2.4. Publish a vector layer

This section begins with using vector data in MapServer. Vector data is similar to vector data in CAD software or illustrator. In addition to the difference of geospatial location, another important aspect is that vector data in GIS has no style information. CAD or illustrator mainly focuses on drawing, and the vector data used need to be distinguished and beautified by width, color and style. In GIS, these works are completed by the map drawing stage. The same data can be drawn in different thematic maps and different application scenarios.

2.4.1. Define vector layers in Mapfile

Mapserver can create an image and save it to a local directory, or deliver it directly to a web browser on demand (as in this example). Note the above map, it is generated by the WebGIS server in the server-side background, and then transmitted to the client. The code behind in the page is:

Instead of browsing it on the HTML page, you can just enter this URL:

http://webgis.pub/cgi-bin/mapserv?map=/owg/mfa1.map&layer=world-country&mode=map

(Remember to replace the hostname or IP address; for example, use the local hostname localhost, or use the IP address directly, such as "127.0.0.1")

MAP
    IMAGETYPE "PNG"
    EXTENT -180 -90 180 90
    SIZE 600 300
    SHAPEPATH "/gdata"
    IMAGECOLOR 255 255 255
    LAYER
        NAME "world-country"
        DATA "wcountry.shp"
        STATUS OFF
        TYPE POLYGON
        CLASS
            NAME "World Countries"
            STYLE
                COLOR 232 232 232
                OUTLINECOLOR 32 32 32
            END
        END
    END
END

Take a look at this Mapfile below. Lines 07 to 19 define the first layer of the map.

The layer starts with LAYER and ends with END. Based on the spatial data of wcountry.shp1, the polygon layer renders the global administrative division data. The NAME keyword specifies the name of the layer. The name itself is optional, but if you use one, it must be no more than 20 characters. The layer name is used in the HTML template layer as a CGI reference. A name must be specified if you want the interactive layer to be able to be toggled on and off from the HTML form.

The STATUS keyword determines whether the layer is rendered, and whether its state can be changed. STATUS the default layer is always rendered and can change state at the same time.

Lines 12 to 18 specify the only class object parameter at this level. A class object is started with the keyword class and terminated by the keyword END (line 18). While this application will use classes more extensively than previous maps, with this layer, you only need to specify a single default class that will include every feature in the shapefile. The name of the class will appear on the legend associated with the map. If a class has no name, it will still be rendered, but it will not appear in the legend. The style object keyword COLOR specifies the color to paint, and OUTLINECOLOR specifies the color to paint the border. Since this layer is a polygon, it will be filled with the specified color. Note that if the layer is a line layer, the COLOR value will specify the color of the line.

2.4.2. URL parameter description

The URL can be broken down into three parts:

The first part, http://webgis.pub/cgi-bin/mapserv?, calls MapServer’s CGI program. On a different system, it might be mapserv or mapserv.exe , or something else. If you directly call to open the above URL, you will get this message:

No query information to decode. QUERY_STRING is set, but empty.

To get the correct information, you need to add additional parameters.

To get the correct information, additional parameters need to be added. The next three parts are the query string. The query string contains CGI parameters (variables and their values), with each parameter separated by a symbol (&).

Now looking at the query string, the first parameter map has a value: /owg/mfa1.map

The next parameter, layer=states, tells mapserv to use the states layer. We named the layer object states in the Mapfile.

Next parameter layer=states Tell mapserv to use the states Layer. We name it layer objects in Mapfile states .

The last parameter, mode=map , tells mapserv what to do to output MapFile in mode. In this case, it tells mapserv to dump the image directly to the web browser (client), without first creating a temporary image on the server.

The values for TYPE can be: 'chart', 'circle', 'line', 'point', 'polygon', 'raster', 'query', 'annotation'.

2.4.3. Map extent

In the Mapfile, the correct range needs to be set in order to display the data. For example, for China-wide data, the set range is: EXTENT 73 8 136 53, the effect is as follows:

The complete Mapfile is:

MAP
    IMAGETYPE "PNG"
    EXTENT -12 25 43 50
    SIZE 550 250
    SHAPEPATH "/gdata"
    IMAGECOLOR 255 255 255
    LAYER
        NAME "topo"
        DATA "land_shallow_topo_8192.tif"
        STATUS OFF
        TYPE RASTER
    END
END

2.4.4. Different modes viewed by MapServer

Note the mode=map above, which is the viewing mode supported by MapServer. MapServer’s mode CGI variable can take values other than map. In addition to the map mode, there is also the browse mode. For example, if you use map=browse, MapServer will dump the image to a temporary directory on the server.

Using browse mode requires specifying the template parameter. Generally, a template file needs to be defined first. Without a template file, browse mode cannot work properly, which will be explained later. In MapServer 6.x, the template parameter allows the use of the OpenLayers keyword, which can be used to view MapServer results.

The following directly uses the default configuration of OpenLayers to view the map. Note that the OpenLayers library of the MapServer website needs to be called, so it will be slower for the first time.

http://webgis.pub/cgi-bin/mapserv?map=/owg/mfa1.map&layer=states&mode=browse&template=OpenLayers

For OpenLayers mode, see:

https://mapserver.org/cgi/openlayers.html

You can try modifying the value of the keyword in MapFile and see the result. These experiments will help you understand these keywords.