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.
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）：
01 MAP
02 IMAGETYPE "PNG"
03 EXTENT -180 -90 180 90
04 SIZE 600 300
05 SHAPEPATH "/gdata"
06 IMAGECOLOR 255 255 255
07 FONTSET "../fonts/fonts.list"
08 LAYER
09 NAME "states_poly"
10 DATA "wcountry.shp"
11 STATUS OFF
12 TYPE POLYGON
13 LABELITEM "NAME"
14 CLASS
15 NAME "States"
16 STYLE
17 COLOR 232 232 232
18 END
19 END
20 END
21 LAYER
22 NAME "states_line"
23 DATA "wcountry.shp"
24 STATUS OFF
25 TYPE LINE
26 CLASS
27 NAME "State Boundary"
28 STYLE
29 COLOR 132 132 32
30 END
31 END
32 END
33 LAYER
34 NAME "wroads"
35 DATA "wroads.shp"
36 STATUS OFF
37 TYPE LINE
38 CLASS
39 NAME "World Road"
40 STYLE
41 COLOR 100 200 100
42 END
43 END
44 END
45 LAYER
46 NAME "wriver"
47 DATA "wriver.shp"
48 STATUS OFF
49 TYPE LINE
50 CLASS
51 NAME "World River"
52 STYLE
53 COLOR 0 0 255
54 END
55 END
56 END
57 END
The roads layer map ( wroads ) is also defined in the Mapfile above and this layer is further added to the map below.
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:
01 MAP
02 IMAGETYPE "PNG"
03 EXTENT -180 -90 180 90
04 SIZE 600 300
05 SHAPEPATH "/gdata"
06 IMAGECOLOR 255 255 255
07 FONTSET "../fonts/fonts.list"
08 LAYER
09 NAME "states_poly"
10 DATA "wcountry.shp"
11 STATUS OFF
12 TYPE POLYGON
13 LABELITEM "NAME"
14 CLASS
15 NAME "States"
16 STYLE
17 COLOR 232 232 232
18 END
19 END
20 END
21 LAYER
22 NAME "states_line"
23 DATA "wcountry.shp"
24 STATUS OFF
25 TYPE LINE
26 CLASS
27 NAME "State Boundary"
28 STYLE
29 COLOR 132 132 32
30 END
31 END
32 END
33 LAYER
34 NAME "wriver"
35 DATA "wriver.shp"
36 STATUS OFF
37 TYPE LINE
38 CLASS
39 NAME "World River"
40 STYLE
41 COLOR 0 0 255
42 END
43 END
44 END
45 LAYER
46 NAME "wroads"
47 DATA "wroads.shp"
48 STATUS OFF
49 TYPE LINE
50 CLASS
51 NAME "World Road"
52 STYLE
53 COLOR 100 200 100
54 END
55 END
56 END
57 END
You can see that although it is not obvious, the river layer is below the road layer.