topojson-simplify | simplify geometries while preserving topology

When I first read this I attempted to address the underlying problem in the comments (for which I missed the mark a bit, as I assumed the code to display the map looked a little different than it likely was - I apologize for the misread). However, on second examination, I realized that while the question is an x,y problem, the attempted solution leads to some interesting questions about D3 and topojson that aren't immediately clear - but the underlying issue remains mixing projected and unprojected data, so I'll attempt to address both the x and the y here:

Topojson

Topojson essentially is a method of storing geojson features or feature classes by encoding topology and, optionally, quantized delta encoded integer coordinates. Topojson itself doesn't change the underlying coordinates, it changes their representation.

In order to use topojson with D3, we need to convert it back to geojson, as D3 geoPaths only accept geojson - topojson.feature() returns geojson.

The Underlying Source of Pain

The ultimate issue doesn't arise from the topojson at all. You have two topojon sources that use different coordinate systems:

• the US json uses two dimensional Cartesian coordinate space (features projected with an Albers projection). The uncompressed coordinates are SVG pixels.

• the Nielsen json uses latitude/longitude coordinate space (points on a 3d globe). The uncompressed coordinates are not Cartesian (unless applying a straight Plate Carree projection: long = x, lat = y).

When the US json is passed through topojson.feature(), the returned geojson will have x,y values within a bounding box of roughly [0,0],[960,600] (in this case). When any unprojected topojson (Nielsen json for exmaple) is passed through topojson.feature(), the returned geojson will have long,lat values within a bounding box of [-180,90],[180,-90] - that is it will have valid long/lat pairs.

These underlying and differing coordinate systems are the issue, not the representation of the coordinates in the topojson.

Combining Topojson

I'm not sure on the details of how you combined the two. A topojson splits geojson into constituent parts - arcs and features (which reference the arcs and hold the feature properties). The arcs are referenced numerically - if you didn't modify the indexes, then you can't just use: