HIGHWAY CONSTRUCTION 15 



and therefore the points at which lateral communication across the 

 high ground separating contiguous valleys can be most readily made. 

 The instruments employed in recoimoitering, are: The compass, 

 for ascertaining the direction; the aneroid barometer, to fix the ap- 

 proximate elevation of summits, etc.; and the hand level, to ascertain 

 the elevation of neighboring points. If a vehicle can be used, an 

 odometer may be added, but distances can usually be guessed or 

 ascertained by time estimates or otherwise, closely enough for pre- 

 liminary purposes. The best maps obtainable and traveling com- 

 panions who possess a local knowledge of the country, together with 

 the above outfit is all that will be necessary for the first inspection. 

 The reconnoissance being completed, instrumental surveys of 

 the routes deemed most advantageous should be made. When the 

 several lines are plotted to the same scale, a good map can be pre- 

 pared from which the exact location of the road can be determined. 

 In making the preliminary surveys the topographical features 

 should be noted for a convenient distance to the right and the left of 

 the line, and all prominent points located by compass bearings. The 

 following data should also be obtained: the importance, magnitude, 

 and direction of all streams and roads crossed; the character of the 

 material to be excavated or available for embankments, the position 

 of quarries and gravel pits, and the modes of access thereto; and all 

 other information that may effect a selection. 



Topography. There are various methods of delineating upon 

 paper the irregularities of the surface of the ground. The method 

 of most utility to the engineer is that by means of " contour lines." 

 These are fine lines traced through the points of equal level over the 

 surface surveyed, and denote that the level of the ground throughout 

 the whole of their course is identical; that is to say, that every part 

 of the ground over which the line passes is at a certain height above 

 a known fixed point termed the datum, this height being indicated 

 by the figures written against the line. 



The intervals between the lines vertically are equal and may 

 be 1, 3, 5, 10 or more feet apart; where the surface is very steep they 

 lie close together. These lines by their greater or less distance apart 

 have the effect of shading, and make apparent to the eye, the 

 undulations and irregularities in the surface of the country. 



