FOREST SURVEY METHOD 



Area and Volume 



Initial survey . The initial survey of North Idaho was made during the 

 years 1932 to 1937 by the "compilation" method as follows; The bounda- 

 ries of non- forest land and forest land classified by type, stand-size, 

 stocking, age, and site classes were delineated on 2-inch-to-the-mile 

 base maps as precisely as possible from intensive cruise data, county 

 assessment records, cutting and fire records, and other available sources. 

 Stand characteristics, such as age and site, which could not be deter- 

 mined from available records, along with some missing data, were deter- 

 mined by field examination in connection with an over-all check of all 

 data. Forest cover classifications were based on prevailing conditions 

 on areas of generally 40 acres or more. 



From 2-inch-to-the-mile townships maps the area of the various cover 

 classes within each ownership was computed by planimeter or 10-acre 

 square count. The area data were tabulated by Land Office sections 

 and summarized by ownership, classification, and county. The possible 

 number of classes for commercial forest land alone were 10 (types) x 

 3 (stand-size classes) x 3 (stocking classes) x 6 (age classes) x 

 5 (site classes) or 2700. 



Timber volume was estimated for each commercial forest delineation 

 either from intensive cruises adjusted to a common standard, field sam- 

 ples, ocular estimates, or modified normal yields correlated with type, 

 stocking, age, and site classes. 



Re survey . Although the initial survey gave very detailed and localized 

 statistics on the forest resource, the cost of repeating periodically 

 such a survey to obtain current statistics would be prohibitive. To 

 offset this cost a sampling method was designed and followed- — with some 

 variations-- for all of the ten counties in North Idaho. 



In resurveying eight of the counties (the design varied for Benewah and 

 Idaho Counties), the areas of nonforest and forest land with various 

 subclasses were derived by correlating initial survey area data with 

 forest cover observed along sample transects. The transects were ran 

 in a random direction to and from Land Office section corners spaced 

 at 4-mile intervals. Cover conditions were observed, classified, and 

 recorded on a strip two chains wide; a 2x2 chain or 0.4-acre unit be- 

 ing the minimum area considered in the classification. 



The transect courses were plotted on the initial survey type naps. The 

 total area of each initial survey classification was reclassified in 

 accordance xvith the composition indicated by the transect sample for 

 that classification. 



For volume determination a location, consisting of three l/5-acre 

 sample plots, was established at each Land Office section corner which 

 controlled the line transects. The measurements from these plots 



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