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    <p>Spatial data is specified in a given coordinate system. This coordinate system is used to project coordinates to a particular point in space, such as a map canvas. There are literally hundreds of different coordinate systems for geographical data. One of the reasons for this is that the earth is not well-behaved with respect to projections -- it's not a perfect sphere and different coordinate systems tend to work best in different places ("work best" refers to how accurate they are once you project a point onto the earth's surface). You can find more information <a href="https://en.wikipedia.org/wiki/Map_projection">on wikipedia</a>.</p> <p>Defining a coordinate system and projection is not necessarily difficult or much work, but it needs to be done before it can be used. This is the reason you weren't able to use your data out of the box -- it uses a coordinate system that D3 doesn't know about (British National Grid). So your options are to convert it into <a href="https://github.com/mbostock/d3/wiki/Geo-Projections">one that D3 does know about</a>, or implement your coordinate system in D3. The former is almost always the easier way to go.</p> <p>The command <code>ogr2ogr output.shp input.shp -t_srs "+proj=longlat +ellps=WGS84 +no_defs +towgs84=0,0,0"</code> converts your input shapefile (a file format for geographical data) <code>input.shp</code> into Universal Transverse Mercator (UTM) WGS84 format (this is what the <code>t_srs</code> string means, more <a href="http://en.wikipedia.org/wiki/World_Geodetic_System">on wikipedia</a>) in <code>output.shp</code>, regardless of the coordinate system used in the input.</p> <p>The target coordinate system argument given to <code>t_srs</code> can be an explicit definition (like in the example above), a reference to a well-known coordinate system or a file that contains the definition. A list of well-known coordinate systems can be found <a href="http://spatialreference.org/ref/epsg/">here</a> for example.</p>
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