测绘工程专业英语翻译高程的确定方法_测绘工程专业英语翻译
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Methods of Elevation Determination
An elevation is a vertical distance above or below a reference datum.Although 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 aigned a vertical value(elevation)of 0.000 ft or 0.000 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 earth’s surface, the term MSL is generally used to describe the datum.MSL is aigned a vertical value(elevation)of 0.000 ft or 0.000 m.Difference 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 commonly 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 engineer’s transit and level rod;plane-table and alidade and level rod;or self-reducing tacheometer and level rod.4.Barometric leveling, by measuring the differences in atmospheric preure 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 outputs.Vertical accuracies from 15 to 50 cm in distances of 60 and 100 km, respectively, have been reported.The equipment cost is extremely high and applications are restricted to very large projects where terrain, weather, time, and acce 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 0.053 to 0.094 m are poible under these conditions.Spirit leveling
The most precise method of determining elevations and most commonly 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 surveyor’s 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 neceary 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 electromagnetic distance measurers and the vertical(or zenith)angle using a theodolite, or the total station that integrate these two instruments into a single instrument.Total stations contain built-in microproceors that calculate and display the horizontal distance from the measured slope distance and vertical height.This latter facility has resulted in trigonometrical leveling being used for a wide variety of heighting procedures, including contouring.The basic concept of trigonometrical leveling can be seen from Figure 2.When measuring the vertical angle α and the horizontal distance S is used, then the difference in elevation hAB between ground points A and B is therefore:
hAB=S×tanα+i – v
where i is the vertical height of the measuring center of the instrument above A and v is the vertical height of the center of the target above B.The vertical angles are positive for angles of elevation and negative for angles of depreion.The zenith angles are always positive, but naturally when greater than 90° they will produce a negative result.Trigonometrical leveling method of determining difference in elevation is limited to horizontal distance le than 300 m when moderate precision is sufficient, and to proportionately shorter distances as high precision is desired.For the distance beyond 300 m the effects of curvature and refraction must be considered and applied.To eliminate the uncertainty in the curvature and refraction correction, vertical-angle observations are made at
both ends of the line as close in point of time as poible.This pair of observations is termed reciprocal vertical-angle observation.The correct difference in elevation between the two ends of the line is the mean of the two values computed both ways either with or without taking into account curvature and refraction.The important notes should be mentioned here is that surveyors used to working with spirit levels have referenced orthometric heights(H)to the “average” surface of the earth, as depicted by MSL.However, the elevation coordinate(h)given by GPS solutions refers to the height from the surface of the ellipsoid to the ground station.
