【正文】 時(shí)它們將產(chǎn)生一個(gè)相反的結(jié)果普通精度要求下三角高程測(cè)量方法測(cè)高差水平距離不能超過(guò) 300m 如果要求高的精度則要相應(yīng)縮短距離因?yàn)槌^(guò) 300m 時(shí)地球曲率和折光影響必需考慮為了消除地球曲率和折光改正的不確定因素垂直角觀測(cè)時(shí)應(yīng)采用在觀測(cè)方向 兩端盡量同時(shí)相向觀測(cè)的方法這種觀測(cè)稱為垂直角對(duì)向觀測(cè)線兩端正確的高程之差是計(jì)算得到的兩高程值的平均值不管計(jì)算有無(wú)考慮球氣差這里需要注意的是測(cè)量者在水準(zhǔn)測(cè)量工作中使用的是參考地球平均表面的正高這個(gè)平均表面描述為 MSL 然而 GPS 方法給出的是地球橢球面到地面站的大地高 Methods of Elevation Determination An elevation is a vertical distance above or below a reference datumAlthough vertical distance can be referenced to any datum in surveying the reference datum that is universally employed is that of mean sea level MSL MSL is assigned a vertical value elevation of 0000 ft or 0000 m All other points on the earth can be described by the elevations above or below zero Permanent points whose elevations have been precisely determined benchmarks are available in most areas for survey use In China 7 years of observations at tidal stations in Qingdao from 1950 to 1956 were reduced and adjusted to provide the Huanghai vertical datum of 1956 In the 1987 this datum was further refined to reflect long periodical ocean tide change to provide a new national vertical datum of 1985 according to the observations at tidal stations from 1952 to 1979 Although strictly speaking the national vertical datum may not precisely agree with the MSL at specific points on the earths surface the term MSL is generally used to describe the datumMSL is assigned a vertical value elevation of 0000 ft or 0000 mDifference in elevation may be measured by the following methods 1 Direct or spirit leveling by measuring vertical distances directly Direct leveling is most precise method of determining elevations and the one monly used 2 Indirect or trigonometric leveling by measuring vertical angles and horizontal or slope distances 3 Stadia leveling in which vertical distances are determined by tacheometry using engineers transit and level rod planetable and alidade and level rod or selfreducing tacheometer and level rod 4 Barometric leveling by measuring the differences in atmospheric pressure at various stations by means of a barometer 5 Gravimetric leveling by measuring the differences in gravity at various stations by means of a gravimeter for geodetic purposes 6 Inertial positioning system in which an inertial platform has tree mutually perpendicular axes one of which is up so that the system yields elevation as one of the outputsVertical accuracies from 15 to 50 cm in distances of 60 and 100 km respectively have been reportedThe equipment cost is extremely high and applications are restricted to very large projects where terrain weather time and access impose special constraints on traditional methods 7 GPS survey elevations are referenced to the ellipsoid but can be corrected to the datum if a sufficient number of points with datum elevations are located in the region surveyed Standard deviations in elevation differences of 0053 to 0094 m are possible under these conditions Spirit leveling The most precise method of determining elevations and most monly use method is direct leveling or spirit leveling which means measuring the vertical distance directly Differential leveling is used to determine differences in elevation between points that are remote from each other by using a surveyors level together with a graduated measuring rod For example to determine the elevations of desired point B with respect to a point of known elevation A see Figure 1 the elevation of which BM is known to be above sea level the level is set up at intermediate point between A and B and rod readings are taken at both locations as a and b respectively Then the elevation of the line of sight of the instrument being horizontal is known to be the line of sight of the instrument HA a The elevation of point B can be determined by equation HB HA a － b In addition to determining the elevation of point B the elevations of any other points lower than the line of sight and visible from the level can be determined in a similar manner But some terms should be mentioned from above a is called Backsight BS which is a rod reading taken on a point of known elevation in order to establish the elevation of the instrument line of sight b is called Foresight FS which is a rod reading taken on a turning point benchmark or temporary benchmark in order to determine its elevation HA a refers to the Height of Instrument HI which is the elevation of the line of sight through the level Owing to refraction actually the line of sight is slightly curved the effects of curvature and refraction for the horizontal distance can be reduced to a negligible amount and no correction for curvature and refraction is necessary if backsight and foresight distances are balanced in practical operation Trigonometric Leveling Trigonometric leveling is used where difficult terrain such as mountainous areas precludes the use of conventional differential leveling The modern approach is to measure the slope distance and vertical angle to the point in question Slope distance is measured using electromagic distance measurers and the vertical or zenith angle using a