Offer packages of map tiles for offline use

问题

I'm making a web application that has to work offline. So far everything works and my last step is to take the map tiles offline. Luckily I know exactly what areas of the map will need to be accessible to users, so I don't have to allow caching of millions of tiles.

The map is split into areas and so the idea is to offer the tiles for these areas as downloadable 'packages.'

For instance, when I'm online, I go to the 'tile packages' page, which offers downloads for several areas. I choose the area which I'm interested in, it downloads the tiles, and when I go offline, I'm able to use these tiles. I only need about 2 zoom levels, one far out for quick navigation, and one more up close for more detail.

I'm using leaflet to serve up the map. Has anyone had to do something like this and could give me some guidance? I really just don't know how to even approach this, and it's the last piece of the puzzle.

回答1:

Sadly you don't point out, what the exact problem is or at which step you fail. So I will try to give a general answer:

Leaflet uses Tiles by different providers to for a slippymap using JS. The map tiles (aka rasterimages) can be offered via an Tile Map Service (TMS) or an slightly different method (for OSM the numbering here described).
So you can create a list of images you want to get and can transfer them by respeciting legal and tecnical terms. For OSM this is for example:

• http://wiki.openstreetmap.org/wiki/Legal_FAQ
• https://wiki.openstreetmap.org/wiki/Tile_usage_policy

So you need to create an server/client script, that is able to do such a bulk transfer (maybe as packed archive file?) and ask to place it at a certain place for your user. I'm not experienced enough in Leaflet and can't tell you how to provide them, beside you might add them to the browsers cache itself, or to use a local server to provide them as localhost.

Anyway, if you have more questions, just ask.

回答2:

So here's what I came up with. I import an area of the map to my database. I then offer this section as a downloadable package. When the user downloads the package, the database is queried and returns all tiles associated with that area in JSON format. The images are stored as blobs. I then pass this array of tiles to a custom leaflet layer which parses the data. Here's the code for the layer:

define([], function() {
    L.TileLayer.IDBTiles = L.TileLayer.extend({
        initialize: function(url, options, tiles) {
            options = L.setOptions(this, options);

            // detecting retina displays, adjusting tileSize and zoom levels
            if (options.detectRetina && L.Browser.retina && options.maxZoom > 0) {

                options.tileSize = Math.floor(options.tileSize / 2);
                options.zoomOffset++;

                if (options.minZoom > 0) {
                    options.minZoom--;
                }
                this.options.maxZoom--;
            }

            this._url = url;

            var subdomains = this.options.subdomains;

            if (typeof subdomains === 'string') {
                this.options.subdomains = subdomains.split('');
            }

            this.tiles = tiles;
        },
        getTileUrl: function (tilePoint) {
            this._adjustTilePoint(tilePoint);

            var z = this._getZoomForUrl();
            var x = tilePoint.x;
            var y = tilePoint.y;

            var result = this.tiles.filter(function(row) {
                return (row.value.tile_column === x
                        && row.value.tile_row === y
                        && row.value.zoom_level === z);
            });

            if(result[0]) return result[0].value.tile_data;
            else return;
        }
    });
});

回答3:

I think you can use a quadtree,i.e. space filling curve. MS Bing Map uses the most simple tile map: http://bcdcspatial.blogspot.de/2012/01/onlineoffline-mapping-map-tiles-and.html?m=1. I think the other maps server also uses a space filling curve, buf it's not so obvious. You may search for ms bings maps quadkey or nick's spatial index hilbert curve. You can also download my php class hilbert curve @ phpclasses.org. You can use it with many different space filling curves and to generate a quadkey. A good start is also the hacker's cookbook. There is a whole chapter dedicated to the hilbert curve.

来源：https://stackoverflow.com/questions/20228241/offer-packages-of-map-tiles-for-offline-use