The Ahney Hand Level. 363 



having the greatest horizontal distance between them. Having 

 corrected the area of Hne and pacing, the draftsman proceeds to 

 correct the vertical control obtained from the field data. 



2. Vertical Closure. Beginning with the precise elevation on the 

 base line stake, the draftsman works across the strip obtaining the 

 elevations of all slope stations. These operations are very simple 

 and can be perfomied with speed. At the end of the strip the 

 draftsman compares his calcidated elevations with the base line 

 stake elevation. If there is a sufficiently large error to correct, this 

 error is distributed along the line, pro ratio to the difference in ele- 

 vation between the slope stations. A large part of the error is 

 thrown between stations having a large difference in elevation than 

 between slope stations having small difference in elevation regard- 

 less of the horizontal distance between slope stations. It is im- 

 perative that the draftsman calculates the elevations of the slope 

 stations only after he has corrected their position horizontally on 

 the strip line. The error due to pacing will affect the vertical 

 control unless the horizontal control is corrected first. 



Having corrected the errors of horizontal and vertical closure 

 the next operation is the location of the contours upon the form 

 line field sketch turned in by the field man. 



The Location of Contours on the Form Line Sketch. 



The form lines are an absolute index to the direction of the 

 contour crossing the strip line and the office man is not to inter- 

 fere with the field sketch. The man in the field was' on the 

 ground when the form lines were drawn and his guess in the field 

 is more valuable than the guess of the man in the office who may 

 be unacquainted with the country being mapped. 



This form line as drawn by the man in the field is the guide 

 line for the contour. A contour drawn at an acute angle to a 

 form line would be revealed as an obvious error to the office man 

 were he at the spot in the field to compare his contour as drawn 

 with the actual topography. In plotting these Abney field notes 

 the draftsman should conform strictly to the form lines drawn 

 by the man in field, having first corrected their horizontal location 

 if an error in horizontal closure has been made by the man in the 

 field. The contours will then represent the true character of the 

 topography of the country mapped. The finished map will have 



