Geodetic Surveying and Plane Surveying
hentaitubefreeSurveying has been traditionally defined as the art and science of determining the position of natural and artificial features on, above or below the earth’s surface; and reprenting this information in analog form as a contoured map, paper plan or chart, or as figures in report tables, or in digital form as a three dimensional mathematical model stored in the computer. As such, the surveyor/geodesist dealt with the physical and mathematical aspect of measurement. The accurate determination and monumentation of points on the surface of the Earth is therefore en as the major task.
Though the surveys are for various purpos, still the basic operations are the same---they involve measurements and computations or, basically, fieldwork and office work. There are many different types of surveys such as land surveys, route surveys, city surveys, construction surveys, hydrographic surveys, etc., but generally speaking, surveying is divided into two major categories: geodetic and plane surveying.
Surveys will either take into account the true shape of the Earth（Geodetic surveys）or tre
at the earth as a flat surface(Plane surveys). Additionally, surveys are conducted for the purpo of positioning features on the ground(Horizontal surveys), determining the elevation or heights of features(Vertical surveys) or a combination of both.
Geodetic Surveying
The type of surveying that takes into account the true shape of the earth is called geodetic surveying.
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厦门英语翻译公司This type of survey is suited for large areas and long lines and is ud to find the preci location of basic points needed for establishing control for other surveys. In geodetic surveys, the stations are normally long distances apart, and more preci instruments and surveying methods are required for this type of surveying than for plane surveying.
广州学大教育Widely spaced, permanent monuments rve as the basis for computing lengths and distances between relative positions. The basic points with permanent monuments are called geodetic control survey points, which support the production of consistent and com
patible data for surveying and mapping projects. In the past, ground-bad theodolites, tapes, and electronic devices were the primary geodetic field measurements ud. Today, the technological expansion of GPS has made it possible to perform extremely accurate geodetic surveys at a fraction of the cost. A thorough knowledge of the principles of geodesy is an absolute prerequisite for the proper planning and execution of geodetic surveys.
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In Geodetic Surveys, the shape of the earth is thought of as a spheroid, although in a technical n, it is not really a spheroid. Therefore, distances measured on or near the surface of the earth are not along straight lines or planes, but on a curved surface. Hence, in the computation of distances in geodetic surveys, allowances are made for the earth’s minor and major diameters from which a spheroid of reference is developed. The position of each geodetic station is related to this spheroid. The positions are expresd as latitudes(angles north or south of the Equator) and longitudes(angles east or west of a prime meridian) or as northings and eastings on a rectangular grid.
A geodetic survey establishes the fundamentals for the determination of the surface and gravity field of a country. This is realized by coordinates and gravity values of a sufficiently large number of control points, arranged in geodetic and gravimetric networks. In this fundamental work, curvature and the gravity field of the earth must be considered.
The type of surveying in which the mean surface of the earth is considered a plane, or in which the curvature of the earth can be disregarded without significant error, generally is called plane surveying. The term is ud to designate survey work in which the distances or areas involved are of limited extent. With regard to horizontal distances and directions, a level line is considered mathematically straight, the direction of the plumb line is considered to be the same at all points within the limits of the survey, and all angles are considered to be plane angles. To make computations in plane surveying, you will u formulas of plane trigonometry, algebra, and analytical geometry. For small areas, preci results may be obtained with plane surveying methods, but the accuracy and precision of such results will decrea as the area surveyed increas in size. For example, the length of an arc 18.5 km long lying in the earth’s surface is only 7mm greater than the su
btended chord and, further, the difference between the sum of the angles in a plane triangle and the sum of tho in a spherical triangle is only 0.51 cond for a triangle at the earth’s surface having an area of 100km2 . It will be appreciated that the curvature of the earth must be taken into consideration only in preci surveys of large areas.
A great number of surveys are of the plane surveying type.
Surveys for the location and construction of highways, railroads, canals, and in general, the surveys necessary for the works of human beings are plane surveys, as are the surveys made to establish boundaries, except state and national. However, with the increasing size and sophistication of engineering and other scientific projects, surveyors who restrict their practice to plane surveying are verely limited in the types of surveys in which they can be engaged. The operation of determining elevation usually is considered a division of plane surveying. Elevations are referred to the geoid. The geoid is theoretical only.
It is the natural extension of the mean a level surface under the landmass. We could ill
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ustrate this idea by digging an imaginary trench across the country linking the Atlantic and Pacific oceans.If we allowed the trench to fill with awater, the surface of the water in the trench would reprent he geoid. So for all intents and purpos, the geoid is the same as mean a level. Mean a level is the average level of the ocean surface halfway between the highest and lowest levels recorded. We u mean a level as a datum or, curiously and incorrectly, a datum plane upon which we can reference or describe the heights of features on, above or below the ground. Imagine a true plane tangent to the surface of mean a level at a given point. At horizontal distances of 1km from the point of tangency, the vertical distances(or elevations) of the plane above the surface reprented by mean a level are 7.8cm. Obviously, curvature of the earth’s surface is a factor that cannot be neglected in obtaining even rough values of elevations. The ordinary procedure in determining elevations, such as balancing backsight and foresight distance in differential leveling, automatically takes into account the curvature of the earth and compensates for earth curvature and refraction, and elevations referred to the curved surface of reference are cured without extra effort by the surveyor.
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