intermediate  TPs  as  you  need  to  solve  the problem. A “turning point” is defined as a point on which both a minus sight (FS) and a plus sight (BS)  are  taken  on  a  line  of  direct  levels. As shown in figure 14-12, if we assume that the  elevation  of  the  BM  is  correct,  the  accuracy of  the  elevation  you  determine  for  the  summit depends upon how accurately you determine the elevation of each intermediate TP. This accuracy depends   upon   a   number   of   things,   the   most important  of  which  are  the  following: 1.  If  you  are  doing  leveling  of  ordinary precision, FS and BS distances should not exceed 300  ft.  Therefore,  the  first  setup  point  for  the instrument should be not more than 300 ft from the BM, and the first TP should be not more than 300 ft from the instrument. To balance shots, you should   place   the   instrument   about   the   same distance from the BM as the distance to the TP. 2.  Obviously,  the  first  setup  point  must  be  one you can observe with a rod held on the BM and also  a  rod  held  on  the  first  TP. 3. Generally, setup points should be used that make rod readings as small as possible. The reason small rod readings are desirable is that, for a rod held out of plumb, each reading on the rod will be in error. The larger the rod reading, the greater the  error.  Suppose,  for  example,  a  rod  is  so  far out  of  plumb  that  it  indicates  12.01  ft  for  a reading  that  should  be  12.00  ft  if  the  rod  were plumb.  For  a  12.00-ft  reading  on  the  rod,  the error is 0.01 ft. For a 2.00-ft reading on the same rod held in the same manner, however, the error would  be  only  about  0.002  ft. 4.  A  TP  must  have  not  only  visibility  and accessibility,  but  also  stability;  that  is,  it  must furnish a firm, nonsettling support for the base of  the  rod.  Suppose  you  select  a  point  in  soft, yielding  ground  as  your  first  TP.  Assume  the elevation of the BM is 312.42 ft. You take a BS on  the  BM  and  read  3.42  ft.  Then,  HI  is 312.42  +  3.42  =  315.84  ft. The rodman shifts the rod to the TP. You take an FS and read 5.61 ft. The elevation of the TP is,  therefore, 315.84  –  5.61  =  310.23  ft. Now, you shift the instrument ahead and take a BS  to  carry  on  the  line  of  levels  to  a  new  TP. But suppose that before you take the BS on the rod, the TP has settled 0.02 ft in the ground. Then you take a BS and read 4.74 ft. There is now an  error  of  0.02  ft  in  the  new  HI,  and  every subsequent HI and elevation of TP will be off by the  same  amount. So BE SURE that each TP is stable. When the use of a point in yielding ground is unavoidable, you need to base the rod on a turning point pin or  turning  point  plate.  A  pin  is  driven  in  the ground;   if   you   don’t   have   a   regular   pin,   a marlinspike  or  a  railroad  spike  makes  a  good substitute. You should use a plate on soil too soft to  support  a  driven  pin. METHODS  OF  LEVELING Leveling  methods  are  subdivided  into  two major   categories:   DIRECT   and   INDIRECT. Direct  leveling  describes  the  method  of  measuring vertical distance (difference in elevation) directly with  the  use  of  precise  or  semi-precise  leveling instruments.  Indirect  leveling  methods,  on  the other  hand,  apply  to  measuring  vertical  distances indirectly   or   by   computation.   Unlike   direct leveling  operations,  indirect  leveling  operations do not depend on lines of sight or intervisibility of   points   or   stations.   Some   of   the   surveying instruments commonly used for indirect leveling methods  are  the  transit  and  theodolite. DIRECT  LEVELING This  method  of  leveling  uses  the  measured vertical distance to carry elevation from a known point to an unknown point. Direct leveling is the most  precise  method  of  determining  elevation  and yields accuracies of third or higher orders. When this   method   is   specified   for   lower   accuracy surveys, direct leveling is sometimes referred to as  “spirit”  or  “fly”  levels.  Fly  levels  are  leveling operations used to rerun original levels to make sure that no mistake has been made. Fly levels use a shorter route and smaller number of turning points than the original survey. Let’s take a look at  some  of  the  processes  involving  direct  leveling. Differential Leveling Differential leveling (also called direct leveling) is  generally  used  in  determining  elevations  of points to establish a chain or network of BMs for future  use.  It  requires  a  series  of  instrument  setups along the survey route; and for setup, a horizontal line of sight is established, using a sensitive level. The  SEABEEs  commonly  use  this  type  of  leveling in   determining   elevation   during   construction surveys. As shown in figure 14-13, the basic procedure used  to  determine  elevations  in  a  differential leveling   operation   is   the   same   as   previously discussed. First, you take a BS on a rod held on 14-12