should check and clear parallax before the firstsighting and should not readjust it until allsightings from the setup are complete.4. Rod improperly plumbed. This error iscaused by a rodman who does not pay attentionto his work. The instrumentman can call attentionto plumbing if it is at a right angle to his line ofsight, but he cannot see it in the direction of lineof sight. The use of a rod level or waving the rodwill avoid this error.5. Unstable object used for a TP. The rodmancauses this error by selecting a poor point ofsupport, such as loose rocks or soft ground. Asthe rod is turned between sights, the weight of therod can shift a loose rock or sink into soft ground.The elevation of the TP as used for the next BScan change appreciably from the value that hadbeen computed from the previous FS. This errorcan be avoided by using the turning pin or pedestalwhen the ground does not present solid points.6. Rod length erroneous. This error results ineither too long or too short rod readings at eachpoint. In a survey predominately over slopes, thiserror will accumulate. The rod length should bechecked with a steel tape at intervals to locate thiserror.7. Unbalanced BS and FS distances. Theunbalanced distances do not cause the error. Itis caused by the effect on the line of sight fromresidual adjustment and leveling errors and theeffect of curvature and refraction errors. Readingsyou take at a long distance will have a greatererror than those at a short distance. Thisunbalance may not be critical on one setup butcan be compounded into a considerable error ifthe unbalance continues over several setups. Bybalancing the sight distances at each instrumentsetup, if possible, and the sums of the BS and FSdistances at every opportunity, you will keep theseerrors to a minimum.8. Earth’s curvature. This produces an erroronly on unbalanced sights in leveling. When theBS distances are constantly greater than FSdistances, or vice versa, a greater systematicerror results, especially when the sights are long.To eliminate this error, you must maintain abalanced sight distance in every BS and FSreading, not just their sum total between BMs (theerror varies directly as the square of the distancefrom the instrument to the rod).9. Atmospheric refraction. This error alsovaries as the square of the distance but oppositein sign ( + or – ) to that caused by the earth’scurvature. The effect of atmospheric refractionis only one-seventh of that caused by the earth’scurvature. In first- and second-order leveling, theeffect of refraction is minimized by taking the BSand FS readings in quick succession and avoidingreadings near the ground. (They should be takenat least 2 ft from the ground.)10. Variation in temperature. If a portion ofthe telescope is shaded and some parts are exposedto the sun’s rays, it produces some warping effecton the instrument that may affect its line of sight.This effect is negligible in ordinary leveling; butin leveling of higher precision, this effect mayproduce appreciable error. This is one of thereasons why surveyors use an umbrella to shieldthe instrument when doing more refined work.BASIC ENGINEERING SURVEYSAND CONSTRUCTION SITE SAFETYAn engineering survey forms the first of achain of activities that will ultimately lead to acompleted structure of some kind, such as abuilding, a bridge, or a highway. An engineeringsurvey is usually subdivided into a DESIGN-DATA SURVEY and a CONSTRUCTIONSURVEY.This section discusses the basic engineeringsurveys commonly performed by an EA surveyparty in support of military construction activities.In addition, various types of occupational hazardsrelating to specific surveying operation are alsopresented in this section together with theprecautions or applicable abatement proceduresthat must be carried out to deter injury to thesurvey crew and/or damage to surveying equip-ment or material.HIGHWAY SURVEYSSurveys for roads and streets involve both fieldwork and office work. The extent of each typeof work depends on the magnitude and complexityof the job. Some phases of the work may be doneeither in the field or in the office, and thedecision as to the exact procedures to be followedwill be influenced by the number of personnelavailable and by the experience and capabilitiesof the individuals involved.Design-Data SurveyThis type of survey is conducted for thepurpose of obtaining information that is essentialfor planning an engineering project or develop-ment and estimating its cost. A typical design-data14-25
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