# Creating a map The topics that we will cover in this chapter include: 1. Creating a map 1. Define GeoJSON 1. Use a projection 1. Generate a `` using a projection and the GeoJSON data In this section we'll generate `` elements from GeoJSON data that will draw a map of the world ## Define GeoJSON GeoJSON is just JSON data that has specific properties that are assigned specific data types. Here's an example: ```javascript { "type": "Feature", "geometry": { "type": "Point", "coordinates": [125.6, 10.1] }, "properties": { "name": "Dinagat Islands" } } ``` In this example, we have one `Feature` who's geometry is a `Point` with the coordinates `[125.6, 10.1]`. It has "Dinagat Islands" as its name. Each `Feature` follows this general structure: ```javascript { "type": STRING, "geometry": { "type": STRING, "coordinates": ARRAY }, "properties": OBJECT } ``` We can also have a `Feature Collection` which is many `Features` grouped together in a `features` array: ```javascript { "type": "FeatureCollection", "features": [ { "type": "Feature", "geometry": { "type": "Point", "coordinates": [102.0, 0.5] }, "properties": { "prop0": "value0" } }, { "type": "Feature", "geometry": { "type": "LineString", "coordinates": [ [102.0, 0.0], [103.0, 1.0], [104.0, 0.0], [105.0, 1.0] ] }, "properties": { "prop0": "value0", "prop1": 0.0 } }, { "type": "Feature", "geometry": { "type": "Polygon", "coordinates": [ [ [100.0, 0.0], [101.0, 0.0], [101.0, 1.0], [100.0, 1.0], [100.0, 0.0] ] ] }, "properties": { "prop0": "value0", "prop1": { "this": "that" } } } ] } ``` This basically follows the form: ```javascript { "type": "FeatureCollection", "features": ARRAY } ``` The `features` property is an array of `feature` objects which we've defined previously. ### Set up the HTML Let's set up a basic D3 page: ```html ``` The only thing different from the setup that we've used in previous chapters is this line: ```html ``` This just loads an external javascript file which sets our GeoJSON data to a variable. Here's what the beginning of it looks like: ```javascript var map_json = { type: "FeatureCollection", features: [ { type: "Feature", id: "AFG", properties: { name: "Afghanistan" }, geometry: { type: "Polygon", coordinates: [ //lots of coordinates ] } } // lots of other countries ] } ``` Note that the `map_json` variable is just a JavaScript object that adheres to the GeoJSON structure (it adds an `id` property which is optional). This is very important. If the object didn't adhere to the GeoJSON structure, D3 would not work as it should. In production, you would probably make an AJAX call to get this data, or at the very least, create your own geoJSON file similar to the one being hosted on rawgit.com. The setup above was created to make learning easier by decreasing the complexity associated with AJAX. ## Use a projection Now let's start our `app.js` file: ```javascript var width = 960; var height = 490; d3.select('svg') .attr('width', width) .attr('height', height); ``` At the bottom of `app.js` let's add: ```javascript var worldProjection = d3.geoEquirectangular(); ``` This generates a projection, which governs how we're going to display a round world on a flat screen. There's lots of different types of projections we can use: https://github.com/d3/d3-geo/blob/master/README.md#azimuthal-projections The line above tells D3 to create an equirectangular projection (https://github.com/d3/d3-geo/blob/master/README.md#geoEquirectangular) ## Generate a `` using a projection and the GeoJSON data Now that we have our projection, we're going to generate `` elements for each data element in the `map_json.features` array. Then we set the fill of each element to `#099`. Add this at the end of app.js: ```javascript d3.select('svg').selectAll('path') .data(map_json.features) .enter() .append('path') .attr('fill', '#099'); ``` Here's what it should look like at the moment if we open index.html in Chrome and view the elements tab in the developer tools: ![](https://i.imgur.com/ljSlk4s.png) We created the `path` elements, but they each need a `d` attribute which will determine how they're going to drawn (i.e. their shape). We want something like: ```javascript d3.selectAll('path').attr('d', function(datum, index){ //somehow use datum to generate the value for the 'd' attributes }); ``` Writing the kind of code described in the comment above would be very difficult. Luckily, D3 can generate that entire function for us. All we need to do is specify the projection that we created earlier. At the bottom of `app.js` add the following: ```javascript var dAttributeFunction = d3.geoPath() .projection(worldProjection); d3.selectAll('path').attr('d', dAttributeFunction); ``` `geoPath()` generates the function that we'll use for the `d` attribute, and `projection(worldProjection)` tells it to use the `worldProjection` var created earlier so that the `path` elements appear as an equirectangular projection like this (This is helpful because we can use different projects to view a round world on a flat screen in different ways): ![](https://i.imgur.com/hX7hOoB.png) ## Conclusion In this section we've covered how to use D3 to create a projection and render GeoJSON data as a map, and we've learned about using different projects to visualize the world. This can be helpful when displaying populations or perhaps average rainfall of various regions. Congratulations! You've made it to the end of this book. Now go off and create amazing visualizations.