New Topographic Survey Methods 435 



engineer is aware who has used one in mountainous regions subject 

 to frequent and sudden atmospheric changes. As the working 

 aneroid goes through the changing atmospheric conditions during 

 the day, it is necessary to correct the readings in proportion to the 

 fluctuations of the camp barograph. 



After the day's field work is completed and the aneroid reading 

 corrected, there follows the difficult work of adjusting contours to 

 form the finished map. It the topographer has been conscien- 

 tious and is skilled at his work, he has gathered a wealth of topo- 

 graphic detail which is all to be lost and wasted when his contours 

 are put through the "juggling" process. The very lack of pre- 

 ciseness and the knowledge that the map must be adjusted greatly 

 discourages careful and conscientious sketching in the field. 



It is very evident that the map cannot be accurate except in a 

 general way. All topography is approximately correct, but would 

 hardly answer on a map on which to make the paper location for a 

 railroad or a flimie. The cost of making surveys using the method 

 outlined above is from 5 cents to 35 cents per acre. 



Such was the condition when the engineers of District 1 of the 

 Forest Service commenced their series of experiments to develop 

 a more efficient method of topographic surveying. Ordinary 

 topographic-survey practice was not applicable. Heavy timber 

 and dense brush and undergrowth prevented efficient use of 

 plane-table method, as it was necessary to get beneath the cover 

 of the forest in order to get the topographic detail required. Also 

 the operations must necessarily be combined and co-ordinated 

 with the timber estimating and appraisal work, and could not, 

 therefore, follow the methods that would be adopted for the pro- 

 duction of a topographic map alone. Low cost was an important 

 consideration. 



The first tests were made with the ordinary Abney level or 

 clinometer familiar to all engineers. While the results were 

 satisfactory, it was a cumbersome method, in which the field men 

 were compelled constantly to refer to reduction tables in order 

 to ascertain their true horizontal position. The instruments as 

 then made were not constructed for such work and it was almost 

 impossible to keep them in adjustment. By constantly experi- 

 menting and making improvements, a new Abney level was 

 developed that is now manufactured expressly for work of this 

 character. Simplified methods, introducing greater speed and 

 accuracy, were worked out. 



