900

2, 300

2, 700

3, 100

3, 550

600

750

850

950

1, 100

1,300

1, 750

2, 100

2, 500

2,850

3, 300

3,750

700

850

950

1, 100

1,250

1,500

1, 950

2, 300

2, 700

3, 000

3,450

3,950

800

950

1, 050

1, 250

1,450

1,750

2, 150

2, 500

2,850

3, 200

3, 600

4, 150

950

1, 100

1,250

1,450

1, 700

2, 000

2, 350

2, 700

3, 000

3,400

3, 800

4, 350

13- TO 17-FOOT CROWN DIAMETER

30 200

35 300

40 400

45 100

200

300

500

750

1,000

1,300

1,800

2,300

2, 800

3,300

3, 700

500

50 600

55 750

60 1 000

65 1 300

70 1 700

75 2 100

80 2 600

85 3 100

90 3 600

95 4 100

100 4 500

400

500

600

700

850

1,000

1, 300

1,700

2, 100

2,500

2, 900

3,450

3,900

4, 400

4, 850

550

700

800

850

1, 000

1,250

1, 600

2, 000

2, 400

2,750

3, 200

3, 700

4, 150

4, 600

5, 150

700

800

900

1,000

1, 150

r, 500

1,850

2, 200

2,600

2,950

3,400

3, 900

4,350

4, 800

5, 350

800

900

1,000

1, 150

1,350

1, 700

2, 050

2,400

2, 750

3, 150

3, 600

4, 050

4, 500

5, 000

5, 550

900

1, 050

1,250

1, 350

1, 600

1,900

2, 250

2, 600

2, 900

3, 300

3, 750

4, 200

4, 650

5, 200

5, 750

1,050

1,200

1, 450

1,600

1,850

2, 150

2, 450

2, 800

3, 100

3, 500

3, 900

4, 400

4, 850

5,400

5, 950

1, 200

1, 500

1, 700

1, 850

2, 100

2, 350

2, 650

3, 000

3, 300

3, 700

4, 100

4, 600

5, 050

5, 600

6, 150

18- TO 22-FOOT CROWN DIAMETER

100

200

350

450

650

850

1, 100

1,600

1,950

2,500

2, 900

3,400

3,900

4, 600

5,200

300

400

500

600

750

850

1, 150

1, 500

1,900

2,400

2,750

3,300

3, 700

4, 200

4,700

5,400

6, 000

500

650

750

850

1, 000

1, 150

1, 500

1, 900

2, 300

2, 700

3, 050

3, 600

4, 050

4, 500

5, 000

5, 800

6, 400

1,350

1, 500

1, 750

1, 950

2, 150

2, 500

2, 650

3, 100

3, 300

3,800

4, 200

4, 750

1,350

1,700

1, 950

2, 100

2, 350

2,550

2,850

3,200

3, 500

3,900

4, 300

4, 800

5, 250

5, 800

6, 350

700 800 900 050 1, 200 1, 400 1,600

800 900 050 200 1, 400 1, 600 1,800

900 1, 050 200 400 1, 600 1, 800 2, 000

1, 050 i, 200 400 600 1, 800 2, 000 2, 200

1, 200 1, 400 600 800 2, 050 2, 250 2, 450

1, 400 1, 650 900 2 100 2 300 2, 500 2,700

1, 800 2, 050 2 250 2, 450 2, 650 2, 850 3, 050

2, 200 2, 400 2, 600 2 750 2, 900 3, 100 3,300

2, 550 2, 750 2, 900 3, 100 3, 300 3, 500 3,700

2,950 ^, 100 3, 250 3, 450 3, 650 3, 850 4,050

3, 300 3, 500 3, 700 3, 900 4, 100 4, 300 4, 500

3, 850 4, 050 4, 250 4, 400 4, 550 4, 750 4, 950

4, 300 4, 500 4, 700 4, 850 5, 050 '^, 300 5,500

4,800 5, 000 5, 200 5, 400 5, 600 '^, 800 6, 000

5, 300 5, 500 5, 700 5, 900 6, 100 6, 300 6,500

6, 000 Ö, 200 6, 400 6, 550 6, 700 6, 850 7, 050

6, 600 Ö, 800 7, 000 7, 200 7, 400 7, 600 7,800

34
TABLE 7. -Aerial volume per acre for Rocky Mountain conijers

23-FOOT AND LARGER CROWN DIAMETER

-Continued

Average stand height

Crown closure percent

(feet)

5 15 25 35 45 55 65 75 85 95

30

35

40

Cu. ft.

100

200

300

400

550

700

850

1,200

1,550

2,050

2,450

2,900

3,400

3,850

4,400

5, 100

6,600

6, 150

Cu.ft.

500

650

750

850

1, 000

1,200

1,550

1,900

2,250

2,750

3, 150

3,600

4,100

4,550

5,100

5,800

6,400

6,900

Cu. ft.

750

850

1,000

1, 100

1,300

1,600

1,950

2,300

2,650

3, 050

3,450

3,950

4,400

4,850

5,400

6,100

6,700

7,200

Cu. ft.

900

1, 050

1,200

1,350

1,600

1,900

2, 250

2,550

2,900

3,300

3, 700

4,200

4,650

5, 150

5,700

6,400

6,900

7,400

Cu. ft.

1, 050

1,250

1,400

1,600

1,850

2,100

2,450

2,750

3,100

3,500

3,900

4,400

4,850

5,350

5,900

6,550

7,050

7,600

Cu. ft.

1, 250

1, 450

1,600

1,800

2,050

2,300

2,650

2,900

3,300

3,650

4,050

4,550

5,050

5,550

6, 100

6,700

7,200

7,750

Cu. ft.

1,450

1,650

1,800

2,000

2,250

2,500

2,850

3,100

3,450

3,800

4,200

4,700

5,250

5,750

6,250

6,850

7,350

7,900

Cu. ft.

1, 650

1, 850

2,050

2,250

2,450

2,700

3,050

3,300

3,650

4,000

4,400

4,900

5,450

5,950

6,400

7,000

7,500

8,050

Cu. ft.

1,900

2, 100

2,300

2,500

2,650

2,900

3,250

3,500

3,850

4,200

4,600

5,100

5,650

6, 150

6,600

7,150

7,700

8,200

Cu. ft.

2, 150

2,350

2,550

45 2,750

50 -_-

2,850

55 3, 100

60 3,450

65 3,700

70 4,050

75 4,400

80 4,800

85 5,300

90 - - 5,850

95 6,350

100_ 6,800

105 7,300

110 7,850

115 8,350

1 Based on 168 field plots taken in southeastern Idaho, southwestern Wyoming, and northeastern Utah. Aggregate

deviation: Table 1.5 percent low. Standard error of estimate: ± 48 percent of average plot volume^ Reprinted from {11).

TABLE 8.—Aerial volume per acre jor Kentucky hardwoods

10- TO 14-FOOT AVERAGE CROWN DIAMETER

Average stand height (feet)

Crown closure percent

15 25 35 45 55 65 75 85 95

30 --- -_

Cu. ft.

300

350

400

550

900

Cu. ft.

375

425

475

675

1,075

Cu. ft.

450

500

550

800

1,250

Cu. ft.

475

550

625

875

1,325

Cu. ft.

500

600

700

950

1,400

Cu. ft.

570

650

770

1,035

1,470

Cu.ft.

635

700

835

1,115

1,535

Cu. ft.

700

750

900

1,200

1,600

Cu. ft.

770

40 800

50 970

60 1,285

70 __ 1,670

15- TO 19-FOOT AVERAGE CROWN DIAMETER

30

40

50

60

70.

80.

350

400

450

600

1, 000

1,500

400 450 500 550

450 500 575 650

525 600 675 750

725 850 950 1,050

1, 150 1,300 1,400 1,500

1,625 1,750 1,825 1,900

620

720

835

1, 100

1,550

1,970

685 750

785 850

920 1,000

1,150 1,200

1,600 1,650

2,035 2,100

820

920

1,085

1,250

1,700

2,170

20- TO 29-FOOT AVERAGE CROWN DIAMETER

40.

50.

60.

70.

80.

90.

100.

750 850

900 1,000

1,200 1,275

1,600 1,675

2,050 2,125

2,500 2,575

3,000 3,075

950 1,035 1, 115 1,200

1,100 1,185 1,270 1,350

1,350 1,420 1,485 1,550

1,750 1,820 1,885 1,950

2,200 2,270 2,335 2,400

2,650 2,720 2,785 2,850

3,150 3,200 3,250 3,300

1,285

1,435

1,620

2,020

2,470

2,920

3,350

35
TABLE 8.—Aerial volume per acre for Kentucky hardwoods ^—Continued

30-FOOT AND LARGER AVERAGE CROWN DIAMETER

Average stand height (feet)

Crown closure percent

15 25 35 45 55 65 75 85 95

40

Cu. ft.

850

1,050

1,300

1,700

2,150

2,600

3,050

3,600

Cu. ft.

1,025

1,200

1,450

1,850

2,275

2,725

3, 175

3,700

Cu. ft.

1,200

1,350

1,600

2, 000

2,400

2,850

3,300

3, 800

Cu. ft.

1,275

1,425

1,650

2,075

2,500

2,925

3,375

3,875

Cu. ft.

1,350

1,500

1,700

2,150

2,600

3,000

3,450

3,950

Cu. ft.

1,420

1,570

1,785

2,220

2,670

3,070

3,500

4,020

Cu. ft.

1,485

1,635

1,870

2,285

2,735

3,135

3,550

4,085

Cu.ft.

1,550

1,700

1,950

2,350

2,800

3,200

3,600

4, 150

Cu. ft.

1 620

50

60

1,770

2 035

70 2 420

80 2 870

90 3 270

100 3 650

110 4 220

1 Gross volumes include the merchantable stems of all trees 5 inches d.b.h. and larger to a 4-inch top diameter i.b.

Expanded from (14) by linear interpolation.

TABLE 9.—Composite aerial volume per acrejor northeast Mississippi ^

10-FOOT AVERAGE CROWN DIAMETER

Average stand height (feet)

Crown closure percent

15 25 35 45 55 65 75 85 95

30

Cu. ft.

190

230

260

300

340

380

Cu. ft.

310

380

440

500

570

640

Cu. ft.

430

530

620

690

800

890

Cu. ft.

560

690

800

890

1,030

1,140

Cu. ft.

690

840

980

1,090

1,260

1,400

Cu. ft.

810

990

1,150

1,290

1,490

1,650

Cu. ft.

940

1,140

1,330

1,490

1, 720

1,910

Cu. ft.

1,060

1,300

1,510

1,680

1,950

2,160

Cu. ft.

1 180

40 1 450

50 1 fi«n

60 1 880

70 2 180

80 9 4.90

15-FOOT AVERAGE CROWN DIAMETER

30

40.

50

60

70.

80.

90.

210

260

310

360

400

440

480

350 500 640

430 610 780

510 730 930

590 830 1,070

660 940 1,200

740 1,040 1,330

800 1,140 1,450

780

950

1,140

1,300

1,460

1,620

1,770

930

1, 130

1,350

1,550

1,740

1,930

2, 100

1,070

1,300

1,550

1, 780

2,000

2,220

2,420

1,220

1,480

1,770

2,030

2,280

2,530

2,760

20-FOOT AVERAGE CROWN DIAMETER

40.

50

60.

70.

80.

90.

100.

25-FOOT AVERAGE CROWN DIAMETER

50.

60.

70.

80.

90.

100.

320 550 770

370 620 880

410 700 980

460 780 1, 100

510 860 1,220

550 940 1,320

1,000

1,130

1,270

1,420

1, 570

1,710

1,220

1,390

1,550

1,740

1,930

2,090

1,450

1,650

1,840

2, 060

2,280

2,480

1,670

1,900

2,130

2,380

2,640

2,860

1,900

2, 160

2,410

2,700

2,990

3,250

1,360

1,650

1,970

2,260

2,540

2,820

3,070

630 820 1,000 1,190 1,370 1,550 1,720

750 970 1, 190 1,410 1,630 1,840 2,040

860 1,110 1,360 1,600 1,850 2,100 2,330

960 1,240 1,520 1,800 2, 080 2,360 2,610

1,070 1,380 1,690 2,000 2,310 2,620 2,900

1, 190 1,530 1,870 2,220 2,560 2,900 3,220

1,290 1,670 2, 040 2,410 2,790 3,160 3,500

2,120

2,410

2,700

3,020

3,350

3,630

1 Gross volumes are inside bark and include the merchantable stem to a variable top not smaller than 3 inches i.b.

Reprmted from (5).

36
TABLE 10.—Composite aerial volume per acre ^ for northern Minnesota 2

Average total height

Crown closure percent

(feet)

5 15 25 35 45 55 65 75 85 95

Cu. ft.

40

80

180

460

740

1,020

1,300

1,580

1,860

2,140

2,420

2,700

2,980

3,260

3, 540

Cu. ft.

120

190

310

590

870

1, 150

1,430

1,710

1,990

2,270

2,550

2,830

3, 110

3,390

3,670

Cu. ft.

200

300

Cu. ft.

280

Cu. ft. Cu. ft.

440

630

Cu. ft.

520

740

Cu. ft. Cu.ft.

680

Cu. ft.

30 360

520

700

980

1,260

1,540

600

850

1,090

1,370

1,650

1,930

2,210

2,490

2,770

760

35 410

570

^50

1, 130

960

1,220

1,500

1,780

2,060

1,070

40 _ __ 440

720

1,000

830

1, 110

1,390

1,670

1,950

2,230

2,510

960

1,240

1,520

1,800

2,080

2,360

2,640

2,920

1,350

45 1,630

50 1,910

55 1,280

1,560

1,840

2,120

2,400

2,680

2,960

3,240

3,520

3,800

1,410

1,690

1,970

2,250

2,530

2,810

3,090

3,370

3,650

3,930

2,190

60 1,820

2,100

2,340

2,620

2,900

3, 180

3,460

3,740

4, 020

4,300

4,580

2,470

65 2,750

70 2,380 3, 030

75 2,660

2,940

3,220

3,500

3,780

4,060

2,790

3,070

3,350

3,630

3,910

4,190

3,050

3,330

3,610

3,890

4, 170

4,450

3,310

80 - _ -- 3,200

3,480

3,760

4, 040

4,320

3,590

85 3,870

90 -- - 4, 150

95 -- 4,430

100 _ - --- - 4,710

1 Gross volumes are inside bark and include all trees 5.0 inches d.b.h. and larger from stump to a variable top diameter

not less than 4.0 inches i.b. Volumes may be converted to rough cords per acre by dividing by 80.

2 Based on 50 1-acre plots in Carlton County, Minn. Heavy lines indicate limits of basic data. Reprinted from (5) •

quent field checks, was within 10 percent of a

ground inventory made by an independent agency.

Adjiosting volumes hy field checks.—When aerial

volume tables are not sufficiently reliable for acceptance of pure photographic estimates and allowance must be made for defective trees, some

of the plots interpreted should be mechanically

selected for field measurement. For example, if

350 plots were interpreted and every 10th plot

selected, 35 plots would be visited in the field. If

the field volumes averaged 600 cubic feet per acre

as opposed to 800 cubic feet per acre for the photo

plots, the adjustment ratio would be 600^800

or 0.75. If the 35 field plots are representative of

the total, the ratio can be applied to the average

photo volume per acre to determine the adjusted

volume. Ratios should be computed by forest

types, because hardwoods are likely to require

larger adjustments than conifers.

The accuracy of aerial cruises depends not only

upon the volume tables but also on the availability

of recent photographs and the inlerpreter's ability

to measure correctly. This last item may be the

greatest single source of error. Each photo variable should be measured twice for an average, or

two interpreters should assess each plot.

Photo estimates of pine cordwood volumes for a tract

in the Georgia Piedmont ^

County: Oconee

Photo No.: BQ-3BB-85, 86

Owner: J. Reynolds

Photo date: 11-1-60

Pine area: 177 acres

Scale: 1,667 ft./inch

Pine Stand Total Crown Crown Pine volumes Stand

stand 2 area height diameter

closure per acre volume

Per- Cubic

Acres Feet Feet cent feet Cords Cords

1 18 40 10 25 380 4. 75 85.5

2 29 30 10 5 60 .75 21. 8

3 11 30 10 5 60 .75 8.3

4 18 40 15 35 610 7.62 137.2

5 5 30 10 15 190 2.37 11. 8

6 11 40 15 35 610 7. 62 83.8

7 9 30 10 65 810 10. 12 91. 1

8 17 40 15 15 260 3. 25 55.2

9 20 50 15 15 310 3.87 77.4

10 27 40 15 25 430 5.37 145.0

11 12 30 10 15 190 2. 37 28.4

Total pine volume (cords) 745. 5

1 Cubic volumes derived from table 9 and divided by 80

for conversion to cords.

2 Stands are numbered as in figure 29.

37
M:^^'Wá,

Figure 29.—USDA panchromatic stereogram of a forest area in the Georgia Piedmont.

Area and pine volume estimates were made for each of the stands numbered. Scale

is about 1,667 feet per inch.

PHOTO STRATIFICATION FOR GROUND CRUISING

A photo-controlled ground cruise combines the

features of both aerial and ground estimating, offering a means of obtaining timber volumes with

maximum efficiency. Photographs are used for

area determination, for allocation of field samples

by forest type and stand-size classes, and for designing the pattern of field work. Tree volumes,

growth, cull percents, form class, and other data

are obtained on the ground by conventional methods. A photo-controlled cruise may increase the

efficiency, while reducing the total cost, of an inventory on tracts as small as 100 acres.

The approach to an inventory of this kind depends largely on the types of strata recognized and

the method of allocating field samples. Cost and

the statistical accuracy required determines the

total number of field samples to be measured. Once

this number has been determined, there are several

ways in which the samples may be distributed

among various photo classifications :

1. By area of each class. Though sometimes used,

this method is often unsatisfactory because

stands of low value may occupy the greatest

acreage while high-value stands may be insufficiently sampled. The opposite extreme may

result if value alone (volume-per-acre classes)

is used.

2. By applying different cruising intensities to

each class. A 20-percent cruise might be used

for high-value stands, 10 percent for medium

value, and 5 percent or less for low-value areas.

This arbitrary method is better than using acreage alone, but it may not be the most economical

and efficient.

3. By statistical methods. A preliminary cruise

or a good estimate of the variability within each

class must be made before the required number

of plots per class can be computed. This approach is best for large tracts; for small ones it

may be costly or unwieldy.

38
4. By a combination of area and value (volume).

This is the preferred approach for small areas.

A suggested procedure adopted from Johnson

(6) and Spurr (18) follows.

Plot allocation hy area and volume.—Assume

that the tract is 1,600 acres and that 400 field plots

are to be distributed among three forest types

(pine, pine-hardwood, and hardwood) and three

volume classes within each type (5, 8, and 12 cords

per acre).

1. Stratify each forest type into volume-per-acre

classes on the basis of personal experience or

with aerial stand-volume tables. Precision is

not required, as the volume classes are used only

as a guide. Tabulate by type and acreage :

PinePine hardwood Hardwood

Cords per acre (acres) (acres) (acres)

5 120 200 100

8 100 500 350

12 80 100 50

Total 300 800 500

2. Multiply the number of acres in each class by

the cord volume per acre. The product is the

number of cords per class:

Pine Pine-hardwood Hardwood

5X120= 600 5X200=1,000 5X100= 500

8X100= 800 8X500=4,000 8X350=2,800

12X 80= 960 12X100=1,200 12X 50= 600

2,360 6,200 3,900

3. Add the cordage in each class to get the total

for the entire area: 2,3604-6,200+3,900 =12,460

cords. Divide by the number of plots to get

the number of cords to be represented by each

plot:

12,460 ^, ,- , 1 ,

-— =31.15 cords per plot

400

4. Divide the volume for each class (item 2) by the

cord volume per plot to get the number of plots

assigned to each class. Eound to the nearest

whole number. Add the plots for each class to

make sure the total (400) is correct.

Pine Pine-hardwood Hardwood

^=19.26(19) »=^2-^«(^^) S-r''-''^''^

^=25.68(26) 1^= 128.41(128) g5=89.88(90)

Ä= 30.82(31) |g= 38.52 (39) ^= 19.26(19)

(76) (199) (125)

Total : 400 plots

Arrangement of ground samples.—If type

boundaries have been accurately delineated and

stands are homogeneous within the recognized

classes, field plots can sometimes be taken along

routes of easy travel without introducing much

bias. Usually, though, it is necessary to lay out

line-plot or strip cruises at right angles to topography. The cruise lines should be drawn to scale

on a type map in such a way that the required

number of samples within each class can be obtained. The number of plots or chains of strip

that a crew can complete per day is the basis for

calculating the lengths of the lines. To minimize

travel, lines may be triangular or U-shaped, beginning and ending near the same starting point on

a road or trail. Compass bearings and distances

can be determined on the map to avoid location

bias in the field. The cruise lines can then be

placed on the photographs for use in the field, if

desired.

Field measurements are taken by conventional

procedures. Cumulative tally sheets or pointsampling may be employed to speed the tree tally.

After the cordage per acre for each volume class

has been determined by field sampling, the values

are multiplied by the appropriate stand acreages.

The result is the total volume on the tract, by forest

types:

Assumed

volume class

(cords per

acre)

Field

Field volumes

plots per acre

(number) (cords)

PINE TYPE

Type

area

(acres)

Volume

per class

(cords)

5

8

L2 _ - -

19 3.9

26 8.2

31 11.3

120

100

80

468.0

820.0

904.0

Total 76 300

PINE-HARDWOOD TYPE

5 32 5.8 200

8 128 7.7 500

12 39 14.1 100

Total 199 800

HARDWOOD TYPE

5 16 4.7 100

8 90 8.6 350

12 19 13.5 50

Total 125 500

2, 192. 0

1, 160. 0

3, 850. 0

1, 410. 0

6, 420. 0

470.0

3, 010. 0

675.0

4, 155. 0

39
LITERATURE CITED

( 1 ) American Society of Photogrammetry.

1952. Manual of photogrammetry. Ed. 2, 876 pp.,

illus. Menasha, Wis. : Banta.

(2)

1960. Manual of photographic interpretation. 868

pp., illus. Menasha, Wis. : Banta.

(3) Avery, T. Eugene.

1958. Composite aerial volume table for southern

pines and hardwoods. Jour. Forestry 56: 741-745,

illus.

(4)

1962. Interpretation of aerial photographs. 192 pp.,

illus. Minneapolis, Minn. : Burgess.

( 5 ) and Meyer, Merle P.

1^59. Volume tables for aerial timber estimating in

northern Minnesota. U.S. Forest Serv. Lake States

Forest Expt. Sta., Sta. Paper 78, 21 pp., illus.

(6) Johnson, Evert W.

1952. Timber volume determinations using aerial

photographs. Ala. Polytech. Inst., 43 pp., illus.

(7)

1954. Ground control for planimetric base maps.

Jour. Forestry 52 : 89-95, illus.

(12)

(8)

1954. ''Shadow-height" computations made easier.

Jour. Forestry 52 : 438^442, illus.

(9) Kramer, P. R., and Sturgeon, E. E.

1942. Transect method of estimating forest area

from aerial photo index sheets. Jour. Forestry 40 :

693-696, illus.

(10) .Minor, C. O.

1951. Stem-crown diameter relations in southern

pine. Jour. Forestry 49 : 490-493, illus.

(11) Moessner, Karl E.

1957. Preliminary aerial volume tables for conifer

stands in the Rocky Mountains. U.S. Forest Serv.

Intermountain Forest and Range Expt. Sta. Res.

Paper 41,17 pp., illus.

1957. How important is relief in area estimates from

dot sampling on aerial photos? U.S. Forest Serv.

Intermountain Forest and Range Expt. Sta. Res.

Paper 42, 16 pp., illus.

(13) and Jensen, C. E.

1951. Timber cruising on aerial photos. U.S. Forest

Serv. Central States Forest Expt. Sta. Tecîh. Paper

126, 27 pp., illus.

(14) , Brunson, D. F.. and Jensen, C. E.

1951. Aerial volume tables for hardwood stands in

the Central States. U.S. Forest Serv. Central States

Forest Expt. Sta. Tech. Paper 122,15 pp., illus.

(15) Pope, Robert B.

1962. Constructing aerial photo volume tables. U.S.

Forest Serv. Pacific Northwest Forest and Range

Expt. Sta. Res. Paper 49, 25 pp., illus.

(16) Sayn-Wittgenstein, Leo.

1961. Recognition of tree species on air photographs

hy crown characteristics. Photogrammetric Engin.

27: 792-809, illus.

(17) Society of American Foresters.

1955. Forestry handbook. 23 sects., illus. N.Y. :

Ronald Press.

(18) Spurr, S. H.

1948. Aerial photographs in forestry. 340 pp., illus.

N.Y. : Ronald Press.

(19) Wilson, R. C.

1949. The relief displacement factor in forest area

estimates by dot templets on aerial photographs.

Photogrammetric Engin. 15 : 225-236, illus.

(20) Zsilinszky, Victor G.

1963. Photographic interpretation of tree species in

Ontario. Ontario Dept. Lands and Forests, 80 pp.,

illus.

40 U.S. GOVERNMENT PRINTING OFFICE: 1969 O—367-547
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Figure 1.—An aerial stereo-pair, from 9- by 9-inch contact prints.

Note that the print showing the time of day is the last one

in the flight line. The principal point (PP) of one photo coincides

with the conjugate principal point (CPP) on the overlapping print.

The flight path of the aircraft is indicated by the lines connecting the

PP and CPP.