URL placeholders are tile indices? How to reference location instead?

[stodge]

The documentation on URL placeholders is minimal, but I assume X,Y are the indices of the tile and Z is the zoom factor. I'm writing my own small custom server as I have my own dataset. My server (Django) doesn't have the concept of tiles and my dataset is not in lat/long coordinate space. How can I turn the X/Y/Z tuple into basic coordinates so I can respond wiht only the required data? Thanks

[RandomEtc]

http://polymaps.org/docs/url.html is indeed a bit minimal :) I've tagged this issue as doc so that we'll remember to clean that up at some point.

Meanwhile, you might try using a {B} parameter in your url template. This will make a tile request using a north,west,south,east latitude and longitude bounding box instead of tile indices.

If your data isn't in latitude and longitude then you need to invent a tile scheme for it. Typically, zoom level 0 is a single 256px tile and contains all your data. Zoom level 1 is 2x2 tiles, so you'd scale your data to fit a 512px tile and then move it depending on the tile indices. A full tutorial for this isn't really in the scope of a github issue thread though, nor a coffee break support session :)

[shawnbot]

+1 what Tom said. The URL format is "{Z}/{Y}/{X}", where Y and X are WGS84 latitude and longitude, respectively. If your dataset isn't in WGS84 you might consider reprojecting it either once permanently or at runtime. GeoDjango does a lot of this, and can build queries that intersect geometries in your database with the bounding boxes of individual tiles. TileStache also has a built-in GeoJSON provider, but that requires that you back onto Postgresql, so it may not work for you.

[stodge]

Thanks. I am using Postgresql/postgis.

The {B} helped, though the svg generated by polymaps is invalid:

Error: Problem parsing d="M-10105303.040000001,NaNL-9822770.062222224 etc

Likely I'm returning dodgy data.

Thanks again.

[RandomEtc]

Are you using GeoJSON? GeoJSON coordinates must be in latitude and longitude degrees to be understood by Polymaps. The NaN will happen with latitude approaching (or over) +/- 90º I think.

You might just have lat/lon mixed up? A GeoJSON coordinate should be [ lon, lat ] (think x,y), even though people commonly say 'latitude and longitude' just to confuse things.

[stodge]

Right I remember reading that. Yes I am using geojson, though I'm using a local coordinate system, not lat/long. My coordinates range from approx -5000,-5000 to 5000,5000. So geojson won't work.

[mbostock]

You don't need to use po.url; that's just the default implementation of the URL template. You can pass in any function that takes a coordinate object (row, column, zoom) and returns a string. There's a bit more details here:

http://polymaps.org/docs/image.html#url