javascript - Scaling d3 v4 map to fit SVG (or at all)

Question

I am trying to make this map of the us scale smaller. Either to my SVG, or even manually.

This is my code in its simplest from:

function initializeMapDifferent(){
    var svg = d3.select("#map").append("svg")
        .attr("width", 1000)
        .attr("height", 500);



    d3.json("https://d3js.org/us-10m.v1.json", function (error, us){

        svg.append("g")
            .attr("class", "states")
            .selectAll("path")
            .data(topojson.feature(us, us.objects.states).features)
            .enter().append("path")
            .attr("fill", "gray")
            .attr("d", d3.geoPath());
    });
}

I have tried something like:

  var path = d3.geoPath()
  .projection(d3.geoConicConformal()
      .parallels([33, 45])
      .rotate([96, -39])
      .fitSize([width, height], conus));

but every time I add anything to my path variable I get NAN errors from the internal parts of D3. Thanks for any help!

Answer 1

Why the data doesn't project properly

The key issue is that your data is already projected. D3 geoProjections use data that is unprojected, or in lat long pairs. Data in the WGS84 datum. Essentially a d3 geoProjection takes spherical coordinates and translates them into planar cartesian x,y coordinates.

Your data does not conform to this - it is already planar. You can see most evidently because Alaska is not where it should be (unless someone changed the lat long pairs of Alaska, which is unlikely). Other signs and symptoms of already projected data may be a feature that covers the entire planet, and NaN errors.

That this is a composite projection makes it hard to unproject, but you can display already projected data in d3.js.

"Projecting" already projected data

Null Projection:

Most simply, you can define your projection as null:

var path = d3.geoPath(null);

This will take the x,y data from the geojson geometries and display it as x,y data. However, if your x,y coordinates exceed the width and height of your svg, the map will not be contained within your svg (as you found in your example with .attr("d", d3.geoPath());).

The particular file in this question is pre-projected to fit a 960x600 map, so this is ideal for a null projection - it was designed with the dimensions in mind. Its units are pixels and all coordinates fall within the desired dimensions. However, most projected geometries use coordinate systems with units such as meters, so that the bounding box of the feature's coordinates may be millions of units across. In these cases the null projection won't work - it'll convert a map unit value to a pixel value with no scaling.

With d3, A null projection is commonly used with geojson/topojson that is preprojected to fit a specified viewport using a d3 projection. See command line cartography for an example (the example uses unprojected source files - the same issues that arise from using a d3 projection on projected data apply in both browser and command line). The primary advantage of preprojecting a file for use with a null projection is performance.

geoIdentity

If all you need is to scale and center the features, you can use a geoIdentity. This is implements a geoTransform but with standard projection methods such as scale, translate, and most importantly - fitSize/fitExtent. So, we can set the projection to a geoIdentity:

var projection = d3.geoIdentity();

This currently does the same as the null projection used above, it takes x,y data from the geojson geometries and displays it as x,y data with no transform - treating each coordinate in the geojson as a pixel coordinate. But, we can apply fitSize to this (or fitExtent) which will automatically scale and translate the data into the specified bounding box:

var projection = d3.geoIdentity()
  .fitSize([width,height],geojsonObject);

or

var projection = d3.geoIdentity()
  .fitExtent([[left,top],[right,bottom]], geojsonObject);

Keep in mind, most projected data uses geographic conventions, y=0 is at the bottom, with y values increasing as one moves north. In svg/canvas coordinate space, y=0 is at the top, with y values increasing as one moves down. So, we will often need to flip the y axis:

var projection = d3.geoIdentity()
 .fitExtent([width,height],geojsonObject)
 .reflectY(true);

This particular dataset: https://d3js.org/us-10m.v1.json was projected with a d3 projection, so its y axis has already been flipped as d3 projections project to a svg or canvas coordinate space.

geoIdentity Demo

var width = 600;
var height = 300;

var svg = d3.select("body").append("svg")
 .attr("width", width)
 .attr("height", height);



d3.json("https://d3js.org/us-10m.v1.json", function (error, us){
  var featureCollection = topojson.feature(us, us.objects.states);
  
  var projection = d3.geoIdentity()
  .fitExtent([[50,50],[600-50,300-50]], featureCollection)

  var path = d3.geoPath().projection(projection)
  
  svg.append("g")
    .attr("class", "states")
    .selectAll("path")
    .data(featureCollection.features)
    .enter().append("path")
    .attr("fill", "gray")
    .attr("d", path);
  
});

<script src="https://cdnjs.cloudflare.com/ajax/libs/d3/4.6.0/d3.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/topojson/2.2.0/topojson.js"></script>