How to make interactive OpenStreetMap layers in TileMill?

Question

If I'm reading the UTFGrid spec right, you can only have 65,501 different interactive features. Or more precisely, every feature needs a unique ID in the range 0-65501.

TileMill uses UTFGrid for its interactivity, so this limit seems to apply. It shows up as non-parseable JSON being generated, JSON handling exceptions etc.

So, what ID can you use for OpenStreetMap features (loaded into a PostGIS database), given that the osm_id column will have values in the hundreds of millions.

Answer 1

If you want a unique id per row it will be easiest to use the row number.

 SELECT (row_number() OVER())-1 AS id, way, (...)

That way you'll get incrementing id starting from 0 and won't have to worry about collision.

Answer 2

I found a simple workaround that seems to work ok. Make a PostGIS layer with an extra column like this:

(SELECT way,....
osm_id % 65500 AS tinyid
FROM planet_osm_point) p

Then set tinyid as the layer's "Unique key field".

That is, take the actual osm_id modulo 65500 to always produce a value in the range 0-65500. This will occasionally cause errors if two features within the same tile have the same tinyid.

(There might be better ways?)