angle. To compute the HI, the rod reading RB and theDE are added to the elevation of B, orHI = RB + DE + Elev. B.2. DEPRESSION ANGLE FORESIGHT (fig, 7-3,view B). The rod is below the instrument, and thevertical angle is minus. The elevation at C equals the HIminus the DE and minus the rod reading RC, orElev. C = HI – DE – RC.3. ELEVATION ANGLE BACKSIGHT (fig. 7-3,view C). The rod is above the instrument, and thevertical angle is plus. The HI at F equals the elevationat C plus the rod reading (RC) and minus the DE, orHI = Elev. C + RC – DE.4. ELEVATION ANGLE FORESIGHT (fig. 7-3,view D). The rod is above the instrument and the angleis plus. The elevation of G equals the HI plus the DE andminus the rod reading (RG), orElev. G = HI + DE – RG.As mentioned earlier in this section, the horizontalor slope distances used for calculating the DE may beobtained using various methods. For each method, thereare requirements and limitations that must be adheredto. These requirements and limitations are discussed asfollows:1. Measured distances obtained by horizontalchaining should be corrected for standard error,temperature, and sag before you compute the DE. Thesecorrections are discussed in chapter 12 of the EA3TRAMAN. Under ordinary circumstances in theSeabees, corrections for earth curvature and refractionare not necessary. However, methods to perform thesecorrections can be found in commercial publications,such as Surveying Theory and Practice, by Davis,Foote, Anderson, and Mikhail.2. Measured distances obtained by slope chainingalso should be corrected as discussed above. In addition,you must convert the slope distance to a horizontaldistance before computing the DE. As an aid incomputations, tables have been developed that providethe following data:a. Inclination corrections for 100-foot tapeb. Differences in elevation forgiven horizontaldistances and gradients from 0° to 45°c. Differences in elevation for given slopedistances and gradients from 0° to 45°d. Horizontal distances for given slopedistances and gradients from 0° to 45°3. When using stadia, you should refer to the stadiaprocedures and formulas described in chapter 8 of thisTRAMAN. With practice, stadia provides a rapid meansof determining the horizontal distances and elevations.4. Electronic distance-measuring devices measurethe straight-line horizontal or slope distance betweeninstruments. When you use the same setup for slopes,replace the electronic equipment with a theodolite andeither a target or a rod to measure the vertical angle. Themeasured vertical angle can be used to convert themeasured slope distance to DE by multiplying by thesine of the vertical angle.LEVEL AND TRAVERSECOMPUTATIONSIn this section we provide information onprocedures used in making level and traversecomputations. We also discuss methods of differentialleveling, including steps to follow in checking levelnotes. Coverage includes information on adjustingintermediate bench marks as well as a level net. Inaddition, we describe several methods of plottinghorizontal control that may be used in determining thebearing of the traverses. These methods include plottingangles by protractor and scale, plotting angles fromtangents, and plotting by coordinates. We point out someof the common types of mistakes that the EA mayencounter in making or checking computations, and weprovide some information about locating mistakes.PRELIMINARIES TO COMPUTATIONSBefore computations are started, a close check onthe field data for completeness and accuracy is required.This includes checking the field notes to ensure that theyaccurately reflect what was actually measured; forexample, a deflection-angle note 79°01'R must bechecked to be sure that the angle actually measured79°01' (by ascertaining that the sum of the angle and theclosing angle is 360° or within allowable differences)and to ensure that the angle was actually turned to theright.A field measurement may itself requiretransformation (called reduction) before it can beapplied as a value in computations; for example, fieldnotes may show plate readings for two-, four-, or7-4