Geographic information data is one of the core foundations of digital twins. It can map the spatial structure and relationship of the real world to the digital world, including static geographic elements and dynamic space-time variables, such as roads, buildings, water systems, traffic flow, and population. Density, environmental quality, etc., it can reflect the spatiotemporal characteristics and dynamic changes of the real world, and provide a basis for analysis, prediction, and optimization of digital twins.
In the construction phase of digital twin applications, geographic information data is the input of digital twin applications, which can be collected and transmitted through various sensors and devices, and processed and stored through cloud computing and big data technology, providing digital twin applications with various, Multi-level, multi-source data, and intelligent analysis through machine learning and algorithms. In the display stage of digital twin applications, geographic information data is the output of digital twins, which can effectively display the results and effects of digital twins, and provide a way for users to make decisions, control, and feedback.
In actual digital twin projects, we usually divide GIS data into vector data and raster data. Vector data is used to represent geometric elements and attribute information such as points, lines, and planes on the map, such as roads, buildings, place names, etc.; raster data is used to represent image information on the map, such as satellite images, topographic maps, and weather maps wait.
1. Vector data
The main common data formats of vector data files are: GeoJSON, ShapeFile, TopoJSON, WKB/WKT (Well-known text), KML/KMZ. Among them, the most common GIS vector data file formats in actual projects are GeoJSON and ShapeFile.
1）GeoJSON
GeoJSON is a map vector data format based on the JSON (JavaScript Object Notation) format. It supports various geometric elements such as points, lines, and polygons, as well as attribute data. It is often used to draw administrative boundaries, buildings, road networks, river systems, etc. in project construction.
Standard or custom administrative boundary data can be obtained in EasyMap, such as East China (Jiangsu Province, Anhui Province, Jiangxi Province, Zhejiang Province, Shanghai City), and the drill-down level supports China-province-city-district, county (not down drill down to town streets).
At the same time, EasyMap supports manual drawing or automatic generation of feature data of points, flying lines, trajectories, and surfaces.
Note: The data source of EasyMap is Gaode open platform
2）ShapeFile
Shapefile is an open map vector data format. It consists of at least three files: a .shp file containing vector geometry data, a .shx file containing index information, and a .dbf file containing attribute data. The shp data is not often used directly in project construction, and the shp data is often used as source data clipping or directly converted to GeoJSON. The shp data can be obtained in the National Geographic Information Resource Directory Service System and OpenStreetMap.
2. Raster data
Common GIS raster data are remote sensing images (satellite images/satellite photos), and in a broader sense, various aerial images (photos) are raster data. Some remote sensing application data or spatial analysis and calculation result data in GIS are generally provided in the form of raster data, and the size of raster data is proportional to the spatial range and resolution of the data.
1) Digital Elevation Model (DEM)
In the process of collecting mountain and river data, altitude information can also be obtained at the same time, which we call elevation data. Digital elevation model refers to the digital simulation of ground terrain through limited terrain elevation data (that is, the digital expression of terrain surface morphology). It is a solid ground model that expresses ground elevation in the form of a set of ordered numerical arrays. The methods of digital elevation model are divided into the following types: photogrammetry, ground survey, digitization of existing topographic maps, and extraction from existing DEM libraries.
2) Digital Surface Model (DSM)
A digital surface model refers to a solid ground model that uses a set of ordered numerical arrays to represent the surface elevation of surface objects, and generally refers to those with a specific scale series, a fixed frame range, and a comprehensive expression of the topography and features of the ground surface. map. Its content is characterized by being comprehensive, balanced, and not highlighting certain elements. It generally includes: survey control points, residential areas, water systems, transportation, pipelines, landforms, vegetation, etc.
3) Digital Orthophoto (DOM)
Digital orthophotos refer to digitized aerial photographs/remote sensing images (monochrome/color) that are scanned and processed by digital elevation models, corrected for projection difference pixel by pixel, mosaiced according to images, and generated by cutting according to the scope of the map Image data. It has the advantages of high precision, rich information, intuitive and realistic, fast acquisition, etc. The production principle of DOM is to apply professional geographic information remote sensing software to eliminate various distortions and displacement errors after radiation correction and geometric correction of the original remote sensing image according to its characteristics. Finally, a satellite remote sensing digital orthophoto map containing geographic information and various themes is obtained.
3. Common map coordinate system
WGS84 World Geodetic Coordinate System: It was established by the U.S. Defense Mapping Agency in 1984. It is the reference base for GPS satellite ephemeris and a kind of protocol earth reference system. If the map data source is MapBox, Cesium, OpenStreetMap, then the coordinate system used by the data is basically WGS84.
GCJ-02 Mars coordinate system: The coordinate system of the geographic information system formulated by the National Bureau of Surveying and Mapping of China. Various map systems published in China must at least use GCJ-02 to encrypt the geographic location for the first time. If the source of the map data is Gaode Map or Tencent Map, then the coordinate system used by the data is GCJ-02.
BD-09 Baidu coordinate system: Encrypted again on the basis of GCJ-02 coordinate system, if the map data source is Baidu map, then the coordinate system used for the data is BD-09.
CGCS2000 National Geodetic Coordinate System: According to the provisions of the National Surveying and Mapping Law, China has established a unified national geodetic coordinate system. If the map data source is Tiantu, the coordinate system used by the data is CGCS2000. The difference between the test and the WGS84 world geodetic coordinate system is about 15cm. The value is not accurate and is for reference only.
4. Online map service
Online map services usually contain two types of map data, vector data and raster data, which can be called and embedded through the API interface. Vector data for online map services is usually provided in the form of vector tiles, which divide map data into a series of small vector tiles for fast loading and display on web pages or mobile devices. Raster data is usually provided as raster tiles, which divide the map data into a series of small raster tiles. Common online map service providers include AutoNavi, Baidu, Tiantu, OpenStreetMap, Mapbox, etc.