测绘学习工程专业英语section2the reliability of a survey and errors

1、Section 2 The Reliability of a Survey and ErrorsSince every technique of measurement is subject to unavoidable error, surveyors must be aware of all sources and types of error and how they combine. If the accuracy of a measurement is defined as the nearness of that value to its true value (a quantit

2、y we can never know) then a surveyor must ensure that the techniques he chooses will produce a result that is sufficiently accurate. He must know, therefore, how accurate he needs to be, how to achieve this accuracy and how to check that the required accuracy has been achieved. Accuracy requiredWhen

3、 surveying to produce a plan, the accuracy required is defined by the scale of the plot, since there should be no plottable error in the survey data. A good draughtsman can plot a length to within 0.25 mm and so, if a plan of an area is required at a scale of 1/1000, i.e., 1 mm on the plan represent

4、s 1 m on the ground, the smallest plottable distance is 0.25 m. Thus, The specifications of surveys for other purposes such as engineering works or property boundary definition might well be determined by engineering tolerances or legal standards. Achieving the specificationTo ensure that the specif

5、ication is achieved the equipment and methods must be chosen such that, not only will they collect the right sort of data, i.e., the correct combination of angles and distances, but that the data will be to the required accuracy. There are several types of error that occur and a knowledge of their i

6、mportance and characteristics is essential in the understanding of the limitations of the techniques of measurement. The salient features will be stated now but further information is given in later chapters. MistakesBlunders or mistakes are often inaccurately referred to as gross errors. Miscountin

7、g the number of tape lengths when measuring a long distance or transposing numbers when booking are two very simple, but all too common, examples of mistakes. These types of mistakes can occur at any stage of a survey, when observing, booking, computing or plotting, and they would obviously have a v

8、ery damaging effect on the results if left uncorrected. However, by following strictly a well-planned observing procedure it is possible to reduce the number that occur and then independent checks at each stage should show up those that have been made. In practice, none should ever go undetected and

9、 uncorrected. Systematic errorsSystematic errors arise from sources which act in a similar manner on observations. The method of measurement, the instruments used and the physical conditions at the time of measurement must all be considered in this respect. Expansion of steel tapes, frequency change

10、s in electromagnetic distance measuring (EDM) instruments and collimation in a level are just a few examples of possible sources of systematic errors.These errors are of vital importance in activities which consist of adding together a succession of individual observations (see sections on leveling

11、and traversing). If all the individual measurements contain the same type of systematic error, which by their nature always act in the same direction, then the total effect is the sum of them all.It must be ensured that measurements are as accurate as required by removing the effects of all factors

12、that, if neglected, would result in a significant error. The errors caused by some factors can be eliminated with the correct observing procedure and others countered by applying corrections. Systematic errors are not revealed by taking the same measurement again with the same instruments. The only

13、way to check adequately for systematic error is to remeasure the quantity by an entirely different method using different instruments.Random errorsRandom errors are really all those discrepancies remaining once the blunders and systematic errors have been removed. Even if a quantity is measured many

14、 times with the same instrument in the same way, and if all sources of systematic error have been removed, it is still highly unlikely that all results will be identical. The differences, caused mainly by limitations of instruments and observers, are random errors.It is found in practice that these

15、errors, although called random, have the following characteristics:(1) small errors occur more frequently than large ones(2) positive and negative errors are equally likely to occur(3) very large errors seldom occur.These characteristics are typical of errors which are normally distributed and it is

16、 assumed that we can use the mathematical theory based on the normal distribution to deal with the errors met with in surveying. Understanding the errors that limit the accuracy of the measurement techniques is but one step to ensuring specifications are achieved, as will be seen when the methods of

17、 survey are described:(a) The survey area is always totally covered with the simplest possible framework of high quality measurements. If the rest of the survey work is carried out within this control the possible damaging accumulation of errors can be contained. This is often termed working from th

18、e whole to the part,(b) Observing procedures are designed so that (i) most mistakes that occur are discovered immediately and (ii) possible sources of systematic errors eliminated. (c) Additional, or redundant, observations are taken so that all data can be checked for the mistakes, systematic error

19、s and random errors that do occur. For example, the three angles of a triangle would be observed although only two are required to define the shape. The third angle could be deduced but, when measured, acts as a check. (d) Many quantities are observed several times. These repeated measurements and t

20、he observation of redundant data serve both as checks and to improve on the precision of the final results. Checking the surveyEven with all the checking procedures the surveyor employs, errors can still occur in the finished plan and for this reason final independent checks are required. For simple

21、 work this would involve inspecting the final plan in the field and comparing some measurements scaled off the plan with their equivalents on the ground. New Words and ExpressionsReliability：可靠性Error：误差，错误is subject to：受影响的accuracy：精度（有时特指最后结果的精度）Accuracy required：要求的精度the scale of the plot：绘图比例尺plottable error：绘图误差draughtsman ：制图员，绘图员represent：表现，描绘，象征plottable distance：可绘距离specification：规格，规范，技术要求，技术条件；说明书，明细表，清单tolerance：限差，容许误差；公差standard：标准，规