Table 13-1.-Control Traverse Order of PrecisionFOURTH ORDER control surveys. The FIRSTis 90°, or 5,400´. The precision of a 1-min transit.ORDER is the highest and the FOURTHORDER, the lowest standard of accuracy.Because of the type of instruments availablein the SEABEEs, most of your surveys may notrequire a precision higher than a third ordersurvey. When the order of precision is notspecified, you may use table 13-1 in this trainingmanual (TM) as a standard for a horizontalcontrol survey when using the traverse controlmethod. For surveys that call for a higher orderof precision, you will have to use theodolites toobtain the required precision.The triangulation control method is discussedfully in Engineering Aid 1 & C, NAVEDTRA10635-C. At present, however, you may havesurvey problems that require the use of thetriangulation method. In such a case, you mayuse table 13-2 in this TM as a guide for the orderof precision if it is not specified in the survey.The practical significance of a prescribed orimplied order of precision lies in the fact that theinstruments and methods used must be capableof attaining the required precision. The precisionof an instrument is indicated by a fraction inwhich the numerator is the inherent error. (In a1-min transit, the inherent error is 1 min.)The denominator is the total number of unitsin which the error occurs. For a transit, this lastthen, is 1/5,400, adequate for a third order survey.Precision of a tape is given in terms of theinherent error per 100 ft. A tape that can be readto the nearest 0.01 ft has a precision of 0.01/100,or 1/10,000—adequate for second order work.Attaining Precision with aLinear Error of ClosureFor a closed traverse, you should attain aRATIO OF LINEAR ERROR OF CLOSUREthat corresponds to the order of precisionprescribed or implied for the traverse. The ratioof linear error of closure is a fraction in whichthe numerator is the linear error of closure andthe denominator is the total length of the traverse.To understand the concept of linear error ofclosure, you should study the closed traverseshown in figure 13-27. Beginning at station C, thistraverse runs N30°E300 ft, thence S30°E300 ft;thence S90°W 300 ft. The end of the closingtraverse,BC, lies exactly on the point ofbeginning, C. This indicates that all angles wereturned and all distances chained with perfectaccuracy, resulting in perfect closure, or anerror of closure of zero feet.However, in reality, perfect accuracy inmeasurement seldom occurs. In actual practice,13-22
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