The Ahney Hand Level. 357 



the reading on the map sheet. He paces along the Hne until a 

 change of slope is encountered. He marks this position on the 

 map sheet where the change in slope occurs. The slope reading 

 now applies to the horizontal distance along the line between the 

 point of the first reading and the position where the change in 

 slope occurred. Having marked the point on the line where 

 there is a change in slope the compassman takes another sight 

 parallel to the slope of the ground along the compass line and 

 pioceeds as before. It is desirable to take several readings at 

 different points along the same degree of slope, in order to get 

 better average approximation of the slope. 



The compassman does not attempt to run a line of levels ; the 

 object of the field work is not to establish a set of absolute ele- 

 vations along the strip line, but in most cases the compassman 

 is working between two base line stakes having precise eleva- 

 tions, and if he preserves the relative value of the slopes in per 

 cents or degrees along his lines between these two bench marks 

 at each end of his line the office draftsman can determine the 

 absolute elevations of the slope station in the office. The field 

 man however does not, attempt to perform this unnecessary work. 

 These datas are as valuable as a set of absolute elevations. 



The compassman sketches the topography passed over along 

 the strip. The guess of the man in the field as to the form of 

 the country is better than the guess of the man in the office. The 

 field man draws fine form lines following points of equal eleva- 

 tion. He does not attempt to draw these form lines separated by 

 known contour intervals. The man in the field is not drawing 

 contours but sketch lines. These sketch lines are supposed to 

 show the man in the office that the points on the sketch line to 

 one side of the strip are of the same elevation as the point of in- 

 tersection of the line of strip and the sketch or form line. By 

 clamping the Abney at O degrees or per cent the form line can be 

 traced to some distance on each side of the line. 



It is an established rule to sketch only the country already 

 passed over and lieing behind the compassman. 



The direction of sketch lines is governed by streams and valley 

 lines, peaks and ridge lines. These points of topographic control 

 are of great assistance to the man in the office when plotting the 

 map ; hence the field data obtained with the Abney include the 



