for areas in Glass 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 corner 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 seg- 

 ments consisted 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 in- 

 terpretation and all sample segments without aerial photo- 

 graphs, were examined and mapped in the field. For each 

 sample segment the forest cover was stratified by com- 

 mercial character, forest type, stand-size, stocking, age, 

 and site classes. The area of these stratifications was 

 determined from the mapped sample segments and as re- 

 fined by line transects was multiplied by a sample factor 

 (the area of a class divided by the area as computed for 

 the sample segments) to get the total area by forest con- 

 dition classes. 



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

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

 in the sample segments of Classes II and III. The plots 

 were randomly located within the sample segments. Plot 

 volumes when averaged for a given forest condition were 

 multiplied by the area to determine the total timber 

 volume . 



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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