The  primary  traverse  ABCD  runs  around  the perimeter  of  the  tract.  Were  this  tract  sufficiently  small and level, then details within the whole tract could be located from only the primary control points; that is, from stations A, A₁, B, B₁, C, C₁, D, and D₁. In this case, however, the size (or perhaps the character) of the terrain made it necessary to establish additional control points within the perimeter of the tract, such as D₂, A₂, and B₂. These stations were established by running traverse lines (called crossties) across the area from one primary traverse station to another. It should be noted that, since each secondary traverse closes on a primary control point, errors cannot accumulate any farther than the distance  between  the  primary  stations. Field notes for the survey sketched in figure 8-1 must contain (1) notes showing the horizontal locations of  the  stations  and  (2)  level  notes  for  determining  the elevations  of  the  stations. Vertical Control In topographic surveying, bench marks serve as starting  and  closing  points  for  the  leveling  operations when  you  are  locating  details.  Although  for  some surveys the datum may be assumed, it is preferable that all elevations be tied to bench marks which are referred to the sea-level datum. In many areas, particularly in the United  States,  series  of  permanent  and  precisely established bench marks are available. As a surveyor, you  must  make  every  feasible  effort  to  tie  in  your surveys to these bench marks to ensure proper location and identification. Often, the established horizontal control marks are used as the bench marks because the level routes generally follow the traverse lines. Vertical  control  is  usually  carried  out  by  direct leveling;  however,  trigonometric  leveling  may  be  used for a limited area or in rough terrain. When you establish the primary vertical control to use in a topographic survey  for  an  intermediate-scale  map,  four  degrees  of precision are used as follows: 1. 0.05 foot ~ distance in miles. This order is used as the standard for surveys in flat regions when the contour interval is 1 foot or less. It is also used on surveys that require the determination of the gradient of streams or to establish the grades for proposed drainage and  irrigation  systems. 2. 0.1 foot i distance in miles.  This order is used in a survey when the contour interval of the map is 2 feet. 3. 0.3 foot ~ distance in miles.  This order is used for a contour interval of 5 feet. 4. 0.5 foot i distance in miles.  This order is used for a contour interval of 10 feet and may be done by stadia leveling, a method that is very advantageous in hilly  terrain.  Stadia  will  be  discussed  later  in  this chapter. You use the third or fourth orders of precision for a large-scale map that generally has a contour interval of 1 or 2 feet. For an extensive survey of a large area, use the third order; for surveys of a smaller area, use the fourth  order. Once the topographic control has been established, your next major step in a topographic survey is to locate the details horizontally and vertically in the vicinity of each control point or station. These details consist of (1) all natural or artificial features that will appear on the  map  and  (2)  enough  ground  points  and  spot elevations  to  make  the  drawing  of  contour  lines possible. The   methods   and   the   instruments   used   in topographic surveys depend upon the purpose of the survey, the degree of precision needed, the nature of the terrain to be covered, the map scale, and the contour interval. For a high degree of accuracy, you should locate  azimuths  with  a  theodolite  or  transit.  Measure horizontal distances with the chain or the electronic distance   measurement   (EDM)   device.   Determine elevations  with  a  level. The following sections discuss two methods that are commonly used to locate topographic details. A third method (topography by plane table) is discussed in the next chapter of this TRAMAN. LOCATING DETAILS BY TRANSIT AND TAPE In  the  EA3  TRAMAN  you  studied  the  procedures used to tie in and locate points, using a transit and tape. These same procedures are used for tying in and locating topographic details. Determine the vertical location (or elevation)  of  the  detail  points,  using  direct  or trigonometric  leveling  procedures.  Horizontally  locate the  details  either  by  directions  or  distances  or  a combination of both. Use a method, or a combination of methods, that requires the least time in a particular situation. Directly measure the dimensions of structures, such  as  buildings,  with  tapes.  When  details  are numerous, assign each one a number in the sketch and key  the  detail  to  a  legend  of  some  kind  to  avoid overcrowding.  For  directions,  use  azimuths  instead  of deflection angles to minimize confusion. Locate details as  follows: 8-2