11000航測(cè)數(shù)字化地形圖技術(shù)設(shè)計(jì)畢業(yè)論文-資料下載頁(yè)
2025-06-06 23:28本頁(yè)面
【正文】 g 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 accuracies from 15 to 50 cm in distances of 60 and 100 km, respectively, have been 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 to 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 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 equationHB=HA + a － bIn 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 LevelingTrigonometric 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 builtin microprocessors 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=Stanα+i – vwhere 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 depression. The zenith angles are always positive, but naturally when greater than 90176。 they will produce a negative result. Trigonometrical leveling method of determining difference in elevation is limited to horizontal distance less 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, verticalangle observations are made at both ends of the line as close in point of time as possible. This pair of observations is termed reciprocal verticalangle observation. The correct difference in elevation between the two ends of the line is the mean of the two values puted both ways either with or without taking into account curvature and 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.33
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