1. Spatial data and common formats
1. Spatial data concept
Spatial data refers to data used to represent the location, shape, size, and distribution characteristics of spatial entities . It can be used to describe targets from the real world. It has the characteristics of positioning, qualitative, temporal and spatial relationships. Spatial data is a kind of data that uses basic spatial data structures such as points, lines, surfaces, and entities to represent the natural world on which people live.
Spatial data structure is a logical structure of spatial data suitable for computer storage, management, and processing. It is the organization and coding form of spatial data in a computer, and is an abstract description of the spatial arrangement and interrelationship of geographic entities. It is an understanding and interpretation of spatial data.
Spatial data structure refers to the arrangement and organization of spatial data. The specific realization of the spatial data structure is to convert graphics data, image data, statistical data and other data into a form suitable for computer storage and processing according to a certain data structure. Different data sources are processed with different data structures, and the content varies greatly. The efficiency of computer processing of data largely depends on the data structure.
2. Spatial data expression
There are two main ways to express geospatial data: raster and vector, forming two forms of raster data and vector data respectively. For example, remote sensing image data based on pixels (pixels) is raster data, and geospatial data information described based on elements such as points, lines, and areas is vector data.
There are various formats of raster data and vector data. Currently, raster data are commonly used in *.img, *.tif formats, etc. For raster data, there are many different encoding methods, and the commonly used encoding methods include direct rasterization. Lattice data encoding, Fairman chain code, run-length encoding, quadtree encoding, etc. In quadtree encoding, there are conventional quadtree and linear quadtree encoding.
Vector data includes *.shp, *.cov, etc. For files with the same suffix, some encoding methods are different, and different software may not be compatible. At present, most GIS software supports the reading and processing of these commonly used data formats, and they are gradually compatible with different formats.
Paper-based literature data is inconvenient to quickly view, save and calculate and analyze. It is often necessary to use a scanner to scan it into an electronic version of the document or manually enter it into an electronic version of the document, for example, statistical data, manually enter it into the electronic Forms are better in format data such as documents, while image data is scanned with a scanner. Current scanners usually scan paper documents into raster files for storage. It is difficult to scan data such as maps into vector files, and manual correction is often required for drawing.
3. Introduction to commonly used spatial data formats
There are many organizational forms of spatial data, such as the file types of Shapefile, Coverage, and Tiff commonly used in ArcGIS, the Coverage data format commonly used by Workstaion, and the types of Geodatabase databases:
Shapefile : A file format for storing GIS data based on files . It is made up of at least three files, shp, dbf, and shx, which store space, attributes, and the relationship between the first two. It is a relatively common data format in GIS . In addition, there are four kinds of files: prj, shp.xml, sbn and sbx: prj stores the coordinate system, shp.xml is the xml metadata file generated after metadata browsing of the shapefile, and sbn and sbx store the space of the shapefile. Index, which can speed up the reading of spatial data. These two files are generated after the data is manipulated, browsed, or connected.
Coverage: A topological data structure, which is discussed in general GIS principle books. The data structure is complex, and the attributes are stored in the Info table by default. At present, there are still some analysis operations in ArcGIS that can only be operated based on this data format.
Coverage : A topological data structure, the data structure is complex, and the attributes are stored in the Info table by default. At present, there are still some analysis operations in ArcGIS that can only be operated based on this data format.
Tiff is a popular high-bit color image format with a complex format, a large amount of storage content, and a large storage space. It has a .tif extension. The data format is a three-level architecture, from high to low: file header, one or more directories and data containing marker pointers called IFD.
The spatial database Geodatabase, a spatial database that truly implements object storage, is based on the data format stored in RDBMS, and there are three types:
- FileGeodatabse is stored in a file, and there is no limit to the file size.
- PersonalGeodatabse is used to store small amounts of data, stored in Access's mdb format, and the file cannot be larger than 4GB.
- ArcSDEGeodatabse stores large data, stored in large databases such as Oracle, SqlServer, DB2, etc., which can achieve concurrent operations, but requires a separate user license.
2. Map projection
1. Map projection concept
Map projection is a theory and method that uses certain mathematical laws to transform the longitude and latitude lines of the earth's surface to a plane .
Map, Map Projection. The theory and method of converting any point on the surface of the earth to the plane of the map using certain mathematical laws. Written conceptualization definition: Map projection refers to the method of establishing a one-to-one correspondence between points on the surface of the earth (or the surface of other planets or celestial spheres ) and points on the projection plane (that is, the map plane). That is to establish the mathematical conversion formula between. It will serve as a basic method for projecting an unflattenable curved surface, that is, the surface of the earth onto a plane, ensuring the regional connection and integrity of spatial information. This projection process will produce projection distortion, and different projection methods have different properties and sizes of projection distortion . (Baidu Encyclopedia)
2. Commonly used map projections and parameters in my country
According to my country's geographic distribution and the characteristics of various projections, the selection of map projections in my country usually adopts the following criteria:
Lambert projection, equiangular cut conic projection, no angular distortion, no distortion on the two standard latitudes, due to the use of sub-zone projection, the latitude difference in each zone is small, so the deformation within our country is almost equal, and the maximum length deformation does not exceed ± 0.03% , the maximum area deformation is not more than ±0.06% . The basic scale topographic map of our country is 1:1 million topographic maps, most of the provinces and regions, and most maps of this scale or smaller scales mostly use Lambert projection.
The Gausske Rüger projection, that is, the equiangular transverse axis tangent to the elliptic cylinder projection, the central meridian is projected as a straight line, its length is not deformed, and is equal to the actual length of the sphere. The remaining meridians are arcs converging to the pole, and the farther away from the central meridian, The greater the deformation. Our basic scale topographic maps 1 : 50 million, or 1 : 25 million, or 1 : 10 million, or 1 : . 5 million and 1 : 2.5 million, or 1 : 1 million and 1 : 5000 usually high Trask Lvge projection.
The Albers projection, that is, the double standard latitude equal-cut conic projection, has no area deformation, and two standard latitudes are not deformed. It maintains the correct area comparison on the map and is suitable for dynamic monitoring of planting area in remote sensing monitoring. The Albers projection and Lambert projection belong to the same projection system (both are secant conic projections). This projection is in China. The central meridian is generally 105°E , and the two standard parallels are 25°N and 47°N .
Albes projection is usually used for provincial-scale data . The same coordinate is selected uniformly for the reference coordinates of spatial data, which is convenient for the unified management and application of later data.
In the process of normalized data processing, one projection parameter can be uniformly adopted, and projection conversion of inconsistent projection data is carried out. If there is no correct projection information, geometric correction methods are used to uniformly process the data with the following projection information:
For example, the bi-standard latitude equal area conic projection ( Albes projection), the projection parameters are set to:
Central longitude: 105º east
The first standard latitude (southern standard latitude): 25º north latitude
Second standard latitude (northern standard latitude): 47º north latitude
Coordinate origin: the intersection of 0º east longitude and 0º north latitude
Geographical Coordinate System: Beijing1954
a = 6378137 ，b = 6356752 ，α = 1 / 298.3
Unit: m
Among them, a represents the long radius of the ellipsoid, b represents the short radius, and α represents the flatness.
In the actual application process, the selection of projection and its parameters are often selected and set according to actual monitoring requirements, and is not limited to a projection coordinate system.
3. Different map projection shapes change
Map projection deformation
3. How to use projection in ARCGIS
In arcgis, there are two main uses of map projection. The first is to define projection, and the second is projection transformation. For the specific use of these two situations, the author has also explained in detail in the blog garden before: define projection The difference between projection conversion and its specific usage method . The following is a description of the use of projection in arcgis
1. The type of projection in ARCGIS
There are two main types of projections in arcgis, the first is Geographic Coordinate Systems, that is, geographic coordinate systems; the second is Projected Coordinate Systems, that is, projected coordinate systems. The former is a coordinate system based on latitude and longitude, which displays latitude and longitude coordinates, and the latter is a plane-based coordinate system, such as the Beijing54 coordinate system established in my country, which is a coordinate system based on the origin of the coordinate.
Under the two kinds of coordinate data sets, there are many local and global coordinate systems, each of which has its own scope of application or zone range.
2. Define the projection
For example, define a geographic coordinate system based on the registration result of latitude and longitude in the previous article. The data itself has no projection information, but has correct coordinate information. You can define a WGS84 geographic coordinate system. The coordinate values between different coordinate systems are If there are certain differences, it can be transformed according to the 7-parameter model.
3. Projection conversion
Projection conversion includes several types, such as geographic coordinate system-projected coordinate system, geographic coordinate system-geographic coordinate system, projected coordinate system-geographic coordinate system, projected coordinate system-projected coordinate system,
If it involves the transformation of different ellipsoids (some geographic coordinate systems are based on different ellipsoids), a 7-parameter model is needed to transform the ellipsoid.
In this case, an ellipsoid transformation model needs to be established, and the Geographic Transformation in the figure above needs to be set up to establish a transformation model. If the accuracy requirements are not high, this error can usually be ignored and the transformation is not strictly performed.
4. Dynamic projection
Dynamic projection means that after adding spatial data to the map window, the map window (data frame in arcgis) will convert all the spatial data of different projections into the projection used by the current window for display. The current window generally defaults to the first data added The projection is used as the projection of the Data Frame (work area). If the first loaded data has no projection, the dynamic projection function will often fail.