Visible map is a toy that lets you
see all of the magic at work behind our maps and the way that we do interaction with
UTFGrid. It works
by spying on the workings of Wax and
Modest Maps while they do the dirty work
of building maps, downloading grids, and producing pretty tooltips. It’s stuff that
we work hard to make invisible, but this is all of that turned inside out so that you
can can see how it works.
UTFGrid is cool because you can throw lots of data on a map and it will still
be fast in your browser. The common approach of web maps is to simply limit the amount of data on the map, like
limiting each view to 100 points or simplifying shapes so that they aren’t huge.
We didn’t like making these concessions and didn’t want to ask everyone who
used TileMill to make them as well. You can throw
arbitrarily huge data sets at the UTFGrid spec, and while they may take a very long
time to process in TileMill, the number of features that we serve in each
tile of data is limited by pixel resolution. You’ll never get more than 4,096
features back, and once they’re downloaded, it’s all as fast as if there
were only one feature.
But this way of making things fast - rasterization -
means that Wax is doing quite a bit of magic. You aren’t hovering over things like
you’re used to - there are no DOM elements for features, no polygons. It’s an
elegant dupe. But, not too hard to unravel. Tinker around with
visible map to see for yourself.