Appendix 


SURVEY METHODS 
HE FOREST survey of South Carolina utilized 
aerial photographs of the Production and Market- 
ing Administration as a basis for preliminary 
estimates of forest area and timber stand-class area. 
| A stratified sample of forest plots was located on the 
photographs and these plots were then carefully cruised 
on the ground. The county was the basic work unit. 
Detailed statistics by survey units and counties are 
given in Forest Survey Releases 25 and 28 (12, 6). 
Area Estimates 

Acreages of forest and other land were estimated with 
the use of a dot grid placed on every third contact 
print along each flight line in each county. The pro- 
portion of dots falling on forest land when applied to 
the gross area of the county, as reported in 1940 by the 
Bureau of the Census, yielded a preliminary estimate of 
the acreage of forest and other land-use classes in each 
county. This estimate was later revised after field checks 
of a sample of both forest and nonforest ground plots 
were made. All together 175,839 of these dots were 
classified. 
Every fifth dot classified as forest in the preceding 
step was further classified into forest type, stand class, 
and density class by careful stereoscopic analysis of a 

one-acre plot surrounding the dot on the photograph. 
About 20,900 of these plots were classified. The pro- 
portion of plots falling in each classification when 
applied to the forest area of the county gave the total 
area in each classification. These areas were revised 
after a sample of plots was checked for proper classifica- 
tion on the ground. 
In estimating the areas of various categories of land, 
there were two possible sources of error: (1) errors in 
classifying the dots and plots or in compiling the data, 
and (2) sampling errors. In this survey every effort 
was made to maintain a high order of accuracy in the 
collection and compilation. Frequent checks were made 
and a continuous program of training was carried out. 
The sampling intensity was sufficient to provide an 

| Timber Supply Outlook in South Carolina 
| 
estimate of the forest acreage of the State with a 
standard error of + 0.7 percent. This indicates the 
probabilities are two out of three that the actual forest 
area is within + 0.7 percent of the value given, provided 
measurement and computing errors have introduced 
no bias. 
Volume Estimates 
Timber cruisers made a detailed on-the-ground tally 
of a proportion of the photo plots in each stand class 
to obtain volume, growth, cull, and mortality data and 
to check the accuracy of the photo classification. Propor- 
tions varied according to the distribution of stand classes ; 
in the Coastal Plain, for example, every 3d large saw- 
timber photo plot, every 8th small saw-timber, every 
17th pole-timber, and every 30th seedling, sapling, and 
denuded plot was taken. The total amounted to 2,363 
Y/4-acre plots, although these were classified by forest 
type and stand class on a 1-acre basis. When the forest 
ground plots were taken, an additional sample of 
569 plots, classified as agricultural on the photographs, 
was taken to provide a check on land-use changes 
since the date of photography. 
In estimating timber volumes, the sources of error 
include (1) errors in classifying field plots and in 
compiling the data, (2) sampling errors, (3) inaccurate 
measurements of tree diameter, height, form, and cull, 
and (4) bias resulting from improper construction or use 
of tree volume tables. As in the case of area determina- 
tions, every effort was made to obtain accurate classi- 
fications, measurements, and final statistics through 
frequent checks and training. The volume tables used 
also were checked and were found to give reasonably 
accurate results. The standard error of estimate of the 
board-foot volume of saw timber in the State is + 1.6 
percent; a corresponding error for the total volume in 
cords was not computed, but it should be smaller. 
Growth Estimates 
Measurements for growth calculations were obtained 
from increment borings made in a mechanically selected 
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