areas of mistakes. Errors cannot be completelyeliminated, but they can be minimized so that theireffect on the survey accuracy will be small andwithin the tolerances permitted.Identifying Leveling MistakesThe leveling mistakes discussed here are notintended to include all possibilities but willgive an idea of the more common ones. Thesurvey party personnel should be aware of thesepossibilities and should be careful to avoid thesemistakes. Some of the common mistakes are asfollows:1. Not setting the rod on the same point foran FS and the following BS. Using a turning pin,pedestal, stake, or marking the location with chalkon hard surfaces will help you to recover theidentical point.2. Neglecting to clamp the target or the rodwhen extended. Any slippage can pass unnoticedand result in a wrong reading that may requirean entire rerun of the line to discover the mistake.The rodman should watch the rod or target forany movement as the clamp is tightened. The rodextension or target should be read again after theclamp has been set.3. Reading the wrong mark. This is a commonmistake. The figures on a rod may be obscuredby brush or may fall in a position in the field ofview so that the instrumentman cannot see twoconsecutive numbers. Under these conditions, hemay read the wrong mark or even read in thewrong direction. This is a great possibility whenan inverting eyepiece is being used. For example,if the figure 2 is the only number visible, theinstrumentman might read “up” the rod—2.1,2.2, 2.3 when actually he should be reading 1.9,1.8, 1.7. Another possibility is miscounting thenumber of divisions. There is no way to check ordiscover these mistakes except to be aware of theirpossibility and to read carefully.4. Recording a reading in the wrong column.In leveling, readings are not entered into thenotebook in a normal sequence, such as left toright across the page. There is always a chancethat one or more values may be recorded in thewrong column. The recorder must be alert toavoid making this mistake.5. Reading the wrong angle sign in trigono-metric leveling. The instrumentman can accidentallycall out a wrong sign in reading the angle. Thistype of mistake can be eliminated by the recorderwatching the telescope as a pointing is made onthe rod. If the wrong one is called out, both therecorder and the instrumentman can resolve itimmediately.6. Recording the wrong sign. The sign variesdepending on whether the rod reading is a BS oran FS, and whether the angle is a depression oran elevation. Also, the difference in elevationcomputation requires a sign reversal if the angleis read for the BS, but not for the FS. Thesevariations can be confusing; the recorder has tobe careful to avoid mistakes. This can be doneby recording the angle and rod reading signs asread. The sign conversion, if needed, shows upwhen you compute the DE. Examining thecomputations to see if all BS DEs have a signopposite to the angle sign is simple.7. Subtracting the BS or adding the FS indifferential leveling. If the BS or FS is recordedproperly (see Number 4 above), you can discoverthe mistake when you add the BS column and theFS column for a computation check.8. Using the wrong horizontal cross hairs.This occurs on an instrument provided with stadiahairs.Identifying Leveling ErrorsGenerally, errors cannot be totally eliminated,but they can be contained within acceptabletolerances. This requires you to use the prescribedmethods and instruments and apply correctionsestablished either mathematically or by experience.Some of the conditions that produce errors arelisted below.1. Instrument not properly adjusted. A smallamount of residual error will always exist in anyadjustment. For the more accurate surveys, theresidual error can be minimized by using BS andFS balancing and, in trigonometric leveling, bytaking direct and reverse (circle left and circleright) readings for the angles.2. Instrument not leveled properly. Unlike theresidual adjustment error that will affect thereadings one way consistently, this is a randomor accidental error. It may affect the line of sightdifferently at each setup. This error can beminimized only by careful leveling each time theinstrument is set up and by recentering the bubblebefore each reading.3. Telescope not focused properly. Misfocusingand parallax in the eyepiece create accidentalerrors that cannot be corrected. The only way toavoid or minimize this error is to take care tofocus properly at each setup. The instrumentman14-24
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