topojson / D3 / Map with longitude + latitude

In a TopoJSON, those numbers in the coordinates are not the actual latitude/longitude values. They have to be transformed. This function transforms the quantized topology to absolute coordinates:

function transformPoint(topology, position) {    position = position.slice();    position[0] = position[0] * topology.transform.scale[0]     + topology.transform.translate[0],    position[1] = position[1] * topology.transform.scale[1]     + topology.transform.translate[1]    return position;};

You'll find the scale and translate at the end of the TopoJSON you linked:

"transform":    {"scale":        [0.000818229038834542,0.0005946917122888551],    "translate":[-6.418556211736409,49.8647494628352]    }

Based on that function, I believe it's easy to write a function that does the reverse:

function transformPointReversed(topology, position) {    position = position.slice();    position[0] = (position[0] - topology.transform.translate[0])    /(topology.transform.scale[0]),    position[1] = (position[1] - topology.transform.translate[1])    /(topology.transform.scale[1])     return position;};

I tried this function I just made and your London coordinates returned me this array:

[7688.309645789168, 2762.1059840278253]

Please, test it in your coordinates to see if it works.

An alternative is overlaying your TopoJSON with a GeoJSON, which does use an absolute coordinates system.

Here is the API reference: https://github.com/mbostock/topojson-specification/blob/master/README.md#22-geometry-objects