438 



Forestry Quarterly 



United States Geological Survey. Secondary control lines are 

 run around a block of sections or topographic unit, stakes or posters 

 on trees being left at periodic intervals from which the strip 

 surveys may be begun or closed. 



The Abney instrimient is used for the control as well as for the 

 strips, it being found to be just about as accurate as direct leveling, 

 and much faster. The work on control lines is, of course, much 

 more carefully executed than on the strip lines, two Abneys 

 being used. 



The control system is so planned that the strip surveys will not 

 be run more than two miles without closing upon a control line. 

 The strip surveys are run along parallel lines usually an eighth or 

 quarter mile apart, depending upon the density of the timber and 

 the degree of detail required. 



It is customary to combine the mapping with either the timber 

 estimating or the soil classification work, thus making one survey 

 serve two purposes and reduce the cost of each. The crew con- 



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Fig. 3. — Locating Contours by the Abney Method 



sists of the topographer, who keeps his direction with a staff 

 compass, operates the Anbey and sketches the topography, and 

 the estimator, who "snubs" the rear end of the tape, making the 

 proper allowance for slope, and estimates the timber on a strip 33 

 feet wide on each side of the line. Experience has shown that the 

 time lost by the estimator in holding the rear end of the tape to be 

 practically negligible. 



The strips are usually belts 10 chains wide, the line being run 

 along the center line and topography sketched for 5 chains on 

 either side. It will be noticed that the method is not a system of 

 determining the elevation of points along a profile line, and inter- 

 polating the intermediate contours, such as the aneroid method, 

 but a number of points are located through which each contour 

 must pass, by taking side shots at right angles to the general 

 direction of the contours, as indicated by the dotted lines and 

 arrows in Fig. 3. Although the skeleton of the topography is 

 thus accurately obtained mechanically, the sketching or filling 

 in of the topographic detail depends entirely upon the skill and 



