angle. To compute the HI, the rod reading RB and the DE are added to the elevation of B, or HI  =  RB  +  DE  +  Elev.  B. 2. DEPRESSION ANGLE FORESIGHT (fig, 7-3, view  B).  The  rod  is  below  the  instrument,  and  the vertical angle is minus. The elevation at C equals the HI minus the DE and minus the rod reading RC, or Elev.  C  =  HI  – DE  – RC. 3. ELEVATION ANGLE BACKSIGHT (fig. 7-3, view  C).  The  rod  is  above  the  instrument,  and  the vertical angle is plus. The HI at F equals the elevation at C plus the rod reading (RC) and minus the DE, or HI  =  Elev.  C  +  RC  –  DE. 4. ELEVATION ANGLE FORESIGHT (fig. 7-3, view D). The rod is above the instrument and the angle is plus. The elevation of G equals the HI plus the DE and minus the rod reading (RG), or Elev.  G  =  HI  +  DE  – RG. As mentioned earlier in this section, the horizontal or slope distances used for calculating the DE may be obtained using various methods. For each method, there are  requirements  and  limitations  that  must  be  adhered to. These requirements and limitations are discussed as follows: 1.  Measured  distances  obtained  by  horizontal chaining  should  be  corrected  for  standard  error, temperature, and sag before you compute the DE. These corrections  are  discussed  in  chapter  12  of  the  EA3 TRAMAN.   Under   ordinary   circumstances   in   the Seabees, corrections for earth curvature and refraction are not necessary. However, methods to perform these corrections can be found in commercial publications, such as  Surveying  Theory  and  Practice,  by  Davis, Foote, Anderson, and Mikhail. 2.  Measured  distances  obtained  by  slope  chaining also should be corrected as discussed above. In addition, you must convert the slope distance to a horizontal distance  before  computing  the  DE.  As  an  aid  in computations, tables have been developed that provide the following data: a.  Inclination  corrections  for  100-foot  tape b.  Differences  in  elevation  forgiven  horizontal distances and gradients from 0° to 45° c.  Differences  in  elevation  for  given  slope distances and gradients from 0° to 45° d.   Horizontal   distances   for   given   slope distances and gradients from 0° to 45° 3. When using stadia, you should refer to the stadia procedures and formulas described in chapter 8 of this TRAMAN. With practice, stadia provides a rapid means of  determining  the  horizontal  distances  and  elevations. 4.  Electronic  distance-measuring  devices  measure the  straight-line  horizontal  or  slope  distance  between instruments. When you use the same setup for slopes, replace the electronic equipment with a theodolite and either a target or a rod to measure the vertical angle. The measured vertical angle can be used to convert the measured  slope  distance  to  DE  by  multiplying  by  the sine of the vertical angle. LEVEL AND TRAVERSE COMPUTATIONS In   this   section   we   provide   information   on procedures  used  in  making  level  and  traverse computations.  We  also  discuss  methods  of  differential leveling, including steps to follow in checking level notes.  Coverage  includes  information  on  adjusting intermediate bench marks as well as a level net. In addition,  we  describe  several  methods  of  plotting horizontal control that may be used in determining the bearing of the traverses. These methods include plotting angles by protractor and scale, plotting angles from tangents, and plotting by coordinates. We point out some of the common types of mistakes that the EA may encounter  in  making  or  checking  computations,  and  we provide some information about locating mistakes. PRELIMINARIES TO COMPUTATIONS Before  computations  are  started,  a  close  check  on the field data for completeness and accuracy is required. This includes checking the field notes to ensure that they accurately reflect what was actually measured; for example,  a  deflection-angle  note  79°01'R  must  be checked to be sure that the angle actually measured 79°01' (by ascertaining that the sum of the angle and the closing angle is 360° or within allowable differences) and to ensure that the angle was actually turned to the right. A  field  measurement  may  itself  require transformation  (called  reduction)  before it can be applied as a value in computations; for example, field notes  may  show  plate  readings  for  two-,  four-,  or 7-4