测绘专业英语课文翻译

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Geodetic Surveying and Plane Surveying

Surveying 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 representing 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 seen as the major task.

Though these surveys are for various purposes, 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 treat the earth as a flat surface(Plane surveys). Additionally, surveys are conducted for the purpose 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.

This type of survey is suited for large areas and long lines and is used to find the precise location of basic points needed for establishing control for other surveys. In geodetic surveys, the stations are normally long distances apart, and more precise instruments and surveying methods are required for this type of surveying than for plane surveying.

Widely spaced, permanent monuments serve as the basis for computing lengths and distances between relative positions. These basic points with permanent monuments are called geodetic control survey points, which support the production of consistent and compatible data for surveying and mapping projects. In the past, ground-based theodolites, tapes, and electronic devices were the primary geodetic field measurements used. 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.

In Geodetic Surveys, the shape of the earth is thought of as a spheroid, although in a technical sense, 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 expressed 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.