SAMPLE PLOTS IX SILVlCULTURAL RESEARCH 15 



areas to which the research findings are to be applied is impossible 

 as regards either stand, sites, or topography. This fact, however, 

 does not lessen the need of care in the survey or in any other fea- 

 ture of plot measurement. 



The smaller the plot, the greater the need of accuracy in the 

 survey. 



In many studies an accuracy of about 1 in 500 will suffice. This 

 may be expected when a staff compass and steel tape are used. 



The usefulness of the compass has decided limitations where 

 magnetic attraction is present, where slopes are steep, where stands 

 are dense, or on plots having more than four sides. The compass 

 is subject, also, to considerable instrumental error. 



When the compass is used with an angle mirror, the results are 

 more precise if only 90° angles are involved. In this case the com- 

 pass is used not as a compass but merely as a convenient sighting 

 instrument. In surveying plots of less than half an acre, the angle 

 mirror can be used alone (65) . 



In capable hands the transit is a highly satisfactory instrument 

 for surveying sample plots, although it is rather expensive and some- 

 what cumbersome and its use consumes much time. By the use of a 

 transit, survey accuracy can be increased to 1 in 1,000. As an 

 added check, the diagonals of rectangular plots may be measured. 10 

 Directions for surveying such as are given in various manuals and 

 texts should be carefully followed. 



Steel tapes should be considered standard for linear measurements. 

 A tape graduated into links instead of feet is convenient for laying 

 out plots containing even acres or simple fractions of an acre. A 

 surveyor's link chain has certain advantages over a tape for laying 

 out small quadrats, because it can be bent around corners and can 

 be fastened at convenient intervals with chaining pins. When the 

 links become worn, however, such a chain is inaccurate. When 

 horizontal distances are to be measured on a slope, a " topographic " 

 tape, used with an Abney level fitted with a corresponding topo- 

 graphic limb, saves time and gives measurements as precise as those 

 obtainable with compass and chain (66). The survey notes should 

 show the Abney readings on all lines. 



On steep, brushy slopes, stadia surveys consume much less time 

 and are often more accurate than direct measurement. 



Instruments used in plot surveys are shown in plate 3. 



Sample-plot areas should be measured horizontally and the data 

 expressed in terms of horizontal area. As slope apparently influences 

 growth, 11 it is important that degree of slope be indicated. 



In many instances preliminary lines can be run accurately with 

 the aid of a reflecting surface such as an aluminum notebook holder 

 or the bottom of a pie pan. If the reflected beam of light is flashed 

 up and down along the line, the head chainman can sight back and 

 thus keep accurately on line. To what distance such a beam can 

 be used depends upon brightness of sky, time of day, direction of line. 



^ 10 A. p J°£" su ,£I G5 ' method in which use is made of plane table, alidade, and tape is 

 described by Thrupp (74). 



"McArdle and Meyer (.$?). taking a slope of from 20 to 30 percent as normal in 

 l)ouglas_ hr stands, found the yield on level ground 5 percent less than normal, that on a 

 6U- to oO-percent slope 5 percent greater than normal, and that on an 80-percent slope 

 10 percent less than normal. The relationship of slope to growth needs much further 

 study. 



