METHODS OF USING EQUIPMENT 411 



120 strides = 7.68 inches. 



Then divide a line 7.68 inches long into 6 equal parts of 20 strides 

 each, by proportion, assuming at any angle a line readily divisible, such 

 as 6 inches (see figure 4). 



If the left-hand inch of A C be divided into 1/10 of an inch, then one 

 unit on the scale will be divided into divisions of 2 strides each, thus 

 giving a scale on paper vs^hich can be transferred to the triangular ruler 

 (see figure 5, A). Similarly, working scales may be constructed for any 

 fraction, whether metric or miles per inch. 



Methods 



For all plain mapping, such as traversing and location of points by 

 intersections and resections, the equipment described above is used in the 

 same manner as the standard reconnaissance plane-table and folding 

 alidade, except that orientation of the plane-table by back sight is used, 

 wherever possible, in preference to compass orientation, since it insures 

 greater accuracy. The methods for this type of work may be found in 

 almost any text-book on surveying or military mapping. 



The results of the work using this equipment were found to be well 

 within the allowable errors, and in locating points by the Italian resection 

 or other three-point methods the accuracy was comparable with that ob- 

 tained in using the more expensive standard equipment. The allowable 

 error for pacing is usually considered to be 2 per cent ; so that if pacing 

 is the method of measurement employed, the error of all the work can 

 not be less than 2 per cent. With the open-sight alidade, however, the 

 degree of accuracy is much greater and locations by intersection will 

 yield closer results, which on a scale of I/IOOOO would not exceed the 

 wddth of a pencil line. 



For topographic work the base outline is constructed in the usual 

 manner and elevations are obtained by means of the slope-card and a 

 slope scale combined with the pace scale. Slope angles are read in de- 

 grees, by means of the slope-card, to critical points or positions located 

 by triangulation. The distances from the observer to these positions are 

 scaled from the map or paced. With the slope angle and horizontal dis- 

 tance, the vertical distance or difference in elevation can be calculated. 

 The elevations may then be recorded on all measured points, and contours 

 interpolated, thereby giving a topographic sketch map of an area. 



