Geographic information system | August more challenges

This is the first day of my participation in the August Challenge. For details, see:August is more challenging

Geographic Information System (GIS)

GIS basics

• Spatial data: used to describe the geographical location, regional scope and spatial connections (such as points, polylines, polygon, etc.) of geographical entities, geographical elements, geographical phenomena, geographical events and geographical processes that arise, exist and develop.
• Attribute data: Used to describe geographic entities. The related attribute characteristics of geographical elements, geographical phenomena, geographical events and geographical processes

• Spatial topology relationship: topology adjacency, topology association, and topology inclusion

• Map projection: A map projection is a method of establishing a one-to-one correspondence between points on the earth’s surface and points on the projected plane (the map plane). Namely to establish the mathematical conversion formula between. It can be used as a basic method of projecting the non-flattening surface, namely the earth surface, into a plane to ensure the connection and integrity of spatial information in the region. This projection process will produce projection deformation, and different projection methods have different properties and sizes of projections

• Elevation system: The height of the ground point from the elevation datum is called elevation. Generally, a country uses only one average sea level as the uniform elevation datum, and the elevation system established from this datum is called the national elevation system, otherwise called the local elevation system. Before 1985, China adopted the “1956 Yellow Sea elevation system” (taking the average sea surface measured at Qingdao tide station from 1950 to 1956 as the elevation datum). In 1985, the national elevation datum of 1985 was put into use (taking the mean sea level of Qingdao Tide Station from 1952 to 1979 as the elevation datum).

• Coordinate system: describes the representation of position in space

• Coordinate system classification: geographic coordinate system, projection coordinate system

• Geographic coordinate system: expressed in latitude and longitude, also known as the geodetic coordinate system

• Projection coordinate system: The transformation of spherical surface coordinates into plane coordinates requires a method called projection. The projection method is not the only one, but also for one purpose, to make the local coordinates the most accurate. So now there are many projection methods, such as Gaussian projection, Mercator projection and so on

• Our commonly used coordinate system: 1954 Beijing coordinate system, 1980 Xi ‘an coordinate system, national 2000 coordinate system, Web84 coordinate system, Mars coordinate system

• Beijing 54 coordinate system: the origin is in Pulkovo, the former Soviet Union, the reference ellipsoid is the Klasovsky ellipsoid, the main parameters are: A =6378254 m, F =1/298.3

• Xi ‘an 80 coordinate system: the origin is in Yongle Town, Jingyang County, Shaanxi Province. The reference ellipsoid is the ellipsoid recommended by the International Union of Geodesy and Geophysics in 1975. The main parameters are: A =6378140 m, the second-order spherical harmonic coefficient of the earth’s gravitational field J2=1/298.3, the GM of the gravitational constant and the mass of the Earth =3.986005×1014m3/s2, and the angular velocity of the earth is W =7.292115×10-5rad/s

• National 2000 coordinate system: China’s latest national geodetic coordinate system, the English abbreviation is CGCS2000. The 2000 National geodetic coordinate system is the embodiment of global geocentric coordinate system in China. Its origin is the center of mass of the whole earth including ocean and atmosphere. The Z-axis points to the agreed polar direction (BIH International Time Bureau) as defined by BIH1984.0, the X-axis points to the intersection of the zero meridian plane and the agreed equator as defined by BIH1984.0, and the Y-axis is determined in the right-handed coordinate system

• Web84 coordinate system: a geocentric coordinate system adopted internationally. The origin of the coordinates is the center of mass of the Earth, the z-axis of the geocentric space rectangular coordinate system points to the direction of the protocol Earth pole (CTP) as defined by BIH (International Time Service Agency) 1984.O, the X-axis points to the intersection of the zero meridian plane of BIH 1984.0 and the equator of CTP, and the Y-axis is perpendicular to the Z-axis and X-axis to form the right-handed coordinate system. Called 1984 World Geodetic Coordinate System

• Gcj-02 Coordinate system: THE GCJ-02 coordinate system released by the National Survey Bureau in 2002, also known as the “Mars coordinate”. Suitable for domestic, domestic map suppliers basically use this coordinate system to encrypt the position. In China, the coordinate system of GCJ-02 must be used in many cases. For example, Google, Tencent, Autonavi are using this coordinate system. Gcj-02 is also the most widely used coordinate system in China. Here’s how the Mars coordinate system works: NBS has developed a system that converts actual coordinates into virtual ones. All digital maps sold in China must be coordinate converted using this system before being marketed. This is production, and this electronic map is called Mars map. In the use process, THE GPS terminal equipment must integrate the encryption algorithm provided by the National Survey bureau to convert the coordinates obtained from the GPS satellite into virtual coordinates, and then look for them on the Mars map. In this way, the map matching on the Mars coordinate system is completed, which protects the security of our country to a certain extent

• BD09 coordinate system: The longitude and latitude projection of BD09 belongs to baidu coordinate system. It is based on the standard longitude and latitude after gCJ-02 addition and deflection, plus Baidu’s own addition and deflection algorithm, that is, two addition and deflection are carried out on the basis of the standard longitude and latitude, which is similar to the principle of Mars coordinate system

• Geographic coordinate system transformation: A coordinate system transformation is required when the latitude and longitude coordinates you obtained do not match the GEOGRAPHIC information system you are using. For example: the current acquisition is a geographical coordinate system in units of latitude and longitude, your current system is a projection coordinate system in units of meters, then you need to carry out the coordinate transformation. (The transformation parameters and THE JS code will be released in a future article.)

OGC- Map data service

• OGC: The Open Geospatial Consortium is a nonprofit international standards organization that has developed a set of standards for data and services, GIS vendors develop according to this standard to ensure interoperability of spatial data, and are committed to providing standardization of software and data and services for the geographic information industry.
• Map service: a way to make maps accessible through the Web. The original map needs to be created first, and then published to the service site, where users can access the corresponding map service through various Web applications.
• Web Map Service (WMS) : Web Map Service (also known as dynamic Map Service), it uses the data with geospatial location information to make maps, where the Map is defined as a visual representation of geographic data, according to the user’s request, return the corresponding Map, including PNG, GIF, JPEG and other raster forms, Or vector form such as SVG or WEB CGM. WMS supports the HTTP protocol, and the operations supported are determined by the URL. The WMS provides the following operations:
• GetCapabitities: Returns service-level metadata, which is a description of the service information content and required parameters.
• GetMap: Returns a map image with clearly defined geospatial reference and size parameters.
• GetFeatureInfo: Returns information about specific elements displayed on the map.
• GetLegendGraphic: Returns legend information for the map.
• The Network Element Service (WFS) (also known as the Element Mapping service) enables users to insert, update, delete, retrieve and discover geographical elements in a distributed environment over HTTP. The service returns element-level GML(Geography Markup Language) data based on HTTP client requests, and provides transaction operations such as adding, modifying, and deleting elements, which is a further step in the Web map service. WFS constructs query conditions through OGC Filter, and supports the query based on spatial geometry relationship, the query based on attribute domain, and of course, the common query based on spatial relationship and attribute domain. WFS provides the following operations:
• GetCapabitities: Returns service-level metadata, which is a description of the service information content and required parameters.
• DescribeFeatureType: Generate a Schema to describe the feature types that the WFS implementation provides. The Schema description defines how the WFS implementation encodes an element instance on input and generates an element instance on output.
• GetFeature: Returns a GML compliant data document based on query requirements.
• LockFeature: In order to ensure the consistency of the element information when the user makes a Transaction request, that is, when one Transaction accesses a data item, other transactions cannot modify the data item, the element data is locked.
• Transaction: An interaction with an element instance. This operation can not only provide element reading, but also support element online editing and transaction processing. The Transaction operation is optional and the server chooses whether to support it or not based on the nature of the data.
• Tile map service (TMS):(tile map Servcie) defines operations that allow users to access the tile map on demand, faster, and with support for modifying coordinate systems. WMTS is probably the first OGC service standard to support RESTful access. The slice Map service is also known as the cache service. The map cache is a very effective way to make the map and image service run faster. When a map cache is created, the server draws the entire map and stores copies of the map images at several different scale levels. The server can then distribute the images when the user requests the map. It is much faster for the server to return cached images each time a map is requested than to draw a map
• Web Map Tile Service (WMTS): OpenGIS Web Map Title Service provides a standardized solution for publishing digital Map services using a pre-defined block approach. WMTS makes up for WMS’s lack of partitioned maps. WMS is an ideal solution for a service that provides customizable maps for dynamic data or user customizable maps in conjunction with THE SLD standard. WMTS sacrifices the flexibility of providing custom maps and instead enhances scalability by providing static data (base maps) with range boxes and scales limited to individual blocks. These fixed sets of blocks make it possible to implement the WMTS service using a Web server that simply returns existing files, and to leverage some standard networking mechanisms such as distributed caching to scale the three types of resources supported by the WMTS interface:
• A ServiceMetadata resource (a response to the GetCapabilities operation in a process-oriented architecture style) that the server must implement. The ServiceMetadata resource describes the capabilities and information contained in the specified server implementation. This operation also supports standard version negotiation between client and server in a process-oriented architecture style.
• Block resources (in response to GetTile operations in a process-oriented architecture style) (must be implemented on the server side). The map block resource represents a small piece of the map representation result of a layer.
• FeatureInfo resources (responses to GetFeatureInfo operations in a process-oriented architecture style) that the server can choose to implement. This resource provides information about feature elements at a specific pixel location in a block map. Similar to the behavior of the GetFeatureInfo operation in WMS, the resource returns relevant information in text form by providing, for example, the topic property name and its value

conclusion

Webgis knowledge points and actual combat will be updated in the future, and questions are welcome to leave a message. The future plate will also be set in the introduction to GIS basic knowledge, WebGIS: Including ArcGIS API, OpenLayers, Leaflet and Secondary development API framework of Audev.net and Baidu, some real time points, path analysis algorithm part, 3d part based on cesiumJS and threeJS, etc.