APPENDIX 2 



PROCEDURES FOR DEVELOPING BASIC DATA ON 

 SOIL AND LAND USE CONDITIONS 



The basic data on soil and land use conditions in Montana were secured from 

 soil surveys. A set of randomized samples to represent every county were 

 selected by the statistical laboratory at Iowa State University. The sample 

 units were located on county base mapso The map showed the boundaries and 

 symbols of land-resource units. Boundaries of federally owned land were 

 shown on the maps. 



Land-resource units were used in the selection of samples. The State SCS 

 office informed the laboratory that it wanted samples drawn by land-resources 

 units fiind indicated whether or not a higher or lower sampling rate was wanted 

 for certain land-resource units. 



Provision was made for identifying all samples by land-resource units so that 

 data could be combined on that basis for study of special problems. It was 

 necessary, therefore, to have a land-resource map of the State, 



A land-resource map shows the geographic distribution of land-resource units. 

 A land-resource unit is a geographic area of land, usually several thousand 

 acres in extent, characterized by a particular combination or pattern of 

 soils (including slope and erosion) j, climate, water resource, land use, and 

 types of farming. Such a unit may occur in one continuous area or as several 

 separate but nearby areas. 



The standard size of sample units was 160 acres. The basic sampling rate was 

 2 percent. As standard procedure, the statistical laboratory selected two 

 separate sets of sample units in each county, each set representing approxi- 

 mately 2 percent of the county. In a county or other area of 250,000 to 

 500,000 acres, a 2 percent sampling provided data of an acceptable degree of 

 reliability. In counties of this size, therefore, it was necessary to map 

 only one set of sample units. In larger areas, the rate was reduced, but in 

 smaller ones it was increased in order to maintain the same degree of 

 reliability. 



The laboratories used the following procedure in selecting sample units; The 

 county, or land-resource area within the county, was divided into blocks 

 (called "strata") which were then farther subdivided into equal-sized sample 

 units. One sample unit was selected at random from each block for each of 

 the two sets of samples. 



In Montana where section lines were easily identified, both on the photographs 

 and on the ground, the designated quarter sections were delineated. 



The laboratory outlined one set of samples in red and the other in blue , on 

 the county map. From this map sample unit boundaries were transferred to 

 the aerial photographs on which the mapping was done. 



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