440 Forestry Quarterly 



a base line for plotting the data from the strip line which is to 

 be mapped. The pivot point of the alidade is inserted at the top 

 of this line as a starting point, the board is held with its face 

 perpendicular to the ground, and sights are taken along the top of 

 the board to the point to which it is desired to obtain the grade 

 (Fig. 2). The same principles govern sighting with this instru- 

 ment that govern sighting with the Abney or any other hand level. 

 When the suspended ruler has ceased swinging on the pivot, the 

 board is gently turned face upward so as not to disturb the 

 position of the ruler, and a pencil line (which is really the line 

 of slope) is drawn in the slot. The weight of the ruler is suffi- 

 cient to keep it in place, if the pivot is properly constructed. 



The length of a perpendicular line from the base line to the 

 slope line measures the elevation of a point on the base line, on the 

 same scale that is used to tally paces on the base line. If a large 

 scale and finely divided cross-section paper are used, the eleva- 

 tions are plotted and read accurately and quickly. Pacing for- 

 ward to a new sighting point on the strip line, after drawing in 

 the line of slope from the starting point, the new sighting point 

 is marked on the base line of the profile sheet, according to scale, 

 and the pivot point of the alidade is inserted where a line drawn 

 at right angles to the base line through this new sighting point 

 intersects the slope line. A new sight along the line of slope is then 

 taken. When completed (Fig. 3), the slope lines on the sheet 

 form a profile of the elevations on the strip lines ; the elevations 

 to the left of the base line are higher, and those to the right of 

 the base line are lower, than the starting point when the cross- 

 section paper is on the right-hand face of the vertically-held 

 sketching board. This throws the starting point to the right- 

 hand edge of the sheet when the profile is held in its proper 

 position. Using the instrument on the left face of the board 

 will, of course, throw the starting point of the left side of the 

 sheet, when the profile is held so as to show the relative positions 

 of the slope lines to the base line. 



The sketching board may be of sufficient length to accommo- 

 date sheets large enough for mapping several miles of line, but 

 it is preferable to have the paper mounted on rollers and reduce 

 the size of the board. A board similar to the army sketching 

 case, with a hand grip at the back, is most convenient. The 

 base line on the sheet may be used for entering the sketches of 



