Author: Demons
The map
Maps are the interface of GIS. It’s a representation of geographic data. It is the cartographic synthesis of all elements of the space environment, the use of map symbols, in accordance with certain mathematical rules projected onto a certain medium of the graphics, in order to convey the distribution of spatial things, mutual connection and development and change information.
What is usually on a map
Common map elements includeMap title, map body, legend, compass, scale, text description and gallery. Other elements include grid lines or grids, map projection names, illustrations or location maps, and data quality information.
scale
Map scale refers to the ratio of distance on a map to the corresponding distance in the field. This definition applies to all units of measurement. Map scale of 1:24000 means that the distance of 1cm on the map means that the distance in the field is 24000cm (240m). Map scale 1:24000 can also be used to represent the distance on the map 1m means the distance in the field is 24000m. The actual geography depends on the unit.
1:24,000 scale is greater than 1:100,000 scale. Moreover, the 1:24,000 scale shows more detail for small areas than the 1:100,000 scale.
So what do we mean by large scale and small scale?
Large scale means something with a small denominator, so mathematically 1/100 is bigger than 1/1000, so 1/100 is large scale.
China defines eight scale topographic maps of 1:5,000, 1:10,000, 1:25,000, 1:50,000, 1:100,000, 1:250,000, 1:500,000 and 1:1,000,000 as the basic national scale topographic maps. Among them, 1:5,000, 1:10,000, 1:25,000, 1:50,000 and 1:100,000 are classified as large scale topographic maps, 1:250,000 and 1:500,000 as medium scale topographic maps, and 1:1,000,000 as small scale topographic maps.
Large scale small range, high precision. Small scale large range, small precision.
Map symbol
The map notation for raster data is simple: regardless of whether the spatial element is described as a point, line, or region, the map notation applies the imaging element. The selection of map symbols for vector data depends on the type of elements. The general rules are:
- The point symbol represents the point element, which is represented by a coordinate pair [{x,y}];
- Line symbols represent line elements, which are represented by multiple coordinate pairs [{x1,y1},{x2,y2}…… {xn,yn}];
- The face symbol represents the face element, and the face element is represented by multiple coordinate pairs (end and end closed) [{x1,y1},{x2,y2}…… {xn,yn},{x1,y1}].
These graph data of point, line and plane stored by coordinates are called the Geometry of geographical elements.
The resolution of the
Represents how many map units (x map units/pixel) 1 pixel represents in the current map range, depending on the spatial reference of the data itself.
Map service
According to OGC standard, map service is specialized in providing shared map data service, responsible for providing map image, element information of designated coordinate points, and function description information of map service according to the request of client program. GIS services commonly used at present include:
- Services published by ArcGIS Server (MapServer tile service, MapServer dynamic service, FeatureServer vector service)
- OGC standard WMS, WMTS, WFS services (commonly used GeoServer publishing)
- Static resource data services (direct Web container publishing, such as IIS, Tomcat, Nginx, Node, etc.)
- Internet online map services (Google Map, Baidu Map, Autonavi, Tencent Map, Bing Map, Tiandi Map, etc.)
OGC
The OGC, or Open Geospatial Consortium, is a more “official” standardization body. Its main purpose is to develop standards related to spatial information, location-based services. OGC currently provides dozens of standards, including commonly used WMS, WFS, WCS, WMTS and so on, as well as some description documents of geographic data information, such as KML, SFS(Simple object description), GML, SLD(geographic data symbolization) and so on. Several common standard service interfaces are described below. Web services specification for OGC
WMS
WMS is one of the simple and important OGC standards. This standard defines three main operations for creating and displaying map images: GetCapabilities, GetMap, and GetFeatureInfo. GetMap is the core operation, which gets a map image.
- GetCapabilities: Returns service-level metadata, which is a description of the service information content and required parameters. Include some basic information of the current service providers and some of the current service ability, and for the use of WMS service client, to be able to get all the information contained in this document, when the client sends a service does not provide image format, the server is not capable of handling this request at this time. When using a WMS service, you need to validate the capabilities provided by the current service so that unnecessary complications can occur during use. Provide the client with the type and scope of spatial information that can be provided by the current map server, specific layer information and display style, supported query mode, default return information format when no query result (exception) and other service description information.
- Example: http://localhost:8080/geoserver/wms? service=WMS&request=GetCapabilities
GetCapabilities Request URL parameters:
| Request parameters | Mandatory (M)/Optional (O) | instructions |
|---|---|---|
| VERSION=version | O | Request version, an optional request parameter in the URL that, when not used, uses the highest WMS version of the service provided by the current service provider |
| SERVICE=WMS | M | Service type |
| REQUEST=GetCapabilities | M | Request name. The operation of the request must be set. You can set request to Getcapabilities, GetMap, and GetFeatureInfo |
| FORMAT=MIME_type | O | Output format of service metadata |
| UPDATESEQUENCE=string | O | A sequence of numbers or strings used to control buffer storage |
- GetMap: Returns a map image whose geospatial reference and size parameters are clearly defined. It is necessary to specify the version number of the WMS specification that the operation itself follows, the specific layer to display, the corresponding coordinate range, the size format of the returned image, etc.
- Example: http://localhost:8080/geoserver/wms? Bbox = -, 24, 130-66, 50 & styles = population&Format = image/png&request = GetMap&layers = topp: states&width = 550 & height = 250 & SRS = EPSG: 4 326
GetMap request parameters:
| Request parameters | Required (M)/Optional (O) | instructions |
|---|---|---|
| VERSION=version | M | Request version |
| REQUEST=GetMap | M | Name of the request |
| LAYERS=layer_list | M | A comma-separated list of one or more layers, optional if the SLD parameter exists |
| STYLES=style_list | M | A comma-separated list of playback styles for the requested layers, optional if the SLD parameter exists |
| CRS=namespace:identifier | M | Coordinate reference system |
| BBOX=minx,miny,maxx,maxy | M | Bounding box zoom in SRS units (lower left, upper right) |
| WIDTH=output_width | M | Width of map image in pixels |
| HEIGHT=output_height | M | The height of a map image in pixels |
| FORMAT=output_format | M | Map output format |
| TRANSPARENT=TRUE | FALSE | O |
| BGCOLOR=color_value | O | Background color in hexadecimal RGB color value (default=0xFFFFFF) |
| EXCEPTIONS=exception_format | O | WMS exception notification format (default=SE_XML) |
| TIME=time | O | Time value of the request layer |
| ELEVATION=elevation | O | Request level elevation |
| Other sample dimension(s) | O | Values for other appropriate dimensions |
| Vendor-specific parameters | O | Optional experimental parameters |
Response to GetMap: The response to a valid GetMap request must be a map containing the requested layer of information with geographic reference coordinates, in the desired style, and with the specified spatial reference frame, range box, size, and transparency. An invalid GetMap request must generate an error message in the Exceptions format of the request (or, in extreme cases, an answer to a network protocol error).
- GetFeatureInfo: Returns information about specific elements that appear on the map. This operation provides the WMS client program with the ability to further query for information about a specific spatial entity. This is often the result of a client submitting a query by specifying a spatial entity on a map previously returned by the WMS server. The GetFeatureInfo operation is supported only by layers that define or inherit the attribute Queryable =”1″. For other layers, the client cannot send a GetFeatureInfo request. If a WMS does not support the request and encounters it, an appropriately formatted service exception should be answered.
GetFeatureInfo Request parameters
| Request parameters | Required (M)/Optional (O) | describe |
|---|---|---|
| version=version | M | Request version |
| request=GetFeatureInfo | M | Name of the request |
| map request part | M | A partial copy of the map request parameters that produce the map whose information needs to be queried |
| query_layers=layer_list | M | A comma-separated list of one or more layers to query |
| info_format=output_format | M | Return format of element information (MIME type) |
| feature_count=number | O | Number of elements whose information needs to be returned (default=1) |
| x=pixel_column | M | The x-coordinate of the element expressed in pixels (with the upper-left coordinate being 0) |
| y=pixel_row | M | The Y coordinate of the element expressed in pixels (with the upper-left coordinate being 0) |
| exceptions=exception_format | O | The WMS report is in the format of the information |
Web Map Server (WMS) uses data with geospatial information to create maps. A map is defined as a visual representation of geographic data. In essence, it is a service that turns data into pictures and slices again after publishing. Different from other services, it is real-time slicing, that is, the web page sends requests to the server, and the server will start slicing, and only after it is cut will it be sent to the web page. Can return corresponding map according to user’s request, including PNG, GIF, JPEG and other raster form or vector form such as SVG and WEB CGM. Support for the network protocol HTTP, supported operations are defined by URL. Disadvantages: Because it is done, so slow. Advantages: It can ensure the timeliness of the data, and at the same time, it is not necessary to re-slice the updated data, and convenient management. Suitable for data with high update frequency and small data volume. WMS request mode: WMS supports standard GET and POST requests in standard HTTP, but for WMS, they are basically GET-based service requests.
WMTS/TMS
The WMTS standard defines operations that allow users to access tile services. TMS is a tile MP Service, also known as Web Map Tile Service (WMTS). Different from WMS, it saves slices locally. The first load is slow, because it saves slices locally in real time according to the request. The second load of slices at the same position will be faster, because it has been saved locally and can be read directly. The application of caching technology can relieve the pressure of data processing on WebGIS server, improve the speed of interactive response, and greatly improve the user experience of online map application client. Disadvantages: not flexible enough to regenerate slices once data is updated. Advantages: Fast loading speed. Do not use this parameter if the data is frequently updated and the geographical range is large.
WFS
Web Element service (WFS) returns element-level GML coding and provides transaction operations such as adding, modifying and deleting elements, which is a further step into Web map service. Compared with WMS, simply speaking, WMS is the basic, is to display the map, WFS is a bit more advanced, on the basis of WMS can be implemented to add, delete, change, search and other operations on the map data.
WCS
Network Coverage Service (WCS). The main function of this service specification is to publish and share image data containing spatial location on the network. Introduction to WMS, WFS, WCS, and WMTS services
Map world
Map Heaven and Earth is the “national geographic information public service platform” built by the National Bureau of Surveying, Mapping and Geographic Information. As the official geographic information service website in China. At present, it is the only public service platform with standard coordinate system and authoritative and credible data in China. The main service standards are issued by OGC standard.
The layer
You can have multiple layers in a service. We can think of a service as a database, where each layer is a table in the database and the elements are the data in the table. As shown below:
project
We often combine one or more related layers to form projects based on actual business scenarios. For example, you can group image 2018, Image 2019, and Image 2020 into the image project.