90 A MANUAL OF TOPOGRAPHIC MKTHODS. 



It must not be understood, however, that it is at all necessary that the 

 survey of the profile should establish the height of all the points located 

 by the traverse. The profile should give the elevation of all valleys and 

 summits, and of all road crossing's. The line should be carried forward 

 and these points measured by as few and as long lines of sight as possible. 

 Often the roof of a house will furnish a datum point for use for a mile or 

 two. Indeed, in an open, settled country the line can frequently be carried 

 forward continuously by using housetops as targets. 



The reduction of the profile must keep pace with the field work, so 

 that on arriving at a check point the amount of the error may be shown at 

 once. If this is not more than one-fourth or one-fifth of the contour 

 interval, it is not considered as of material account. If, however, it reaches 

 half a contour interval, the work should be examined, and if the error be 

 not discovered the line should be resurveyed. 



The heights, as determined, should be written in ink upon the plane- 

 table sheet in their proper places. 



THE ANEROID. 



In the great majority of traverse work heights are measured with 

 aneroids. The aneroid consists of a vacuum box of thin corrugated metal, 

 which is compressed by an increase and expanded by a decrease in the 

 pressure of the atmosphere. A train of mechanism magnifies this trifling 

 movement enormously and moves an index upon a graduated dial. This 

 dial is graduated to feet of elevation and also to inches of barometric 

 pressure.- 



Several sizes of aneroids are made; that having a diameter of 1\ 

 inches is on the whole found the most satisfactory. 



Owing mainly to its extreme delicacy the aneroid is a very uncertain 

 instrument. It should be used differentially only, and for small differences 

 in height and small intervals of time. Its indications should be checked 

 by reference to known elevations whenever opportunity is afforded during 

 the day, and at the beginning and ending of each day's work. 



On commencing work the movable scale on the aneroid should be set 

 at the known height of the starting point and a note made of its reading 

 on the inch scale. Elevations should then be read directly from the scale 



