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5f93d46 Jan 21, 2015
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API

PolarMap.js contains two parts: the lower-level Leaflet.PolarMap plugin and the basic PolarMap library.

This document will explain the capabilities of the Leaflet.PolarMap and PolarMap APIs.

PolarMap

Initializing a map with the basic PolarMap library is simple. Once the document is loaded (using JQuery or body onload), initialize PolarMap with the container element's ID.

    <!-- index.html -->
    <div id='mapContainer'></div>
    // app.js
    var map = PolarMap('mapContainer');

That is all. PolarMap will create a Leaflet map in the element, with the following features enabled:

  • Support for Arctic Connect projections EPSG:3571–3576 in a layer switching control
  • Controls for rotating the maps through available projections
  • Support for permalinks in the browser URL location

Additionally, the PolarMap function can accept an options object.

PolarMap Options

  • key: geosearch
    • value: true or false
    • default value: false
    • Adds a map control for geospatial location search. Requires the L.GeoSearch plugin to be included in the page along with a provider before it will work.
  • key: locate
    • value: true or false
    • default value: false
    • After the map is loaded, the user location will be automatically detected using the W3C GeoLocation API. PolarMap will then automatically switch to the projection closest to the user's longitude.
  • key: permalink
    • value: true or false
    • default value: true
    • Will save the user's current projection, latitude and longitude, and zoom level in the page's URL. Will automatically update on map pan/zoom or projection change.

These options can only be changed when the PolarMap function is first called.

Aside from specifying custom initialization options, the returned object (map) exposes methods for map customization.

PolarMap Methods

  • method: addLayer
    • arguments: Leaflet LayerGroup, Options object
    • Options object
      • key: name
        • value: Name for layer, if added to layer switch control.
      • key: switcher
        • value: true or false
        • default value: false
        • If true, will display the layer in the PolarMap Layer switching map control.
  • method: getBaseLayer
    • arguments: none
    • description: Returns the base tile layer currently used on the map

Leaflet.PolarMap

The base classes in Leaflet.PolarMap:

  • L.PolarMap.Map
  • L.PolarMap.TileLayer
  • L.PolarMap.LAEATileLayer
  • L.PolarMap.Control.Rotation
  • L.PolarMap.Util.Hash

L.PolarMap.Map

Very similar to L.Map, with a few differences. Instead of initializing with an array for layers, pass an L.PolarMap.TileLayer to baseLayer.

    var map = L.PolarMap.map('map_container', {
        baseLayer: L.PolarMap.tileLayer(…)
    });

L.PolarMap.Map also supports the other options for L.Map, aside from crs and layers.

L.PolarMap.Map.loadTileProjection()

This method can be used to dynamically update the map with a new tile layer, even if the tile layer has a different projection.

    map.loadTileProjection(L.PolarMap.tileLayer(…))

The object loaded in must be at least an L.TileLayer object. It is recommended it also defines the options set in L.PolarMap.TileLayer and L.PolarMap.LAEATileLayer.

L.PolarMap.TileLayer

A subclass of L.TileLayer. Should be initialized with additional options to define the tile layer's CRS and custom parameters. Be sure to define the projection in Proj4JS first.

    // See L.TileLayer documentation for URL template
    var url = "http://{s}.tiles.arcticconnect.org/osm_3571/{z}/{x}/{y}.png";

    // Extent is dependent on projection. The following is used for EPSG:3571 to
    // EPSG:3576.
    var extent = 11000000 + 9036842.762 + 667;

    var tileLayer = L.PolarMap.tileLayer(url, {
        // Name is required for storing the layer id in the permalink if the
        // PolarMap permalink module is active.
        name: "ac_3571",

        // CRS for the tile layer.
        crs: "EPSG:3571",

        // Minimum/maximum zoom level for the tile provider.
        minZoom: 0,
        maxZoom: 18,

        // Is the tile layer provided by a TMS? FYI: OSM is not a TMS.
        tms: false,

        // The tile origin for the projection. LAEA is based on the extent, may
        // vary for other projection systems.
        origin: [-extent, extent],

        // The tile resolution at the lowest zoom level. May vary for other
        // projection systems.
        maxResolution: ((extent - -extent) / 256),

        // The bounds for the tile layer projection. Will cause Leaflet to avoid
        // loading tiles outside these bounds. May vary for other projection
        // systems.
        projectedBounds: L.bounds(L.point(-extent, extent),L.point(extent, -extent)),

        // Whether the World map should be drawn continuously. May not make
        // sense to use for certain projections.
        continuousWorld: false,

        // Whether to wrap tiles. May not make sense to use for certain 
        // projections.
        noWrap: true,

        // Attribution for the tile layer. This is required for certain tile
        // providers.
        attribution: 'Map &copy; <a href="http://arcticconnect.org">ArcticConnect</a>. Data &copy; <a href="http://osm.org/copyright">OpenStreetMap</a> contributors',

        // If the tile layer is part of a set of projections that form a 
        // complete circle, then prev/next can link to the next CCW/CW layer in
        // the series when using the L.PolarMap.Control.Rotation module.
        // Can be omitted if the module is not in use.
        prev: L.PolarMap.tileLayer(…),
        next: L.PolarMap.tileLayer(…)
    });

The class does not enforce any defaults required for L.PolarMap.Map, but may in the future.

L.PolarMap.LAEATileLayer

A subclass of L.PolarMap.TileLayer. Has presets for an LAEA projection already set in the options. You will be required to initialize with the URL string and include name, crs, and optionally proj4def in the options hash.

A set of LAEA Tile Layer objects have been preset with Arctic Connect tile layers for LAEA projections 3571–3576.

  • L.PolarMap.layer3571
  • L.PolarMap.layer3572
  • L.PolarMap.layer3573
  • L.PolarMap.layer3574
  • L.PolarMap.layer3575
  • L.PolarMap.layer3576

They do not need to be called as a function as they are already initialized.

L.PolarMap.Control.Rotation

A map control that adds two buttons: one to rotate the map clockwise, the other to rotate counter-clockwise. Requires custom glue code to determine which tile layer to switch to on button press.

    var tiles = {
        "id_1": L.PolarMap.tileLayer(…),
        "id_2": L.PolarMap.tileLayer(…)
    };

    var map = L.PolarMap.map('map_container', {
        baseLayer: tiles["id_1"]
    });

    var getBaseLayer: function () {
        var foundLayer = null;

        for (var layerName in tiles) {
            if (tiles.hasOwnProperty(layerName)) {
                if (map.hasLayer(tiles[layerName])) {
                    foundLayer = tiles[layerName];
                }
            }
        }
        return foundLayer;
    };

    var rotationControls = L.PolarMap.Control.rotation({
        onRotateCW: function() {
            map.loadTileProjection(getBaseLayer().next);
        },

        onRotateCCW: function() {
            map.loadTileProjection(getBaseLayer().prev);
        }
    });

    rotationControls.addTo(map);

Assuming a map has been created using a Tile Layer with prev and next defined, then rotation controls can be used to switch between the tile layers. Note that a way to find the tile layers is required (see the getBaseLayer function) that can specifically get the base layer from the L.PolarMap.Map object; the map object does not provide such a function at this time.

L.PolarMap.Util.Hash

Very similar to the Leaflet Hash plugin, it works identically except provides support for setting the base layer in addition to the latitude/longitude/zoom level. To do this, the getBaseLayer and setBaseLayer options must be set in the constructor.

    var getBaseLayer: function () {
        var foundLayer = null;

        for (var layerName in tiles) {
            if (tiles.hasOwnProperty(layerName)) {
                if (map.hasLayer(tiles[layerName])) {
                    foundLayer = tiles[layerName];
                }
            }
        }
        return foundLayer;
    };

    var setBaseLayer: function (name) {
        for (var layerName in tiles) {
            if (tiles.hasOwnProperty(layerName)) {
                if (tiles[layerName].options.name === name) {
                    map.loadTileProjection(tiles[layerName]);
                }
            }
        }
    };

    L.PolarMap.Util.hash(map, {
        getBaseLayer: function () {
            return getBaseLayer().options.name;
        },

        setBaseLayer: function (name) {
            setBaseLayer(name);
        }
    });

Note that glue code for determining the base layer is required.

Once instantiated, the permalink code is activated and will track the map pans and zooms. When loading a URL with the projection, latitude, longitude, and zoom level in the params then these values will override any defaults set for the map object.