Overview of the mission of mapping science and the shape and size of the earth

(I) The task of modern surveying and mapping

The task of modern surveying and mapping is: to study the science of collecting, measuring, describing and utilizing the information of the earth's environment on which human beings depend for their survival. Its contents include: spatial positioning, the shape of the earth and gravity field; obtaining the natural form of the earth and its cosmic bodies in outer space, man-made facilities and information related to their attributes; and making all kinds of topographic maps, thematic maps and setting up geographic information systems to provide basic information for the study of natural phenomena on the earth and related social phenomena for the sustainable development of the society.

(2) Classification of Surveying and Mapping

Surveying and mapping is divided into the subdisciplines of geodesy, photogrammetry and remote sensing, cartography, engineering surveying and marine surveying and mapping in accordance with the scope of the study, the object of the study, and the use of different technical means.

1. Geodesy

Geodesy is the study and determination of the shape of the earth, size, gravity field, overall and local movements and geometric position of points on the earth's surface and its changes in the theory and technology of the discipline.

Geodesy is an important theoretical basis for various branches of surveying and mapping, and its basic task is to establish the national plane control network, elevation control network and gravity control network, accurately determine the spatial three-dimensional position of the control points and the relationship between each other's positions, study and determine the shape and size of the earth, the earth's external gravitational field and its changes, the earth's tides, plate movement and crustal deformation and earthquake prediction, and so on, in order to provide a better service for the national economy, construction and social development, national security, and the development of the national economy. It provides geodetic infrastructure, information and technical support for national economic construction and social development, national security and research in earth and space sciences. Modern geodesy contains three basic branches, namely, geometric geodesy, physical geodesy and space geodesy.

2. Photogrammetry and Remote Sensing

Photogrammetry and Remote Sensing is the study of the use of photography or remote sensing means to obtain the image data of the target, from which geometric or physical information is extracted, and the results of the surveying and mapping results are expressed in the form of graphs, images, and figures.

Photogrammetry, whose main photographic object is the earth's surface, is used to map the topography of the country at various basic scales and to provide basic data for various geographic 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 the image acquired on the ground, photogrammetry can be divided into aerial photogrammetry, aerospace photogrammetry, and ground (close-up) photogrammetry.

3. Map Cartography

Map Cartography is a discipline that utilizes the results of surveying information to study the basic theory of analog and digital maps, the technical methods of designing, compiling and reproducing, and their applications. With the development of computer mapping technology and map database, map cartography has now developed into the study of spatial geographic environment information and spatial information system science.

4. Engineering Surveying

Engineering surveying is the study of engineering construction and natural resources development, in the planning, exploration and design, construction and operation and management of the various stages of control surveying, large-scale topographic mapping, cadastral surveying and mapping, construction sampling, equipment installation, deformation monitoring, and the analysis and forecasting of the theory and technology of the discipline. It is the 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. Marine Surveying and Mapping

Marine Surveying and Mapping is the object of marine water and seabed, the study of marine positioning, determination of marine geodetic level and the average sea surface, seabed and sea surface topography, marine gravity, marine magnetism, the marine environment, such as the geographical distribution of natural and social information, and the preparation of a variety of nautical charts of theoretical and technological disciplines. The contents include marine geodesy, hydrographic survey, seabed topographic survey and the preparation of nautical charts.

With the development of photoelectric technology, artificial Earth satellite technology and computer technology and its application in surveying and mapping, the operation mode and application field of surveying and mapping have undergone significant changes, and the traditional categorization according to the operation mode or the application field can not be fully suited to the status quo of surveying and mapping science.

In recent decades, China's surveying and mapping has developed greatly. The national coordinate system and elevation system have been established and unified; the geodetic control network, national level network, basic gravity network and satellite positioning network have been established throughout the country; the overall leveling of the national geodetic network and level network and the surveying and mapping of the national basic map have been accomplished; the global positioning system (GPS) has been widely used; the domestic GIS software is becoming mature day by day; the level of surveying and mapping science and technology is catching up rapidly and starting to lead the international level of surveying and mapping science and technology in some aspects. The level of surveying and mapping science and technology is rapidly catching up and in some aspects beginning to lead the international level of surveying and mapping science and technology.

(C) the shape and size of the earth

The shape of the earth's natural surface is extremely complex, there are mountains, hills, plains, rivers, lakes, seas and so on. It is difficult to determine the shape and size of the Earth if one considers it only in terms of its complex natural shape. However, from the overall point of view of the Earth, its surface area of the ocean accounts for about 71%, the land area accounts for about 29%, the highest mountain on land is Mount Everest, its altitude is 8844.43m, but it is compared with the average radius of the Earth (about 6371km) is still insignificant. Therefore, the total shape of the Earth can be regarded as a closed body surrounded by seawater. That is to say, we can imagine that the Earth is a closed body surrounded by a stationary sea water surface (i.e., a sea water surface with no waves or tides) that extends to the interior of the continents. The surface of water at rest is called the level surface. There are an infinite number of level surfaces, one of which coincides with the mean sea water surface and extends to the interior of the continent is called the geoid. It is a continuous closed surface without folds or angles. The shape surrounded by the geoid is called the geoid. It is commonly believed that the geoid represents the shape of the entire Earth.

Any point on the Earth is subject to two simultaneous forces, the centrifugal force generated by the rotation of the Earth and the gravitational force. The combined force of these two forces is called gravity, and the line of action of gravity is also known as the plumb line.

The plumb line is the datum line for measuring work. A plumb ball is suspended by a thin rope, and the direction indicated when it is at rest is the direction of gravity at the point of suspension, which is also called the direction of the plumb line (Figure 1-1).

Figure 1-1 Plumb line direction

Level surface is a curved surface, through a point on the level surface and the plane tangent to the level surface is known as the horizontal plane over the point. The physical characteristic of a level surface is that it is perpendicular to the direction of its plumb line everywhere. The direction of the plumb line is also known as the direction of gravity.

Because of the uneven distribution of material inside the earth, the direction of the plumb line at each point on the ground changes irregularly, so the earth's level surface is actually a slightly undulating and irregular smooth surface, as shown in Figure 1-2. Obviously, it is quite difficult or even impossible to calculate various measurement data and process the results and maps on such a surface. However, after a long period of precise measurements, people have found that the earth body is very close to a rotating ellipsoid with slightly flattened poles, and this rotating ellipsoid, which is very close to the shape and size of the earth body, is called the earth ellipsoid. It is a mathematical surface, with a to represent the long radius of the earth's ellipsoid, b to represent its short radius, the flatness f of the earth's ellipsoid is

Geological surveyor: the basics

Figure 1-2 Schematic diagram of the geoid

So the elements of the earth's ellipsoid can be expressed in terms of a and f. Its value used to be determined by arc measurement and gravity measurement, and modern combination of satellite geodetic data can produce more accurate results. There are many elements of the Earth ellipsoid derived and adopted by countries around the world, and Table 1-1 lists several typical geometric parameters of the Earth ellipsoid for reference.

Table 1-1 geometric parameters of the Earth's ellipsoid

Because of the Earth's ellipsoid is very small flatness, so when the measurement area is not large, the ellipsoid can be approximated as a sphere, the radius of which takes the value of 6371km.