An overview of the tasks of surveying and mapping and the shape and size of the earth

(1) The mission of modern surveying and mapping

The mission of modern surveying and mapping is to study the science of collecting, measuring, describing and utilizing the earth's environmental information that humans rely on for survival. Its contents include: spatial positioning, earth shape and gravity field; obtaining the natural form of the earth and its outer space stars, man-made facilities and information related to their properties; making various topographic maps, thematic maps and establishing geographical information systems. To study natural phenomena and related social phenomena on the earth and provide basic information for the sustainable development of society.

(2) Classification of Surveying and Mapping

Surveying and mapping are divided into geodesy, photogrammetry and remote sensing, and cartography according to the different research scope, research objects and technical means used. Science, engineering surveying and oceanographic surveying and other branches of science.

1. Geodesy

Geometry is the study and determination of the shape, size, gravity field, overall and local motion of the earth, and the geometric positions and changes of surface points on the earth. Theoretical and technical subjects.

Geodesy is an important theoretical foundation for all branches of surveying and mapping. The basic task is to establish a national plane control network, elevation control network and gravity control network, and accurately determine the three-dimensional spatial position and mutual positional relationship of control points. , research and determine the shape and size of the earth, the earth's external gravity field and its changes, earth tides, plate movement and crustal deformation, and earthquake prediction, etc., to provide geological information for national economic construction and social development, national security, and earth science and space scientific research. Measurement infrastructure, information and technical support. Modern geodesy contains three basic branches, namely geometric geodesy, physical geodesy and spatial geodesy.

2. Photogrammetry and Remote Sensing

Photogrammetry and remote sensing is the study of using photography or remote sensing to obtain image data of target objects and extract geometric or physical information from them. A discipline that uses graphics, images and digital forms to express surveying and mapping results.

The main photographic object of photogrammetry is the earth's surface, which is used to survey and map the country's topographic maps at various basic scales and provide basic data for various geographical information systems and land information systems.

The development of photogrammetry has gone through three stages: analog, analytical and digital photogrammetry. According to the different positions of acquiring images on the ground, photogrammetry can be divided into aerial photogrammetry, aerospace photogrammetry and ground (close-range) photogrammetry.

3. Cartography

Cartography is the study of the basic theories of analog maps and digital maps, the technical methods of design, compilation and reproduction, and the use of survey results and data. The disciplines to which it is applied. With the development of computer mapping technology and map databases, cartography has now developed into a science that studies spatial geographical environment information and spatial information systems.

4. Engineering Surveying

Engineering Surveying is the study of engineering construction and natural resource development, and it involves control measurements and large-scale measurements at all stages of planning, exploration design, construction and operation management. Theoretical and technical disciplines such as topographic map surveying and mapping, cadastral surveying and mapping, construction setting out, equipment installation, deformation monitoring, analysis and forecasting, etc. It is a direct application of surveying and mapping in the national economy and national defense construction, and can be divided into general engineering surveying and precision engineering surveying.

5. Ocean Surveying and Mapping

Ocean Surveying and Mapping is based on ocean water bodies and seabeds, studying ocean positioning, measuring ocean geoid and mean sea level, seabed and sea surface topography, A discipline that covers the geographical distribution of natural and social information such as ocean gravity, ocean magnetism, and ocean environment, as well as the theory and technology for compiling various nautical charts. Content includes marine geodesy, hydrography, seabed topographic survey and nautical chart preparation.

With the development of optoelectronic technology, artificial earth satellite technology and computer technology and their application in surveying and mapping, major changes have taken place in the working methods and application fields of surveying and mapping. Traditionally, according to the working methods or applications The classification of fields is no longer fully suitable for the current situation of surveying and mapping.

In recent decades, my country's surveying and mapping industry has made great progress. Established and unified the national coordinate system and elevation system; established the geodetic control network, national leveling network, basic gravity network and satellite positioning network throughout the country; completed the overall adjustment of the national geodetic network and leveling network, and the surveying and mapping of the national basic map Work. The GPS global positioning system is widely used, domestic GIS software is becoming more and more mature, and the level of surveying and mapping technology is rapidly catching up with and in some aspects beginning to lead the international level of surveying and mapping technology.

(3) Shape and size of the earth

The shape of the earth’s natural surface is extremely complex, including mountains, hills, plains, rivers, lakes, seas, etc. It is difficult to determine the shape and size of the Earth if we consider it solely from its complex natural shape. However, from the perspective of the earth as a whole, its surface ocean area accounts for about 71%, and its land area accounts for about 29%. The highest mountain on land is Mount Everest, with an altitude of 8844.43m. However, it is different from the average radius of the earth (about 6371km ) is still insignificant compared to. Therefore, the overall shape of the earth can be regarded as a closed shape surrounded by sea water. That is to say, imagine that the earth is a closed shape in which a static sea surface (that is, a sea surface without waves and tides) extends to the interior of the continent and is finally surrounded. The surface of water when it is at rest is called the level plane.

There are an infinite number of horizontal planes, one of which coincides with the mean sea level and extends into the interior of the continent is called the geoid. It is a continuous closed surface without wrinkles or edges. The shape surrounded by the geoid is called a terrestrial body. It is generally believed that the terrestrial body can represent the shape of the entire earth.

Any point on the earth is subject to two forces at the same time. One is the centrifugal force generated by the rotation of the earth; the other is the gravity. The resultant force of these two forces is called gravity, and the line of action of gravity is also called the plumb line.

The plumb line is the baseline for measurement work. A vertical ball is hung with a string. The direction it indicates when it is stationary is the direction of gravity of the hanging point, also called the plumb line direction (Figure 1 -1).

Figure 1-1 Plumb line direction

The horizontal plane is a curved surface. The plane that passes through a certain point on the horizontal plane and is tangent to the horizontal plane is called the horizontal plane passing through that point. The physical characteristic of the level plane is that the level plane is perpendicular to its plumb line everywhere. The plumb line direction is also called the gravity direction.

Due to the uneven distribution of materials inside the earth, the vertical direction of each point on the ground changes irregularly, so the geoid is actually a slightly undulating and irregular smooth surface, as shown in Figure 1- 2 shown. Obviously, it is very difficult or even impossible to calculate various measurement data and process the results and drawings on such a surface. However, after long-term and precise measurements, people have found that the earth's body is very close to a rotating ellipsoid with slightly flattened poles. This rotating ellipsoid whose shape and size is very close to the earth's body is called the earth's ellipsoid. It is a mathematical surface, with a representing the long radius of the earth's ellipsoid, and b representing its short radius, then the oblateness f of the earth's ellipsoid is

Geological surveying engineer: basic knowledge

Figure 1-2 Schematic diagram of the geoid

So the elements of the earth's ellipsoid can be represented by a and f. In the past, its value was determined by radian measurement and gravity measurement. Nowadays, more accurate results can be obtained by combining satellite geodetic data. There are many earth ellipsoid elements derived and adopted by countries around the world. Table 1-1 lists several typical earth ellipsoid geometric parameters for reference.

Table 1-1 Geometric parameters of the Earth ellipsoid

Since the oblateness of the Earth ellipsoid is very small, when the measurement area is not large, the ellipsoid can be approximately regarded as a circle. Treated as a ball, its radius is 6371km.