Multiple Map Layers

The "Roads" layer can only be defined when customising map settings. However, it is possible to overlay data from multiple sources on top of each other using a "Group" layer. This is useful where background data is stored in a Tile Service but additional layers are only available in a separate WMS service.

Group Layers are also likely to be needed when working with local vector or raster data as one would typically not want to show all layers at all zoom levels.

The group layer sets overall properties that are shared between layers within that group and also determines the zoom levels at which different layers are displayed. A typical configuration for a Group layer might look like this:

{
    "roads":{
        "type": "Group",
        "projection": "EPSG:27700",
        "projectionDef": "+proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +datum=OSGB36 +units=m +no_defs +nadgrids=ostn02 ",
        "attribution": "(c) Crown Copyright",
        "maxExtent": [290000,190000,510000,410000],
        "resolutions" : [
            33.866734, 
            16.933367, 
            8.466684,  
            4.233342,  
            2.11667,   
            1.058335,  
            0.5291675, 
            0.264584,   
            0.132292,  
            0.066146
        ],
        "layers": {
            "Overview" : {
                "minimumResolution": 2.11667,
                "order": 1,
                "type": "GeoPackageTile",
                "url": "raster.gpkg",
                "table": "overview"
            },
            "Place Labels" : {
                "maximumResolution": 0.264584,
                "order": 3,
                "type": "GeoPackageFeature",
                "url": "labels.gpkg",
                "table": "CartographicText",
                "column": "geometry",
                "style": {
                    "label": "${textstring}",
                    "labelAlign": "cm"
                }
            },
            "Features" : {
                "maximumResolution": 1.058335,
                "order": 2,
                "type": "GeoPackageFeature",
                "url": "areas.gpkg",
                "table": "TopographicArea",
                "column": "geometry",
                "style": {
                    "fillColor": "orange",
                    "fillOpacity": 0.5, 
                    "strokeOpacity": 0.2, 
                    "strokeWidth": 1
                },
                "styleBy": "featurecode",
                "styles": {
                    "10021": {"fillColor": "red"},
                    "10053": {"fillColor": "brown"},
                    "10172": {"fillColor": "grey"},
                    "10111": {"fillColor": "green"}
                },
                "info": "Feature Type: ${featurecode}"
            }
        }
    }
}