Server/client communication from map data

[LouisDuVerdier]

Given the following terms:

• pixmap : a square PNG image whose size never changes (e.g., 100x100px)
• latitude : latitude in decimal form (not hour/min/sec) of the concerned resource's center
• longitude : longitude in decimal form (not hour/min/sec) of the concerned resource's center
• scale : ratio in degrees per pixel indicating the zoom level (i.e., doing pixmap.width * scale allows to get the real longitude of the image)

Request from the client :

Request GET HTTP : http://adress.server.com/service?service=map&latitude=48.8534100&longitude=2.3488000&width=800&height=800&scale=0.01875

This request asks for all tiles in a range of 800x800px at a scale of 0.01875 (that means 1px = 0.01875 degree, and so that 800px = 15 degrees), with 48.8534100 for the latitude and 2.3488000 for the longitude of the center (Paris' coordinates). It is expected to get a given number of n tiles, depending on the fixed size of a tile (64 tiles if a tile's size is 100x100px, for this request).

JSON response from the server :

{
    "tiles": [
        {
            "latitude": 48.8534100, // latitude of the tile's center
            "longitude": 2.3488000, // longitude of the tile's center
            "scale": 0.01875, // tile's scale (should be the same as the scale given in the client's request)
            "pixmap": "http://adress.server.com/images/00011283840.png" // static URI of the image
        },
        {
            "latitude": 48.8534100,
            "longitude": 2.3488000,
            "scale": 0.01875,
            "pixmap": "http://adress.server.com/images/00011283841.png"
        },
        {
            "latitude": 48.8534100,
            "longitude": 2.3488000,
            "scale": 0.01875,
            "pixmap": "http://adress.server.com/images/00011283842.png"
        },
        {
            "latitude": 48.8534100,
            "longitude": 2.3488000,
            "scale": 0.01875,
            "pixmap": "http://adress.server.com/images/00011283840.png"
        }
    ]
}

Is this proposal realizable from the server point of view?

[kit-ty-kate]

I globally agree with that. Just two point I want to ask and comment:

• Why repeating the scale for every tile ? I guess it should alway be the same one for the whole request (the one you've asked for).
• Do you think 100x100 px is enough for a tile ? Because it will have a huge difference between a phone with low screen resolution and a big desktop screen. Maybe a dynamic tile size would be a good idea.

[LouisDuVerdier]

OK for the first point (it's just the data organization, so no problem since all data remain here). For the second point, I've seen OSM uses some 256x256px tiles in its view (http://tile.openstreetmap.fr/) so we should probably do the same to simplify our work. A dynamic size would be a bit more complicated to implement, so I would prefer a fixed size. But if you want, we can add a tileSize arg to the GET request.

[kit-ty-kate]

I'm ok for the fixed size tile then.

[LouisDuVerdier]

I have encountered a practical issue about all this: the world is spherical, it's not a rectangle, so the x-scale is not the same as the y-scale (cf http://wiki.openstreetmap.org/wiki/Slippy_map_tilenames#X_and_Y). In this article, it is said that there is a few zoom levels. Would it be better to use them? How does it work at your side?

[LouisDuVerdier]

OK, other try: because of the OSM tiling system (url/zoom/x/y.png), we should use the same system to simplify the work. A GET HTTP request would then be: http://adress.server.com/service?service=map&zoom=7&x=10&y=5.

The JSON answer:

{
    "x": 10, // the tile's x coord, according to http://wiki.openstreetmap.org/wiki/Slippy_map_tilenames#Derivation_of_tile_names
    "y": 5, // the tile's y coord, according to http://wiki.openstreetmap.org/wiki/Slippy_map_tilenames#Derivation_of_tile_names
    "zoom": 7, // the zoom level, according to http://wiki.openstreetmap.org/wiki/Slippy_map_tilenames#Zoom_levels
    "pixmap": "http://adress.server.com/images/00011283840.png" // static URI of the image
}

The client would then make the required number of calls, according to its needs. It looks to be also the way GoogleMaps works, and it makes possible for each tile to know easily the exact size and coordinates.

Is that ok for you? I think it is the easiest for you.

[kit-ty-kate]

Sorry for the delay I had a terrible week-end and I couldn't setup the osm database. So for the zoom level I think it's the same as in osm. But for the coordinates, I can't see precisely what the values reflects. It seems that it begins with the full coordinates (lat. and long.) and deals with offsets after but I'm not sure.

[LouisDuVerdier]

What I understand is that for each zoom level, you will have a set of images representing the world, e.g. 4096x4096 for the zoom level 12. Each tile will then represent 0.0879° x 0.0415° of long/lat respectively, and the maximum x value will be 4096. A longitude of 42° should be the same as an x value of 479 (well, not really 479 because we have to apply the formula of http://wiki.openstreetmap.org/wiki/Slippy_map_tilenames#Derivation_of_tile_names to get the exact value, but you know what I mean).

[LouisDuVerdier]

Resolved.