[Mori city] random talk on GIS data (II)

In the first chapter “GIS Data rambling （ One ）” It briefly introduces GIS The concept of vector data and grid data in data , And common GIS Data file type . In this ramble, we will continue to talk about map services .

Spatial database

GIS The history of development is to follow IT The development history of technology , For example, various vector data mentioned above 、 The files of raster data belong to spatial data . Due to the big data attribute of geographic data , The data volume of an urban geographic information system may reach dozens GB, If we consider the storage of image data , Maybe hundreds GB. Massive data of geographic information system , Bring the system operation 、 Data organization and storage 、 A series of technical difficulties such as network transmission , Naturally, it also increases the difficulty of data management .

in order to Better management GIS data It developed Spatial database , With the development of technology, many mainstream databases basically support spatial data expansion , such as MySQL、Oracle、PostGIS etc. .

Map service

With spatial database management GIS data , How to connect these data ？ You can't guide them one by one GeoJSON Well .OGC【 Open Geospatial Information Alliance （Open Geospatial Consortium-OGC）】 Some services are designed for the data source mentioned above, such as WMS、WMTS、WFS wait .

stay GIS Of Web Application , No matter the original data is stored in the file , It's better to store it in the database , These standard map services can be used in the network http(s) Protocol for access sharing . commonly GIS Large factory Server Product or open source GIS Server（ such as GeoServer） All support publishing these map services .

WMS（Web Map Service）

The main capability of this service is to GIS data （ Whether the original data is vector data or raster data ） Issued as “ picture ”. That is, according to the client request parameters （ Mainly bbox Range ）, The server returns the corresponding “ picture ”.

WMS Three common operations ：

• GetMap： According to the request parameters , Return a picture

• GetFeatureinfo： According to the coordinates in the request parameters , Return the queried object attribute information

• GetCapabilities： Return the metadata of the service

take “ Site ”、“ The railway ”、“ lakes ” Data release WMS service , You can use the WMS Service basis bbox Range request corresponding data rendered map image . Most map application front-end library pairs WMS Services are supported by corresponding layers .

WMS Sometimes called “ dynamic ” Map service , The server will send the requested parameters from the client （ Such as layer name 、BBox Within the scope of ） Go to the database or file to search and query the data in the corresponding range , Then render the image according to the configured style , Finally back to the client .

If the client requests frequently or the concurrent requests are high , The map layer contains a huge amount of data 、 When the configured style is complex , The load borne by the server will increase . therefore , Later, a map tile caching mechanism was developed . To put it simply, we should follow certain rules “ Rendering ” The map image after is stored on the server , When the client calls again, the server will directly return “ cache ” Pictures of the , No need to... Again “ dynamic ” Rendering , So there it is Map tiles service .

Map tile service

Map tiles It refers to the map within a certain range according to a certain size and format , By zoom level or scale bar , A square grid picture cut into rows and columns , The sliced square grid image is called tiles （tile）.

Tile Map pyramid model is a multi-resolution hierarchical model , From the bottom of the tile pyramid to the top , Resolution is getting lower , But the geographical scope of the representation remains the same .

Google The map uses Web Mecator Projection and tile grading cutting scheme , It has also become the de facto standard of Internet maps .

WMTS（Web Map Tile Service）

WMTS It's a kind of OGC Defined map tile service standard ,WMTS yes OGC The proposed cache technology standard , That is to say, the map that is cut into tiles of a certain size is cached on the server side , Only these pre-defined single tile services are provided to clients , Put more data processing operations such as layer overlay on the client , So as to alleviate GIS The pressure of server-side data processing .

There is also Google Tile、 OSGeo Of TMS（Tile Map Service） wait , Although different specifications , But basically, it can be in uearth API Try to use TileLayer Load these map tile Services .

How to use this data in low code loading ？ The lake can be 、 Railway and other data utilization GeoServer Published as WMTS and TMS Map service , In low code ThingJS Online development and utilization uearth API Of TileLayer Loading .

Whether it's WMS Service or map tile （ section ） service , The final call is “ picture ” Not the data itself . Although the browser has little pressure to render , But the style is not flexible , Only in low code ThingJS Adjust the effect of the overall picture , Inconvenient effect adjustment （ If you adjust the map style configuration of the server, you need to slice again ）. So there's... Again WFS Service to directly provide the operation of vector data .

WFS（Web Feature Service）

WFS Provides the client with Operate on vector data The ability of , In addition to returning query data , Also provides new 、 modify 、 Standard interface for deleting data . It means that what you see is a vector graph , Not a picture .

The difference between vector graphics and picture elements is that they will not be distorted with the change of magnification , Vector elements facilitate interaction and editing 、 Modify the shape, etc .

How to use this data in low code loading ？

utilize GeoServer Put the data （ Like a railway ） Issued as WFS The service queries directly on the front end , After returning the result （ It's usually geojson Format ）, use ThingJS/uearth relevant API Drawing .

Tool recommendation

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