2.6. Build a map with multiple layers

MapServer builds maps by stacking layers together. On every render, it is placed at the top of the stack. Each layer displays features selected from a single dataset. Features to display can be selected using Unix regular expressions, string comparisons and logical expressions. You can think of layers as topics due to the similarity of data and the similarity of style parameters such as scale, color, and labels. The display of layers is under interactive control, allowing the user to select which layers to render. While it is not possible to generate layers dynamically, it is possible to populate empty layers with dynamic data and manipulate them via URL.

2.6.1. View examples

To make the map richer, continue adding new data (river data) using a new LAYER object. The definitions of each layer are relatively independent and have nothing to do with each other.

Here is the map file（mfml2.map）：

MAP
    IMAGETYPE "PNG"
    EXTENT -180 -90 180 90
    SIZE 600 300
    SHAPEPATH "/gdata"
    IMAGECOLOR 255 255 255
    FONTSET "../fonts/fonts.list"
    LAYER
        NAME "states_poly"
        DATA "wcountry.shp"
        STATUS OFF
        TYPE POLYGON
        LABELITEM "NAME"
        CLASS
            NAME "States"
            STYLE
                COLOR 232 232 232
            END
        END
    END
    LAYER
        NAME "states_line"
        DATA "wcountry.shp"
        STATUS OFF
        TYPE LINE
        CLASS
            NAME "State Boundary"
            STYLE
                COLOR 132 132 32
            END
        END
    END
    LAYER
        NAME "wroads"
        DATA "wroads.shp"
        STATUS OFF
        TYPE LINE
        CLASS
            NAME "World Road"
            STYLE
                COLOR 100 200 100
            END
        END
    END
    LAYER
        NAME "wriver"
        DATA "wriver.shp"
        STATUS OFF
        TYPE LINE
        CLASS
            NAME "World River"
            STYLE
                COLOR 0 0 255
            END
        END
    END
END

The roads layer map ( wroads ) is also defined in the Mapfile above and this layer is further added to the map below.

2.6.2. Order of layers

Pay attention to the order of the layers. In this map, the river layer is above the road, which is inconsistent with the usual mapping principles. Usually, the road layer is placed above the river layer.

Swap the position when passing parameters, the effect is as follows, you can see that it is the same as the map above. The order of the layers defined in the Mapfile cannot be changed by passing parameters to the URL.

If you want to modify the order of layers, you can only modify it in the Mapfile:

MAP
    IMAGETYPE "PNG"
    EXTENT -180 -90 180 90
    SIZE 600 300
    SHAPEPATH "/gdata"
    IMAGECOLOR 255 255 255
    FONTSET "../fonts/fonts.list"
    LAYER
        NAME "states_poly"
        DATA "wcountry.shp"
        STATUS OFF
        TYPE POLYGON
        LABELITEM "NAME"
        CLASS
            NAME "States"
            STYLE
                COLOR 232 232 232
            END
        END
    END
    LAYER
        NAME "states_line"
        DATA "wcountry.shp"
        STATUS OFF
        TYPE LINE
        CLASS
            NAME "State Boundary"
            STYLE
                COLOR 132 132 32
            END
        END
    END
    LAYER
        NAME "wriver"
        DATA "wriver.shp"
        STATUS OFF
        TYPE LINE
        CLASS
            NAME "World River"
            STYLE
                COLOR 0 0 255
            END
        END
    END
    LAYER
        NAME "wroads"
        DATA "wroads.shp"
        STATUS OFF
        TYPE LINE
        CLASS
            NAME "World Road"
            STYLE
                COLOR 100 200 100
            END
        END
    END
END

You can see that although it is not obvious, the river layer is below the road layer.