Map is a graphic language form for recording geographic information. From the point of view of historical development, geographic information system emerged from the map and became another new carrier form of map information. It has the functions of storage, analysis, display and transmission. In particular, computer cartography provides a complete set of methods for digital expression, operation and display of map features, and provides a geographic information system, the traditional graphic output design provides technical support; at the same time, maps are still one of the important sources of data for current GIS. But there are essential differences between the two: the map emphasizes data analysis, symbolization and display, while the geographic information system focuses on information analysis. The theory and method of cartography have an important influence on the development of geographic information systems, especially the spatial data in GIS shows that many technologies are derived from cartography, but their means of expression are more flexible and rich.
Computer graphics is a relatively new field in cartography and an important part of the output and display of GIS. The main content of computer graphics is to implement map design and drawing through computer programs and corresponding drawing hardware. The map symbol can already be drawn entirely by the computer. Many processing of map features, such as classification, spatial interpolation, etc., can be implemented by a computer. Significant progress has also been made in the use of computers for cartographic synthesis of linear features. However, the results of computer graphics synthesis are still relatively mechanical, and it is necessary to computerize more expert experience, especially the use of computers to simultaneously map multiple content is not mature. Computer graphics is a technology that uses computer to process graphical information and human-machine communication processing with graphical information, it is the basis of GIS algorithm design, GIS is continuously developed with the development of computer graphics technology.
The map should be able to be scaled by the scene on the ground it represents, the scale is the ratio of the length of the line on the map to the horizontal projection length of the corresponding distance on the ground. If the map is marked with a scale of 1:50000, it means that the length of the figure on the map is equivalent to 1/50,000 of the length of the corresponding ground object. This scale is basically suitable for all parts of the map for large scale maps. Therefore, it can be considered that the scale is a sign of the degree of linear reduction of the map, and it is one of the basic components that constitute the mathematical elements of the map. The relationship between various objects on a map requires that the map be constructed according to mathematical rules, that is, firstly, the natural surface of the earth is projected vertically onto the ellipsoid (or sphere) of the earth, and then the ellipsoid (or sphere) of the earth is projected onto the plane according to mathematical rules. This method of mathematically mapping the ellipsoid (or spherical) of the earth to a plane is called a map projection. According to the mathematical foundation established by this method, the points on the surface of the earth and the corresponding points on the map plane can maintain a certain functional relationship, so that the relationship and distribution law of the spatial elements can be accurately expressed on the map, the direction, distance and area between them make the map regional and scalability.
Map symbol is the basic means of expressing the content of the map, which not only can represent the spatial position, shape, quality and quantity characteristics of things, but also can represent the interrelationship between various things and the overall characteristics of the area. Graphic symbols are composed of three basic factors: shape, size and color, and have systematic characteristics; annotation is an important part of the map symbol, and it also has the difference in shape, size, and color. According to the characteristics of the distribution of things, the map symbols are divided into spots, dots and lines. These symbols are used to represent the distribution, quality and quantity characteristics of the thematic elements, as well as dynamic changes. Many representations are formed: range method, bottom method, bottom method, contour method, fixed point symbol method, linear symbol method, moving line method, point value method, localized statistical graph method, color level statistical graph method and positioning statistical graph method. The main methods for expressing terrain are contour line method, layered color method, and shading method.
The map symbol is the basic means of representing the content of the map, which consists of graphics and text with different shapes, different sizes, and different colors. The map symbol is the language of the map and is a graphical language. Compared with the word language, the biggest feature is the intuitive image. In the case of a single symbol, it can represent the spatial location, size, quality, and quantity characteristics of a thing; for the same kind of symbols, it can reflect the distribution characteristics of various elements; the sum of the various symbols can indicate the relationship between the various elements and the overall characteristics of the region. Therefore, map symbols not only have the basic functions of determining the spatial location, distribution characteristics, and quality and quantity characteristics of objective things, but also have the special functions of interconnecting and co-expressing the overall elements of the geographical environment.
The formation process of map symbols can be said to be an agreed process, after a long period of inspection, there is a convention to achieve the degree of vulgarity, which is familiar and recognized by the vast number of useers. The role of the map symbol is that it can ensure that the spatial position of the objective object represented has a high geometric precision, thus providing measurability; the quality and quantity characteristics of the thing can be expressed by not using proportional symbols or semi-proportional symbols.
The map uses the graphical symbols that are easily felt by people to represent the ground scene, the use of symbols has the following effects:
Selectively represent the main things in the geographical environment, so the ground conditions on the map of the smaller scale can still be seen at a glance. For those important ground scenes that cannot be scaled due to downsizing, they can be exaggerated with scales that are not proportional;
The flat graphical symbol indicates the undulation of the ground, which can also be said to be in a two-dimensional plane, which can express the three-dimensional space condition, and can measure its length, height and slope;
In addition to symbolizing the shape of the ground scenery, it can also show the invisible essential features of the scenery, such as seabed topography, seabed geology, sea water temperature and salinity, etc.
Symbols can be used to represent many natural and socio-economic phenomena on the ground, such as air pressure, rainfall, political area and population movement. In addition, it can also show the relationship between things and constraints, such as the relationship between forest distribution and wood processing industry. There are also explanatory words and figures on the map, which are also an important part of the map to indicate the name, quality and quantity of the ground scenery.
Cartographic synthesis is the trade-off and simplification of the objective things in the drawing area, the summarized map shows the main things and the essential characteristics. The scale, purpose and theme of the map, the geographical features of the map area and the graphical size of the symbol are the main factors affecting the map summary, the map summary is mainly reflected in the content selection, quantity simplification, quality simplification and shape simplification.
The most important and basic feature of maps is the spatial structure of the ground objects expressed in a reduced form, this feature indicates that the map cannot express all the things on the ground without any omission. The ground conditions represented on the map are the result of the generalization. The map is reduced compared to the actual ground, the ground scenes displayed on the map are few in number, and are graphically small and simplified, this is because the content displayed on the map is subject to trade-offs and simplifications. From 1:10000 scale to 1:50000 scale map, if the original content is not traded off and simplified, the reduced map is neither clear nor easy to read. When the landscape is reduced or the original map is reduced to a smaller scale map, the live or original content is selected and simplified according to the needs of the map or the theme, so as to express on a limited map, the theory and method of the basic characteristics of the cartographic area and the main features of the geographic elements are called map synthesis (map summarization). Cartographic synthesis is the choice and simplification of objective things, the choice is to select the most important things from a large number of objective things to represent on the map, and to discard the secondary things; simplification is the shape, quantity and quantity of objective things, simplification of quality characteristics; shape simplification is to remove the fragment of the contour shape to highlight the overall characteristics of the thing; the simplification of the quantity and quality characteristics is to reduce the number of classification and grading to narrow the difference from the objective things. Of course, the trade-offs and simplifications are not arbitrary, instead, based on the scale, purpose and geographic features of the map, the elements on the map and their intrinsic connections are analyzed, the purpose of selection is to emphasize the characteristics of the main things and the essence, while discarding the secondary things and the non-essential features. Cartographic synthesis can be divided into proportional summaries and purpose summaries. The former is because the scale of the map is reduced, the graphics are reduced, and some of the graphics are narrowed down to be difficult to express clearly, so that they must be selected and simplified; the latter is because the importance of objective things is not entirely determined by the size of its graphics, so its selection and simplification is not entirely determined by the scale, but also by the compiler’s judgment of the importance of objective things to determine whether to select and simplify.
From the development of geographic information system, we can see that the generation and development of geographic information system are closely related to cartographic information system, the similarity between them is based on the expression, display and processing of spatial database. From the point of view of system composition and function, a geographic information system has all the components and functions of the computer-aided cartography system, and the geographic information system also has the function of data processing. Map is a kind of graphic image, a geographic model that scientifically abstracts and symbolically represents the real world according to geographical thought, a product of geographical thinking and an efficient carrier of physical world geographic information, map can record and transmit systematically the historical, present and planned geographic landscape information of physical world from different aspects and themes.