测绘工程专业英语.doc

精品文档From the discussion thus far it can be stated with absolute certainty that all measured values contain errors , whether due to lack of refinement in reading , instrumental imperfection or human limitations . Some of these errors result from physical conditions that cause them to occur in systematic way , whereas others occur with apparent randomness . Accordingly , errors are classified as either systematic or random . But before defining systematic and random errors , it is helpful to define mistakes . These three terms are defined as follows:1、 Mistake . Mistake or blunders (gross errors) actually are not errors because they usually are so gross in magnitude compared to the other two types of errors . Carelessness , inattention , improper training , bad habits ,poor judgement , adverse measuring or observing conditions , and various negative attitudes and emotions are the traces or the common reasons for mistakes . They are not classified as errors and must be removed form any set of observations . Typical example of mistakes are omitting a whole tape length when measuring distance , sighting the wrong target in a round of angles , writing down 27.55 for 25.75 in recording . Therefore great care must be taken to obviate them Mistakes will never be completely eliminated from measurements , but surveyors careful , attentive , conscientious attitude can reduce the mistakes in most cases . Through proper training and development of good work habits , development and maintenance of positive attitudes , and understanding the theory and practically eliminated.2、 systematic errors . Systematic errors are defined as those errors whose magnitude and algebraic sign can be calculated and applied as a correction to the measured quantity , or these errors follow some physical law and thus can be predicted . Some systematic errors are removed by some correct measurement procedures (e , g, , balancing backsight and foresight distance in differential leveling to compensate for earth curvature and refraction ) . Others are removed by deriving corrections based on the physical conditions that were responsible for their creation (e ,g , applying a computed correction for earth curvature and refraction on a trigonometric leveling observation ).Surveyors should know how to deal with systematic errors . The first requirement is to recognize and accept the possible existence of errors . Next , identify the various sources that might be affecting a reading systematically , then , determine what the system is . Is it a constant , linear, or in proportion to the size of the quantity being measured ? Or , does it follow some other mathematical relationship ? Is there some physics involved ? Once systematic errors discovered and quantified , the errors can be essentially compensated by certain processes of measuring or corrected to reduce their effect . Careful calibration of all instruments is an essential part of controlling systematic errors 3、 Random errors . Random (also known as a accident) errors are introduce into each measurement mainly because of human and instrument imperfections as well as uncertainties in determining the effects of the environment on measurements . After all mistakes and systematic errors have been removed from the measured values , the random errors remain . In general , random errors are unavoidable and relatively small . They usually do not follow any physical law , but it can be deal with and estimated according to the mathematical laws of probability . Examples of random errors are (a) imperfect centering over a ground point during distance measurement with an EDM instrument , (b) bubble not centered at the instant a level rod is read , and (c) small errors in reading graduated scalesUnderstanding the nature of random errors helps to understand why random errors never really fully corrected , since the observation of the physical phenomena contains personal , random errors . Thus , measurement have uncertainties or random errors that remain unquantifiable . Random errors