from curbs, 3 feet from fire hydrants, 12 feet from the nearest  track  of  a  railroad  track,  and  7  feet  from railway  sidings. When you are staking pole locations, the center of each pole is marked with a hub on the line; the hub may be offset. On the guard stake, you put the pole number,  the  line  elevation,  and  the  distance  from  the top of the hub to the top of the pole obtained from the profile. TOWER  LINE  SURVEYS.—   High-voltage lines are often supported by broad-based steel towers. For a tower line, construction economy requires that changes in direction be kept at a minimum. That is because a tower located where a line changes direction must withstand a higher stress than one located in a straight direction part of the line. In general, tower construction is cheaper in level country than in broken country; however, the line may be run over broken country to minimize changes in direction, to make the distance shorter, or to follow a line where the cost of obtaining right-of-way is inexpensive. Lines should be   located   adjacent   to   existing   roads,   whenever practical, to provide easier access for construction and future maintenance. When a change in direction in a tower line is unavoidable, it should be made gradually in  as  small-angular  increments  as  possible.  Suppose, for example, a change in direction of 90° is required. Instead of an abrupt change in direction of 90°, towers should be set so as to cause the line to follow a gradual curve in a succession of chords around an arc of 90°. Route  Surveys  for  Drainage When man-made structures are erected in a certain area, it is necessary to plan, design, and construct an adequate drainage system. Generally, an underground drainage  system  is  the  most  desirable  way  to  remove surface  water  effectively  from  operating  areas.  An open  drainage  system,  like  a  ditch,  is  economical; however, when not properly maintained, it is unsightly and unsafe. Sometimes, an open drainage system also causes erosion, thus resulting in failures to nearby structures. Flooding caused by an inadequate drainage system is the most prevalent cause leading to the rapid deterioration  of  roads  and  airfields.  The  construction and  installation  of  drainage  structures  will  be discussed later in this chapter. At this point we are mainly interested in drainage systems and types of drainage. DRAINAGE SYSTEM.—Sanitary  sewers  carry waste  from  buildings  to  points  of  disposal;  storm sewers carry surface runoff water to natural water courses or basins. In either case the utility line must have a gradient; that is, a downward slope toward the disposal point, just steep enough to ensure a gravity flow  of  waste  and  water  through  the  pipes.  This gradient  is  supplied  by  the  designing  engineer. Natural  Drainage.—To   understand   the   con- trolling considerations affecting the location and other design  features  of  a  storm  sewer,  you  must  know something  about  the  mechanics  of  water  drainage from  the  earth’s  surface. When rainwater falls on the earth’s surface, some of the water is absorbed into the ground. The amount absorbed  will  vary,  of  course,  according  to  the physical characteristics of the surface. In sandy soil, for instance, a large amount will be absorbed; on a concrete surface, absorption will be negligible. Of the water not absorbed into the ground, some evaporates, and some, absorbed through the roots and exuded onto the leaves of plants, dissipates through a process called  transpiration. The water that remains after absorption, evapora- tion, and transpiration is technically known as runoff. This term relates to the fact that this water, under the influence of gravity, makes its way (that is, runs off) through natural channels to the lowest point it can attain.  To  put  this  in  terms  of  a  general  scientific principle,  water,  whenever  it  can,  seeks  its  own  level. The  general,  final  level  that  unimpeded  water  on  the earth’s surface seeks is sea level; and the rivers of the earth, most of which empty into the sea, are the earth’s principal drainage channels. However, not all of the earth’s runoff reaches the great oceans; some of it is caught  in  landlocked  lakes,  ponds,  and  other  non- flowing inland bodies of water. Let’s consider, now, a point high in the mountains somewhere. As rain falls in the area around this point, the runoff runs down the slopes of a small gully and forms a small stream, which finds a channel down- ward through the ravine between two ridges. As the stream proceeds on its course, it picks up more and more  water  draining  in  similar  fashion  from  high points in the area through which the stream is passing. As  a  result  of  this  continuing  accumulation  of  runoff, the stream becomes larger until eventually it either becomes or joins a large river making its way to the sea—or it may finally empty into a lake or some other inland  body  of  water. In   normal   weather   conditions,   the   natural channels  through  which  this  runoff  passes  can 10-3