areas in Class II, and 10-mile intervals for areas in Class 

 III. Thus, the three classes were subdivided into units 

 containing approximately 16, 49, and 100 square miles 

 respectively, each unit centered on one of the section cor- 

 ner control points. 



3. The control points were transferred to aerial photo index 

 maps from which photographs were selected to give photo 

 coverage for a segment of each unit containing forest land. 

 For Class-I units the sample segments consisted of four 

 Land Office sections (2,560 acres) centered on the control 

 point, for Class-II and -III units the sample segments con- 

 sisted of one section (640 acres) lying northeast of the 

 control point. 



4. All sample segments containing commercial forest land, in- 

 cluding those with doubtful forest cover by photo interpre- 

 tation and all sample segments without aerial photographs, 

 were examined and mapped in the field. For each sample 

 segment the forest cover was stratified by commercial 

 character, forest type, stand-size, stocking, age, and site 

 classes. The area of these stratifications was determined 

 for each mapped sample segment and was multiplied by sample 

 factors (the area of a class divided by the area sampled in 

 that class) to get the total area by forest condition 

 classes . 



5. Timber volume was tallied on three l/5-acre sample plots in 

 each sample segment of Class I and on two l/5-acre plots in 

 each sample segment of Classes II and III, that contained 

 commercial forest land. The plots were randomly located in 

 the commercial forest area stratification falling closest 

 to the control point of the sample segment. Total volumes 

 were derived by multiplying average acre volumes for each 

 forest condition and sample segment class by the appropriate 

 areas . 



ACCURACY OF THE DATA 



In determining the extent of various cover types and stand-condition 

 classes, there are two possible sources of error: (1) errors in 

 classifying the cover of the field samples and in compiling the field 

 data, and (2) sampling errors. The former result from mistakes of 

 judgment or technic and the complexity of the cover which not in- 

 frequently grades from one class into another with no clearly defined 

 boundaries. These errors were minimized by the exercise of care and 

 skill, but it is seldom possible to evaluate them. An effort was 

 made to maintain a high order of accuracy and uniformity of standards 

 in the classification, collection, and compilation of sample data, by 

 field checks, by a continuing program of training, and by cross checks 

 in the office . 



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