PREPARATORY MAPS AND DATA 15 



to read alike at the office at the beginning of each half day. 

 One barometer remains in the office, where it is read at three- 

 minute intervals with the times noted. The other is taken 

 through the city by horse and buggy and readings taken at 

 every street intersection, at intermediate points where the 

 surface grade changes, at water levels, etc. The times of read- 

 ing are also noted, while the distances are estimated from the 

 number of revolutions of the buggy wheel. The office readings, 

 when plotted, show graphically the variations due to changes 

 in barometric pressure alone and give the corrections from the 

 initial reading to be applied to the field readings. About 

 20 miles of street can be covered in a day and the elevations 

 should check to within a few feet of the true elevations, the 

 difference being too small to invalidate any preliminary plans 

 or conclusions. Mr. Fuertes, referring to some topographical 

 work done in this way outside of a certain city, gives the cost as 

 $457 for field work and $136 for office mapping, a total of $593. 

 The area covered was 9500 acres, so that the cost of the fielp 

 work was at the rate of 4.8 cents per acre and the mapping 1.5 

 cents per acre, a total cost of 6.3 cents per acre. This served 

 to provide a topographical map with contours at 2o-foot inter- 

 vals and on the scale used (one mile to the inch) made maps 

 very similar to those of the United States Geological Survey. 



It will be necessary to have another set of maps to show 

 details of location not possible on the small-scale maps 

 details which are not needed until actual construction begins. 

 This work is therefore usually carried along with the construc- 

 tion. These latter maps (see Plate II*), made on a scale of 

 40 to 60 feet to an inch, are plotted on separate sheets about 

 20X30 inches, as nearly as the size of paper at hand makes 

 convenient. The paper should be of parchment or a similar 

 thin paper from which blue-prints can be made; if this is not 

 procurable, a medium weight of bond paper will serve the pur- 

 pose. These large-scale sheets are of great use to the field- 

 party, who, when engaged in staking out the line on the ground, 



* Copied from Engineering News, Vol. XXXV. p. 2. 



