l No. 88 

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THREE SCALES OF AERIAL PHOTOGRAPHY 

COMPARED FOR MAKING STAND MEASUREMENTS 

Three scales of aerial photography were tested in an 

attempt to determine the best scale to use in forest surveying. This was done by comparing photo measurements of average tree height, average crown diameter, and crown-closure 

percent. These stand variables were selected for testing 

because of their applicability in making aerial estimates of 

timber volume. 1 

During the winter of 1956, a small test area in 

Hancock County, Maine, was photogr&phed on infrared film at 

scales of 1: 1,000, 1: 5,000, and 1 :15,840.' As hardwoods 

were leafless at that time, photo interpretation was concentrated on predominantly coniferous stands (spruce, fir, 

hemlock, and northern whitecedar). On each of twelve 1/5- 

acre plots, three photo-interpreters determined (1) average 

total height of the three tallest softwoods, (2) average 

crown diameter of the three tallest softwoods, and (3) crown 

closure percent of the softwood stand. 

Analyses of variance were employed to compare photo 

measurements by interpreters and photo scales ; "t" tests 

were used to evaluate differences between photo and field 

measurements of the three variables. Results of these tests 

are summarized in tables 1 to 4. 

Tree height.--Table 1 shows a significant difference 

among scales and among interpreters for photo estimates of 

average total height, but the interaction of scales and 

interpreters was not significant. The analysis also reveal- 

'smith, J. Harry G. Problems and potential uses of photo-mensurational techniques for estimating volume of some immature stands of Douglas- fir and western 

hemlock. Photogram. Engin. 23: 595- 599. 1957. 

'Appreciation is expressed to the Maine Forest Service, the James W. Sewall 

Company, Old Town, Me., and the U.S. Dept. Agriculture Forest Insect Laboratory, 

Beltsville, Mi., for supplying aerial photographs used in this study. 
ed a significant difference between mean photo heights and 

I corresponding field heights. This difference was negative 

for all scales and interpreters, that is, all photo heights 

were lower than the field average of 53 feet. 

The "t" test summary (table 2) indicated that there 

is no statistical choice of photo scales for measuring tree 

heights. The mean estimate of 48 feet on 1 :5,000 photos was 

closest to the true field value, but it is doubtful that the 

slight gain in accuracy over 1:15,840 photos would justify 

the increased cost of using the larger scale. 

Crown diameter.--As in the case of tree heights, 

there was a significant difference between scales for photo 

estimates of average crown diameter (table 1). However, the 

significant interaction of scales and interpreters indicates 

that no single scale was consistently best for all interpreters. / 

Table 1 also shows a significant difference between 

photo and field measurements of crown diameter. Photo estiTable 1.--Summary of analyses of variance for photo measurements 

(x = significant at 1-percent level) a 

Table 2.--Summary of "t" tests for photo versus field measurements 

of mean total height, in feet1 

)dean field height: 53.33 feet. 

**Difference significant at the 5-percent level. 

Difference significant at the 1-percent level. 

t 

Scale 

Source of variation 

Between field and photo estimates x x -- Among photo scales x x x 

Among interpreters x -- -- Interaction: scales x interpreters -- x -- 

Crown 

diameter 

Total 

height 

1:1,000 43.92** 44.17** 45.58** 44.56** 

1 :5,000 46.67* 45.58** 52.17 48.14* 

1 : 15,840 41.08** 40.75** 49.00 43.61** 

Means 43.89** 43.50** 48.92* 45.44** 

I 

Interpreter 

A 

Crown 

closure 

Means 
Table 3.--Summary of "t" tests for photo versus field measurements 

of mean crown diameter, in feet1 

;#Aean field crown diameter: 16.08 feet. 

**Difference significant at the 5-percent level. 

Difference significant at the 1-percent level. 

1 

Scale 

1 

Table 4,--Summary of "t" tests for photo versus field measurements 

of crown closure percent 1 

Interpreter 

A I 3 1 C 

i~ield crown closure : 55.00 percent. 

**Difference significant at the 5-percent level. 

Difference significant at the 1-percent level. 

Means 

1 

Scale 

mates, with one exception, were consistently lower than the 

field average of 16 feet (table 3). Interpreters "A" and I 11 B" obtained their best estimates on 1:15,840 photos, while 

interpreter "c" did best on the 1:1,000 scale. Although the 

interaction prohibits a generalization for scale averages, 

it appears that average crown diameters can be determined as 

accurately on 1:15,840 photos as on larger print scales. 

1 : 1,000 12.92** 11.25** 14.00 12.72** 

1:5,000 12,75** 14.50 12. OO** 13.08** 

1 :IS, 840 17.75 15.08 13.83* 15.56 

Means 14.47 X3.61** 13.28+* 13.79** 

Crown closure percent,--Photo measurements of this 

variable differed significantly by scales (table 1). Crown1:1,000 47.42 52.00 52.67 50.69 

1:5,000 55.83 61.25 58.75 58.61 

1:15,840 66.67* 70.83** 57.92 65.14* 

Means 56.64 61.36 56.44 58.15 

closure estimates on 1:1,000 photos were about 4 percent 

lower than the field average of 55 percent. On 1:5,000 

photos, estimates averaged about 4 percent higher than the 

field value. Neither differed significantly from the field 

value (table 4). On 1:15,840 photos, the average crownclosure estimate was 65 percent. This figure was signifit 

Means 

Interpreter 

A B C 
cantly different from the 55-percent field average, but Was 

still within generally acceptable limits. 

Conclusions.--None of the three aerial-photo scales 

tested showed a marked superiority for evaluation of average 

total heights, crown diameters, or crown-closure percent. 

Though the 1w:51000 scale provided the most accurate results 

for two of the three variables tested, measurements on 

1:15,840 prints were within a range acceptgb1k ' to many 

interpreters. Until the advantages of larger photo scales 

can be more clearly substantiated, the 1:15,840 scale is 

therefore recommended to northeastern foresters as the best 

buy for their photo dollar. 

--EARL J. ROGERS, GENE AVERY, AND ROX--A,CYMN 

~mon ofL~o~63t Economics Research, 

Forest Service, U. S. Dept. Agriculture 

Washington, D. C. 

AGRICULTURE-FOREST SERVICE-UPPER DARBY